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Physics

New submissions

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New submissions for Thu, 22 Aug 19

[1]  arXiv:1908.07522 [pdf, other]
Title: The effects of clinically-derived parametric data uncertainty in patient-specific coronary simulations with deformable walls
Subjects: Medical Physics (physics.med-ph); Computational Physics (physics.comp-ph); Fluid Dynamics (physics.flu-dyn)

Cardiovascular simulations are increasingly used for non-invasive diagnosis of cardiovascular disease, to guide treatment decisions, and in the design of medical devices. Quantitative assessment of the variability of simulation outputs due to input uncertainty is a key step toward further integration of cardiovascular simulations in the clinical workflow. In this study, we present uncertainty quantification in computational models of the coronary circulation to investigate the effect of uncertain parameters, including coronary pressure waveform, intramyocardial pressure, morphometry exponent, and the vascular wall Young's modulus. We employ a left coronary artery model with deformable vessel walls, simulated via an ALE framework for FSI, with a prescribed inlet pressure and open-loop lumped parameter network outlet boundary conditions. Stochastic modeling of the uncertain inputs is determined from intra-coronary catheterization data or gathered from the literature. Uncertainty propagation is performed using several approaches including Monte Carlo, Quasi MC, stochastic collocation, and multiwavelet stochastic expansion. Variabilities in QoI, including branch pressure, flow, wall shear stress, and wall deformation are assessed. We find that uncertainty in inlet pressures and intramyocardial pressures significantly affect all resulting QoIs, while uncertainty in elastic modulus only affects the mechanical response of the vascular wall. Variability in the morphometry exponent has little effect on coronary hemodynamics or wall mechanics. Finally, we compare convergence behaviors of statistics of QoIs using several uncertainty propagation methods. From the simulation results, we conclude that the multi-wavelet stochastic expansion shows superior accuracy and performance against Quasi Monte Carlo and stochastic collocation methods.

[2]  arXiv:1908.07552 [pdf]
Title: Control of resonant ionization as a function of time delay between two XUV few-femtosecond pulses. Quantitative application to helium
Subjects: Atomic Physics (physics.atom-ph); Chemical Physics (physics.chem-ph)

It is shown that it is feasible to use ultrashort time delay between two XUV femtosecond pulses in order to control two photon resonant ionization. The proposal is demonstrated on the spectrum of Helium, in terms of nonperturbative solutions of the time dependent Schroedinger equation. Comparison with results from second order time dependent perturbation theory provides additional insight.

[3]  arXiv:1908.07556 [pdf, other]
Title: DNS Study of the Global Heat Release Rate During Early Flame Kernel Development under Engine Conditions
Subjects: Fluid Dynamics (physics.flu-dyn)

Despite the high technical relevance of early flame kernel development for the reduction of cycle-to-cycle variations in spark ignition engines, there is still a need for a better fundamental understanding of the governing in-cylinder phenomena in order to enable resilient early flame growth. To isolate the effects of small- and large-scale turbulent flow motion on the young flame kernel, a three-dimensional DNS database has been designed to be representative for engine part load conditions. The analysis is focussed on flame displacement speed and flame area in order to investigate effects of flame structure and flame geometry on the global burning rate evolution. It is shown that despite a Karlovitz number of up to 13, which is at the upper range of conventional engine operation, thickening of the averaged flame structure by small-scale turbulent mixing is not observed. After ignition effects have decayed, the flame normal displacement speed recovers the behavior of a laminar unstretched premixed flame under the considered unity-Lewis-number conditions. Run-to-run variations in the global heat release rate are shown to be primarily caused by flame kernel area dynamics. The analysis of the flame area balance equation shows that turbulence causes stochastic flame kernel area growth by affecting the curvature evolution, rather than by inducing variations in total flame area production by strain. Further, it is shown that in local segments of a fully-developed planar flame with similar surface area as the investigated flame kernels, temporal variations in flame area rate-of-change occur. Contrasting to early flame kernels, these effects can be exclusively attributed to curvature variations in negatively curved flame regions.

[4]  arXiv:1908.07562 [pdf, other]
Title: Cylindrical vector beam generator using a two-element interferometer
Subjects: Optics (physics.optics)

We realize a robust and compact cylindrical vector beam generator which consists of a simple two-element interferometer composed of a beam displacer and a cube beamsplitter. The interferometer operates on the higher-order Poincare sphere transforming a homogeneously polarized vortex into a cylindrical vector (CV) beam. We experimentally demonstrate the transformation of a single vortex beam into all the well-known CV beams and show the operations on the higher-order Poincare sphere according to the control parameters. Our method offers an alternative to the Pancharatnam-Berry phase optical elements and has the potential to be implemented as a monolithic device.

[5]  arXiv:1908.07588 [pdf]
Title: Improving indistinguishability of single photons from colloidal quantum dots using nanocavities
Subjects: Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

Colloidal quantum dots have garnered active research interest as quantum emitters due to their robust synthesis process and straightforward integration with nanophotonic platforms. However, obtaining indistinguishable photons from the colloidal quantum dots at room temperature is fundamentally challenging because they suffer from an extremely large dephasing rate. Here we propose an experimentally feasible method of obtaining indistinguishable single photons from an incoherently pumped solution-processed colloidal quantum dot coupled to a system of nanocavities. We show that by coupling a colloidal quantum dot to a pair of silicon nitride cavities, we can obtain comparable performance of a single photon source from colloidal quantum dots as other leading quantum emitters like defect centers and self-assembled quantum dots.

[6]  arXiv:1908.07598 [pdf, other]
Title: Unitary Light-2D matter Interaction and Universal Absorption in a Monolayer $\mbox{WS}_2$ Van der Waals Heterostructure Cavity
Subjects: Optics (physics.optics); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

A direct bandgap and large exciton binding energies provide an optical response dominated by excitons in monolayer transition-metal-dichalcogenides (TMDs). While various approaches have been applied to light-exciton interaction in TMDs, the achieved strength have been far below unity, and a complete picture of its underlying physical mechanisms and fundamental limits has not been provided. Here, we introduce a TMD-based Van der Waals heterostructure cavity that provides unitary excitonic absorption, and excitonic complexes emission that are observed at ultra-low excitation powers. Our results are in full agreement with a quantum theoretical framework introduced to describe the light-exciton-cavity interaction. We find that the subtle interplay between the radiative, non-radiative and dephasing decay rates plays a crucial role, and unveil a universal absorption law for excitons in 2D-systems. This enhanced light-exciton interaction paves the way to new possibilities for 2D semiconductors-based optoelectronic devices and allows more fundamental studies on phase-transitions of excitonic complexes.

[7]  arXiv:1908.07604 [pdf, other]
Title: Unscented Kalman filter (UKF) based nonlinear parameter estimation for a turbulent boundary layer: a data assimilation framework
Comments: Presented on International Symposium on Particle Image Velocimetry (13., 2019, M\"unchen)
Journal-ref: In Proceedings of the 13th International Symposium on Particle Image Velocimetry, 2019
Subjects: Fluid Dynamics (physics.flu-dyn)

A turbulent boundary layer is an essential flow case of fundamental and applied fluid mechanics. However, accurate measurements of turbulent boundary layer parameters (e.g., friction velocity $u_\tau$ and wall shear $\tau_w$), are challenging, especially for high speed flows (Smits et al., 2011). Many direct and/or indirect diagnostic techniques have been developed to measure wall shear stress (Vinuesa et al., 2017). However, based on different principles, these techniques usually give different results with different uncertainties. The current study introduces a nonlinear data assimilation framework based on the Unscented Kalman Filter that can fuse information from i) noisy and gappy measurements from Stereo Particle Image Velocimetry, a Preston tube, and a MEMS shear stress sensor, as well as ii) the uncertainties of the measurements to estimate the parameters of a turbulent boundary layer. A direct numerical simulation of a fully developed turbulent boundary layer flow at Mach 0.3 is used first to validate the data assimilation algorithm. The algorithm is then applied to experimental data of a flow at Mach 0.3, which are obtained in a blowdown wind tunnel facility. The UKF-based data assimilation algorithm is robust to uncertain and gappy experimental data and is able to provide accurate estimates of turbulent boundary layer parameters.

[8]  arXiv:1908.07610 [pdf]
Title: Design of automatic focus detection system during dental laser operation using biphoton
Comments: 6 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:1907.10757
Subjects: Medical Physics (physics.med-ph); Applied Physics (physics.app-ph)

Dental laser has been developed recently and gradually replaced the conventional dental treatment methods, especially in dental caries removal. The utility of laser in dentistry contributes to reduce the pain during surgery and decrease the risk of sequelae after surgery. Although bringing about several beneficial properties such as excellent directionality, high speed, ultra-tiny active area and tunable energy concentration, laser is still quite limited in dental surgery because of the difficulty in realtime high precision detection of laser focus during the operation. In this study, a cutting-edge optical system for real-time focus detection during dental laser operation employing the quantum entanglement of photon pairs created via spontaneous parametric down-conversion (SPDC) is presented. By examining the biphoton correlation through the probability distribution of the photons on the detectors, which varies according to the displacement of the main pump photon stream, the defocusing distance of the specimen which is used as the sample tooth can be determined precisely. The proposed technique can be applied to design automatic focus detection system for use in dental laser operation.

[9]  arXiv:1908.07622 [pdf, other]
Title: Prospects of Earth-skimming neutrino detection with HAWC
Authors: Hermes León Vargas (for the HAWC Collaboration)
Comments: Presented at the 36th International Cosmic Ray Conference (ICRC 2019)
Subjects: Instrumentation and Detectors (physics.ins-det); High Energy Physics - Experiment (hep-ex)

Searches for Earth-skimming neutrinos using volcanoes have not yet been achieved, but it is a promising technique for the detection of neutrinos above 1 PeV. The HAWC observatory is located in the vicinity of the highest mountain in Mexico, the Pico de Orizaba volcano, which shields the detector from quasi-horizontal very high-energy (VHE) muons. The large amount of shielding, up to 8 km of rock, enables the suppression of the large VHE muon background and makes neutrino detection possible. In this work we present the first steps towards the implementation of the Earth-skimming technique for the indirect measurement of tau neutrinos with HAWC. The results include the description of the charged lepton tracking reconstruction algorithm developed for this study and the initial analysis of the background of VHE muons using half a year of data.

[10]  arXiv:1908.07627 [pdf, other]
Title: Control of molecular ultracold plasma relaxation dynamics by mm-wave Rydberg-Rydberg transitions
Subjects: Plasma Physics (physics.plasm-ph); Chemical Physics (physics.chem-ph)

Resonant mm-wave fields drive $n_0f(2) \rightarrow (n_0 \pm 1)g(2)$ transitions in a state-selected $n_0f(2)$ Rydberg gas of NO. This transformation produces a clear signature in the selected field ionization spectrum and dramatically increases the intensity of corresponding features in the spectrum of the $n_0f(2)$ Rydberg series observed in UV-UV double resonant transitions via the $A ~^2\Sigma^+, ~N'=0$ state. Here, $n_0$ refers to the principal quantum number of an $f$ Rydberg state converging to the $N^+=2$ rotational state of NO$^+ X~ ^1\Sigma^+$. Enhancement owing to transitions from $\ell=3$ ($f$) to $\ell=4$ ($g$) appears both in the electron signal detected at early time by the field ionization of Rydberg molecules and, 40 $\mu$s later, as the late-peak signal of plasma in a state of arrested relaxation. Similar stabilization and enhanced intensity also occurs for shorter-lived interloping complex resonances, $44p(0) - 43d(1)$ and $43p(0) - 42d(1)$. We conclude from these observations that avalanche alone does not guarantee a plasma state of arrested relaxation. But rather, the formation of an arrested phase requires both avalanche-produced NO$^+$ ions and a persistent population of long-lived Rydberg molecules.

[11]  arXiv:1908.07633 [pdf, other]
Title: Dissipative dynamics of atomic and molecular Rydberg gases: Avalanche to ultracold plasma states of strong coupling
Subjects: Plasma Physics (physics.plasm-ph); Chemical Physics (physics.chem-ph)

Not long after metastable xenon was photoionized in a magneto-optical trap, groups in Europe and North America found that similar states of ionized gas evolved spontaneously from state-selected, high principal quantum number Rydberg gases. Studies of atomic xenon and molecular nitric oxide entrained in a supersonically cooled molecular beam subsequently showed much the same final state evolved from a sequence of prompt Penning ionization and electron-impact avalanche to plasma, well-described by coupled rate-equation simulations. But, measured over longer times, the molecular ultracold plasma was found to exhibit an anomalous combination of very long lifetime and very low apparent electron temperature. This review summarizes early developments in the study of ultracold plasmas formed by atomic and molecular Rydberg gases, and then details observations as they combine to characterize properties of the nitric oxide molecular ultracold plasma that appear to call for an explanation beyond the realm of conventional plasma physics.

[12]  arXiv:1908.07638 [pdf, other]
Title: Coupled rate-equation hydrodynamic simulation of a Rydberg gas Gaussian ellipsoid: Classical avalanche and evolution to molecular plasma
Journal-ref: Chemical Physics 514, 55-66 (2018)
Subjects: Chemical Physics (physics.chem-ph); Plasma Physics (physics.plasm-ph)

An ellipsoidal volume of Rydberg molecules, entrained in a supersonic molecular beam, evolves on a nanosecond timescale to form a strongly coupled ultracold plasma. We present coupled rate-equation simulations that model the underlying kinetic processes and molecular dissociation channels in both a uniformly distributed plasma and under the conditions dictated by our experimental geometry. Simulations predict a fast electron-driven collisional avalanche to plasma followed by slow electron-ion recombination. Within 20 $\mu$s, release of Rydberg binding energy raises the electron temperature of a static plasma to $T_e = 100$ K. Providing for a quasi-self-similar expansion, the hot electron gas drives ion radial motion, reducing $T_e$. These simulations provide a classical baseline model from which to consider quantum effects in the evolution of charge gradients and ambipolar forces in an experimental system undergoing responsive avalanche dynamics.

[13]  arXiv:1908.07661 [pdf]
Title: Designing Anisotropic Microstructures with Spectral Density Function
Subjects: Applied Physics (physics.app-ph); Materials Science (cond-mat.mtrl-sci)

Materials' microstructure strongly influences its performance and is thus a critical aspect in design of functional materials. Previous efforts on microstructure mediated design mostly assume isotropy, which is not ideal when material performance is dependent on an underlying transport phenomenon. In this article, we propose an anisotropic microstructure design strategy that leverages Spectral Density Function (SDF) for rapid reconstruction of high resolution, two phase, isotropic or anisotropic microstructures in 2D and 3D. We demonstrate that SDF microstructure representation provides an intuitive method for quantifying anisotropy through a dimensionless scalar variable termed anisotropy index. The computational efficiency and low dimensional microstructure representation enabled by our method is demonstrated through an active layer design case study for Bulk Heterojunction Organic Photovoltaic Cells (OPVCs). Results indicate that optimized design, exhibiting strong anisotropy, outperforms isotropic active layer designs. Further, we show that Cross-sectional Scanning Tunneling Microscopy and Spectroscopy (XSTM/S) is as an effective tool for characterization of anisotropic microstructures.

[14]  arXiv:1908.07663 [pdf]
Title: Attenuation of an Ultrasound Contrast Agent Estimated from Transient Solution of Linearized Rayleigh-Plesset Equation
Authors: Lang Xia
Comments: 11 pages
Subjects: Fluid Dynamics (physics.flu-dyn)

The attenuation of low-intensity acoustic waves in the suspension of ultrasound contrast agents (UCAs, microbubbles) is determined by the oscillation of the microbubbles in the medium. This bubble-induced attenuation is a linear phenomenon and can be estimated via a linearized Rayleigh-Plesset equation (RPE). In the material characterization, theoretical attenuation is estimated from steady state oscillation of an UCA and immediately compared with experimental attenuation data that are usually measured by shot-pulse ultrasound. However, discrepancy could exist in the characterization if the UCA does not ring up to steady state oscillation. In this article, we investigate the situation where the transient solution of the RPE is not negligible and discuss its impact on the modeling of the shell parameters of an UCA. We provide a formula for attenuation estimation considering the contribution due to transient oscillation.

[15]  arXiv:1908.07680 [pdf, ps, other]
Title: Effects of alpha-proton differential flow on proton temperature anisotropy instabilities in the solar wind: Wind observations
Comments: 24 page, 5 figures, accepted for publication by ApJ
Subjects: Space Physics (physics.space-ph)

Plasma kinetic waves and alpha-proton differential flow are two important subjects on the topic of evolution of the solar wind. Based on the Wind data during 2005-2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha-proton differential flow Vd. As Vd rotates from anti-Sunward direction to Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of Vd further illustrates that large kinetic energies of alpha-proton differential flow correspond to high occurrence rates of ECWs. These results are well consistent with theoretical predictions for effects of alpha-proton differential flow on proton temperature anisotropy instabilities.

[16]  arXiv:1908.07682 [pdf]
Title: Depth thermography: non-invasive 3D temperature profiling using infrared thermal emission
Comments: Main text + supplementary
Subjects: Optics (physics.optics); Applied Physics (physics.app-ph)

We introduce a technique based on infrared thermal emission, termed depth thermography, that can remotely measure the temperature distribution beneath the surface of certain objects. Depth thermography utilizes the thermal-emission spectrum in the semitransparent spectral region of the target object to extract its temperature as a function of depth, in contrast with conventional thermography, which uses the spectrally integrated thermally emitted power to measure the surface temperature. Coupled with two-dimensional imaging, e.g., using an infrared hyperspectral camera or scanning a single-pixel spectrometer, this technique can yield volumetric temperature distributions. We carried out a proof-of-concept experiment on an asymmetrically heated fused-silica window, extracting the temperature distribution throughout the sample. Depth thermography may enable noncontact volumetric temperature measurements of microscopic objects such as multilayer electronic devices or macroscopic volumes of liquids and gasses as well as simultaneous all-optical measurements of optical and thermal properties.

[17]  arXiv:1908.07685 [pdf, other]
Title: Characterising Quantum Devices at Scale with Custom Cryo-CMOS
Subjects: Applied Physics (physics.app-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)

We make use of a custom-designed cryo-CMOS multiplexer (MUX) to enable multiple quantum devices to be characterized in a single cool-down of a dilution refrigerator. Combined with a packaging approach that integrates cryo-CMOS chips and a hot-swappable, parallel device test platform, we describe how this setup takes a standard wiring configuration as input and expands the capability for batch-characterization of quantum devices at milli-Kelvin temperatures and high magnetic fields. The architecture of the cryo-CMOS multiplexer is discussed and performance benchmarked using few-electron quantum dots and Hall mobility-mapping measurements.

[18]  arXiv:1908.07698 [pdf, other]
Title: Symmetry-preserving WENO limiters
Subjects: Computational Physics (physics.comp-ph)

Weighted essentially non-oscillatory (WENO) reconstruction schemes are presented that preserve cylindrical symmetry for radial flows on an equal-angle polar mesh. These new WENO schemes are used with a Lagrangian discontinuous Galerkin (DG) hydrodynamic method. The solution polynomials are reconstructed using the WENO schemes where the DG solution is the central stencil. A suite of challenging test problems are calculated to demonstrate the accuracy and robustness of the new WENO schemes.

[19]  arXiv:1908.07744 [pdf, other]
Title: Generation and characterization of few-pulse attosecond pulse trains at 100kHz repetition rate
Comments: 7 pages, 4 figures, submitted version
Subjects: Optics (physics.optics)

Many experiments in attosecond science will benefit from attosecond pulses at high repetition rates with sufficient photon flux for pump-probe experiments. We use 7fs, 800nm pulses from a non-collinear optical parametric chirped pulse amplification (NOPCPA) laser system to generate few-pulse attosecond pulse trains (APTs) in the extreme ultraviolet (XUV) at a repetition rate of 100kHz. The pulse trains have been fully characterized by recording FROG-CRAB (Frequency-Resolved Optical Gating for Complete Reconstruction of Attosecond Bursts) traces with a velocity map imaging spectrometer.

[20]  arXiv:1908.07745 [pdf, other]
Title: Random Mode Coupling Assists Kerr Beam Self-Cleaning in a Graded-Index Multimode Optical Fiber
Subjects: Optics (physics.optics)

In this paper, we numerically investigate the process of beam self-cleaning in a graded-index multimode optical fiber, by using the coupled-mode model. We introduce various models of random linear coupling between spatial modes, including coupling between all modes, or only between degenerate ones, and investigate the effects of random mode coupling on the beam self-cleaning process. The results of numerical investigations are in complete agreement with our experimental data.

[21]  arXiv:1908.07756 [pdf]
Title: Gravity inversion using a directional filtering method
Subjects: Geophysics (physics.geo-ph)

The calculation of the underground density field from measured gravity data has been done by a variety of methods of varied types. The use of the vector gravity components is here addressed in order to develop one accurate gravity inversion method. The equation to directly calculate the Cartesian components of the gravity field from the density field is transformed in one non-exact correction equation. The discrete vector equations are transformed into scalar equations by using one vector function as directional filter. The used equations have the property that the convergence of the calculated density field to the exact values may only happen if the same occurs to the gravity field. The equations of the method so admit asymptotic exact solutions to the gravity inversion problem. The use of a smooth directional filter allows the equilibrated influence of all the gravity data over all the calculated density deviation corrections. The unknown density field is calculated by means of one iterative under-relaxed method. Two synthetic gravity fields are inverted by means of the presented method. The calculated shallow density fields are very accurate while the far density fields apparently converge to the correct values at very low convergence rates.

[22]  arXiv:1908.07760 [pdf, other]
Title: Turbulence vs. fire hose instabilities: 3-D hybrid expanding box simulations
Comments: 12 pages, 10 figures
Subjects: Space Physics (physics.space-ph); Solar and Stellar Astrophysics (astro-ph.SR); Plasma Physics (physics.plasm-ph)

The relationship between a decaying plasma turbulence and proton fire hose instabilities in a slowly expanding plasma is investigated using three-dimensional (3-D) hybrid expanding box simulations. We impose an initial ambient magnetic field along the radial direction, and we start with an isotropic spectrum of large-scale, linearly-polarized, random-phase Alfvenic fluctuations with zero cross-helicity. A turbulent cascade rapidly develops and leads to a weak proton heating that is not sufficient to overcome the expansion-driven perpendicular cooling. The plasma system eventually drives the parallel and oblique fire hose instabilities that generate quasi-monochromatic wave packets that reduce the proton temperature anisotropy. The fire hose wave activity has a low amplitude with wave vectors quasi-parallel/oblique with respect to the ambient magnetic field outside of the region dominated by the turbulent cascade and is discernible in one-dimensional power spectra taken only in the direction quasi-parallel/oblique with respect to the ambient magnetic field; at quasi-perpendicular angles the wave activity is hidden by the turbulent background. These waves are partly reabsorbed by protons and partly couple to and participate in the turbulent cascade. Their presence reduces kurtosis, a measure of intermittency, and the Shannon entropy but increases the Jensen-Shannon complexity of magnetic fluctuations; these changes are weak and anisotropic with respect to the ambient magnetic field and it's not clear if they can be used to indirectly discern the presence of instability-driven waves.

[23]  arXiv:1908.07767 [pdf]
Title: Electronic interaction of slow hydrogen and helium ions in the nickel-silicon system
Comments: 16 pages, 4 figures, accepted manuscript for Physical Review A
Subjects: Atomic Physics (physics.atom-ph)

Electronic stopping cross sections (SCS) of nickel, silicon and nickel-silicon alloys for protons and helium (He) ions are studied in the regime of medium and low energy ion scattering, i.e., for ion energies in the range from 500 eV to 200 keV. For protons, at velocities below the Bohr velocity the deduced SCS is proportional to the ion velocity for all investigated materials. In contrast, for He ions non-linear velocity scaling is observed in all investigated materials. Static calculations using density functional theory (DFT) available from literature accurately predict the SCS of Ni and Ni-Si alloy in the regime with observed velocity proportionality. At higher energies, the energy dependence of the deduced SCS of Ni for protons and He ions agrees with the prediction by recent time dependent DFT calculations. The measured SCS of the Ni-Si alloy was compared to the SCS obtained from Bragg's rule based on SCS for Ni and Si deduced in this study, yielding good agreement for protons, but systematic deviations for He projectiles, by almost 20%. Overall, the obtained data indicate the importance of non adiabatic processes such as charge exchange for proper modelling of electronic stopping of in particular medium energy ions heavier than protons in solids.

[24]  arXiv:1908.07789 [pdf, ps, other]
Title: Two-frequency approach to the theory of atmospheric acoustic-gravity waves
Comments: 5 pages, 3 figures
Subjects: Atmospheric and Oceanic Physics (physics.ao-ph)

The propagation of acoustic-gravity waves (AGWs) in the stratified isothermal atmosphere is analyzed using methods of the oscillation theory. It is shown that AGW in the atmosphere can be considered as an oscillatory process occurring at two eigenfrequencies. This consideration makes it possible to explain some of the observed properties of AGWs. The solutions for perturbed hydrodynamic velocity versus time and spectral characteristics are obtained in a real, but not complex, variables.

[25]  arXiv:1908.07797 [pdf, other]
Title: Ultra-thin, entirely flat, Umklapp lenses
Comments: 4 pages, 5 figures. 1 page of supplemental material with 2 additional figures
Subjects: Optics (physics.optics)

We design ultra-thin, entirely flat, dielectric lenses using crystal momentum transfer, so-called Umklapp processes, achieving the required wave control for a new mechanism of flat lensing; physically, these lenses take advantage of abrupt changes in the periodicity of a structured line array so there is an overlap between the first Brillouin zone of one medium with the second Brillouin zone of the other. At the interface between regions of different periodicity, surface, array guided, waves hybridise into reversed propagating beams directed into the material exterior to the array. This control, and redirection, of waves then enables the device to operate as a Pendry-Veselago lens that is one unit cell in width, with no need for an explicit negative refractive index. Simulations using an array embedded in a slab of silicon nitride ($\text{Si}_3\text{N}_4$) in air, operating at visible wavelengths between $420 - 500\text{THz}$ demonstrate the effect.

[26]  arXiv:1908.07800 [pdf, other]
Title: The influence of posture, applied force and perturbation direction on hip joint viscoelasticity
Subjects: Medical Physics (physics.med-ph)

Limb viscoelasticity is a critical factor used to regulate the interaction with the environment. It plays a key role in modelling human sensorimotor control, and can be used to assess the condition of healthy and neurologically affected individuals. This paper reports the estimation of hip joint viscoelasticity during voluntary force control using a novel device that applies a leg displacement without constraining the hip joint. The influence of hip angle, applied limb force and perturbation direction on the stiffness values was studied in ten subjects. No difference was detected in the hip joint stiffness between the dominant and non-dominant legs, but a small dependency was observed on the perturbation direction. Both hip stiffness and viscosity increased monotonically with the applied force magnitude, with posture to being observed to have a slight influence. These results are in line with previous measurements carried out on upper limbs, and can be used as a baseline for lower limb movement simulation and further neuromechanical investigations.

[27]  arXiv:1908.07815 [pdf]
Title: Impact of a doping-induced space-charge region on the collection of photo-generated charge carriers in thin-film solar cells based on low-mobility semiconductors
Comments: Main text (29 pages, 7 figures) and supplemental material (7 pages, 4 figures)
Subjects: Applied Physics (physics.app-ph); Materials Science (cond-mat.mtrl-sci)

Unintentional doping of the active layer is a source for lowered device performance in organic solar cells. The effect of doping is to induce a space-charge region within the active layer, generally resulting in increased recombination losses. In this work, the impact of a doping-induced space-charge region on the current-voltage characteristics of low-mobility solar cell devices has been clarified by means of analytical derivations and numerical device simulations. It is found that, in case of a doped active layer, the collection efficiency of photo-generated charge carriers is independent of the light intensity and exhibits a distinct voltage dependence, resulting in an apparent electric-field dependence of the photocurrent. Furthermore, an analytical expression describing the behavior of the photocurrent is derived. The validity of the analytical model is verified by numerical drift-diffusion simulations and demonstrated experimentally on solution-processed organic solar cells. Based on the theoretical results, conditions of how to overcome charge collection losses caused by doping are discussed. Furthermore, the presented analytical framework provides tools to distinguish between different mechanisms leading to voltage dependent photocurrents.

[28]  arXiv:1908.07823 [pdf, other]
Title: Sound waves move matter
Authors: Davison E. Soper
Comments: 6 pages with one figure
Subjects: Classical Physics (physics.class-ph); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

A recent Letter has reported that sound waves can carry gravitational mass. I analyze this effect in a Hooke's law solid, considering a wave packet moving in the $z$ direction with an amplitude that is independent of $x$ and $y$. The analysis shows that, at second order in an expansion around small amplitude vibrations, there is a small net motion of material, and thus mass, in the direction opposite to the wave packet propagation. This is a straightforward consequence of Newton's laws.

[29]  arXiv:1908.07824 [pdf]
Title: Deep Recurrent Architectures for Seismic Tomography
Comments: Published in the 81st EAGE Conference and Exhibition, 2019
Subjects: Geophysics (physics.geo-ph); Image and Video Processing (eess.IV); Signal Processing (eess.SP)

This paper introduces novel deep recurrent neural network architectures for Velocity Model Building (VMB), which is beyond what Araya-Polo et al 2018 pioneered with the Machine Learning-based seismic tomography built with convolutional non-recurrent neural network. Our investigation includes the utilization of basic recurrent neural network (RNN) cells, as well as Long Short Term Memory (LSTM) and Gated Recurrent Unit (GRU) cells. Performance evaluation reveals that salt bodies are consistently predicted more accurately by GRU and LSTM-based architectures, as compared to non-recurrent architectures. The results take us a step closer to the final goal of a reliable fully Machine Learning-based tomography from pre-stack data, which when achieved will reduce the VMB turnaround from weeks to days.

[30]  arXiv:1908.07825 [pdf, other]
Title: On the thermodynamics-based equilibrium beach profile derived by Jenkins and Inman (2006)
Subjects: Geophysics (physics.geo-ph)

Based on the second law of thermodynamics, Jenkins and Inman (2006 J. Geophys. Res., 111, C02003) proposed that an equilibrium beach profile described by an elliptic cycloid maximises the rate of wave energy dissipation. However, here we i) highlight that the solution proposed by Jenkins and Inman (the elliptic cycloid) is difficult to recover due to important information being absent; and ii) show that, in fact, other curves can be proposed (e.g. a line) that yield larger rates of energy dissipation as formulated by the aforementioned authors, thus invalidating their hypothesis. Combined, these two crucial aspects associated with the reproducibility and validity of the research invite further scrutiny of the work and conclusions reached by Jenkins and Inman (2006). This paper also serves as an appendix to Maldonado (under review).

[31]  arXiv:1908.07826 [pdf, other]
Title: Finite element modeling of dynamic frictional rupture with rate and state friction
Subjects: Geophysics (physics.geo-ph); Computational Physics (physics.comp-ph)

Numerous laboratory experiments have demonstrated the dependence of the friction coefficient on the interfacial slip rate and the contact history, a behavior generically called rate and state friction. Although numerical models have been widely used for analyzing rate and state friction, in general they consider infinite elastic domains surrounding the sliding interface and rely on boundary integral formulations. Much less work has been dedicated to modeling finite size systems to account for interactions with boundaries. This paper investigates rate and state frictional interfaces in the context of finite size systems with the finite element method in explicit dynamics. We investigate the long term behavior of the sliding interface for two different friction laws: a velocity weakening law, for which the friction monotonously decreases with increasing sliding velocity, and a velocity weakening-strengthening law, for which the friction coefficient first decreases but then increases above a critical velocity. We show that for both friction laws at finite times, that is before wave reflections from the boundaries come back to the sliding interface, a temporary steady state sliding is reached, with a well-defined stress drop at the interface. This stress drop gives rise to a stress concentration and leads to an analogy between friction and fracture. However, at longer times, that is after multiple wave reflections, the stress drop is essentially zero, resulting in losing the analogy with fracture mechanics. Finally, the simulations reveal that velocity weakening is unstable at long time scales, as it results in an acceleration of the sliding blocks. On the other hand, velocity weakening-strengthening reaches a steady state sliding configuration.

[32]  arXiv:1908.07841 [pdf, other]
Title: Ranking Viscous Finger Simulations to an Acquired Ground Truth with Topology-aware Matchings
Subjects: Geophysics (physics.geo-ph); Computational Geometry (cs.CG); Computer Vision and Pattern Recognition (cs.CV); Image and Video Processing (eess.IV)

This application paper presents a novel framework based on topological data analysis for the automatic evaluation and ranking of viscous finger simulation runs in an ensemble with respect to a reference acquisition. Individual fingers in a given time-step are associated with critical point pairs in the distance field to the injection point, forming persistence diagrams. Different metrics, based on optimal transport, for comparing time-varying persistence diagrams in this specific applicative case are introduced. We evaluate the relevance of the rankings obtained with these metrics, both qualitatively thanks to a lightweight web visual interface, and quantitatively by studying the deviation from a reference ranking suggested by experts. Extensive experiments show the quantitative superiority of our approach compared to traditional alternatives. Our web interface allows experts to conveniently explore the produced rankings. We show a complete viscous fingering case study demonstrating the utility of our approach in the context of porous media fluid flow, where our framework can be used to automatically discard physically-irrelevant simulation runs from the ensemble and rank the most plausible ones. We document an in-situ implementation to lighten I/O and performance constraints arising in the context of parametric studies.

[33]  arXiv:1908.07849 [pdf, other]
Title: Semi-supervised Sequence Modeling for Elastic Impedance Inversion
Comments: A manuscript in Interpretation. arXiv admin note: text overlap with arXiv:1905.13412
Subjects: Geophysics (physics.geo-ph); Machine Learning (cs.LG); Signal Processing (eess.SP); Machine Learning (stat.ML)

Recent applications of machine learning algorithms in the seismic domain have shown great potential in different areas such as seismic inversion and interpretation. However, such algorithms rarely enforce geophysical constraints - the lack of which might lead to undesirable results. To overcome this issue, we have developed a semi-supervised sequence modeling framework based on recurrent neural networks for elastic impedance inversion from multi-angle seismic data. Specifically, seismic traces and elastic impedance (EI) traces are modeled as a time series. Then, a neural-network-based inversion model comprising convolutional and recurrent neural layers is used to invert seismic data for EI. The proposed workflow uses well-log data to guide the inversion. In addition, it uses seismic forward modeling to regularize the training and to serve as a geophysical constraint for the inversion. The proposed workflow achieves an average correlation of 98% between the estimated and target EI using 10 well logs for training on a synthetic data set.

[34]  arXiv:1908.07850 [pdf, other]
Title: Statistics of rigid fibers in strongly sheared turbulence
Comments: 7 pages, 6 figures, Phys. Rev. Fluids (Rapid Communication)
Journal-ref: Phys. Rev. Fluids 4, 072301(R) 2019
Subjects: Fluid Dynamics (physics.flu-dyn)

Practically all flows are turbulent in nature and contain some kind of irregularly-shaped particles, e.g. dirt, pollen, or life forms such as bacteria or insects. The effect of the particles on such flows and vice-versa are highly non-trivial and are not completely understood, particularly when the particles are finite-sized. Here we report an experimental study of millimetric fibers in a strongly sheared turbulent flow. We find that the fibers show a preferred orientation of $-0.38\pi \pm 0.05\pi$ ($-68 \pm 9^\circ$) with respect to the mean flow direction in high-Reynolds number Taylor-Couette turbulence, for all studied Reynolds numbers, fiber concentrations, and locations. Despite the finite-size of the anisotropic particles, we can explain the preferential alignment by using Jefferey's equation, which provides evidence of the benefit of a simplified point-particle approach. Furthermore, the fiber angular velocity is strongly intermittent, again indicative of point-particle-like behavior in turbulence. Thus large anisotropic particles still can retain signatures of the local flow despite classical spatial and temporal filtering effects.

[35]  arXiv:1908.07852 [pdf]
Title: Perfect proton selectivity in ion transport through two-dimensional crystals
Subjects: Applied Physics (physics.app-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Defect-free monolayers of graphene and hexagonal boron nitride were previously shown to be surprisingly permeable to thermal protons, despite being completely impenetrable to all gases. It remains untested whether small ions can permeate through the two-dimensional crystals. Here we show that mechanically exfoliated graphene and hexagonal boron nitride exhibit perfect Nernst selectivity such that only protons can permeate through, with no detectable flow of counterions. In the experiments, we used suspended monolayers that had few if any atomic-scale defects, as shown by gas permeation tests, and placed them to separate reservoirs filled with hydrochloric acid solutions. Protons accounted for all the electrical current and chloride ions were blocked. This result corroborates the previous conclusion that thermal protons can pierce defect-free two-dimensional crystals. Besides importance for theoretical developments, our results are also of interest for research on various separation technologies based on two-dimensional materials.

[36]  arXiv:1908.07864 [pdf, other]
Title: Large Area Photo-Detection System using 3-inch PMTs for the Hyper-Kamiokande Outer Detector
Authors: Stephane Zsoldos
Comments: 6 pages, 6 figures, presented at the 5th International Workshop on New Photon-Detectors (PD18). To be published in the JPS Conference Proceedings
Subjects: Instrumentation and Detectors (physics.ins-det); High Energy Physics - Experiment (hep-ex)

Hyper-Kamiokande, scheduled to begin construction as soon as 2020, is a next generation underground water Cherenkov detector, based on the highly successful Super-Kamiokande experiment. It will serve as a far detector, 295 km away, of a long baseline neutrino experiment for the upgraded J-PARC beam in Japan. It will also be a detector capable of observing --- far beyond the sensitivity of the Super-Kamiokande detector --- proton decay, atmospheric neutrinos, and neutrinos from astronomical sources.
An Outer Detector (OD) consisting of PMTs mounted behind the inner detector PMTs and facing outwards to view the outer shell of the cylindrical tank, would provide topological information to identify interactions originating from particles outside the inner detector. Any optimization would lead to a significant improvement for the physics goals of the experiment, which are the measurement of the CP leptonic phase and the determination of the neutrino mass hierarchy.
An original setup using small 3" PMTs is being designed for the Hyper-Kamiokande OD. They would give better redundancy, spatial, and angular resolution, as they would be twice or three times more photosensors that the original 8" design proposal of the experiment, and for a reduce cost. Several 3" PMTs candidates considered for the Hyper-Kamiokande OD have been characterized at Queen Mary University London. They all show a very low dark counts and good collection efficiency, which makes them excellent choice to be used in the experiment.

[37]  arXiv:1908.07865 [pdf]
Title: Defining best practices in the management of geothermal exploration data
Journal-ref: World Geothermal Congress, 2020, Reykjavik, Iceland
Subjects: Geophysics (physics.geo-ph)

The objective of this work is to define best practices in the management of geothermal exploration data. This study builds on a questionnaire to survey the geothermal data management practices in mature geothermal markets. The inquiry targeted public Regulatory entities with overview of geothermal resources as well as public and private developers. Topics covered in the questionnaire range from the country status to the database set up. The questionnaire focused on the specifications, usage and investments required for installing/maintaining information systems capable of managing exploration data. In addition, information on the different regulatory frameworks and company policies for managing/sharing exploration data has been gathered to identify the requirements imposed on the design of information systems. The responses were analyzed to identify commonalities in data management practices. They reveal that installing an Information System (IS) is the best practice to systematically and securely manage exploration data. They also provide recommendations with respect to the regulatory framework, data types, data collection methodologies, data storage, data quality control, data accessibility and dissemination, IS architecture, financial investments and human resources required to develop a state-of-the art IS. These results will guide the design of future technical assistance programs for beneficiaries of World Bank support to geothermal exploration activities and it is our belief that they will be beneficial for the geothermal sector at large.

[38]  arXiv:1908.07932 [pdf, other]
Title: Detection of symmetry using a crystallographic image processing algorithm
Authors: Paul Plachinda
Comments: 14 pages, 15 figures
Subjects: Popular Physics (physics.pop-ph); Physics Education (physics.ed-ph)

This article presents an automated method to quantify and detect symmetry elements in 2D patterns by means of image processing. Escher's woodcuts, a widely recognized didactic tool for crystallographic education of students, were used to demonstrate this approach. We also discuss peculiarities in the detection of black and white symmetry, color symmetry, and detection of the "hidden" and "broken" symmetry elements by means of the phase origin map approach.

[39]  arXiv:1908.07936 [pdf, ps, other]
Title: Obliterating Thingness: An Introduction to the "What" and the "So What" of Quantum Physics
Comments: Foundations of Science (2019)
Subjects: Popular Physics (physics.pop-ph)

This essay provides a short introduction to the ideas and potential implications of quantum physics for scholars in the arts, humanities, and social sciences. Quantum-inspired ideas pepper current discourse in all of these fields, in ways that range from playful metaphors to sweeping ontological claims. We explain several of the most important concepts at the core of quantum theory, carefully delineating the scope and bounds of currently established science, in order to aid the evaluation of such claims. In particular, we emphasize that the smallest units of matter and light, as described in quantum physics, are not {\em things}, meaning that they do not obey the logic we take for granted when discussing the behavior of macroscopic objects. We also highlight the substantial debate that exists within physics about the interpretation of the equations and empirical results at the core of quantum physics, noting that implicit (and contested) philosophical commitments necessarily accompany any discussion of quantum ideas that takes place in non-technical language.

[40]  arXiv:1908.07938 [pdf, other]
Title: Multiperspective Conformance Analysis of Central Venous Catheter Installation Procedure
Subjects: Medical Physics (physics.med-ph)

Training and practice play a key role in a medical students' attainment of surgical procedural skills. It is beyond doubt that good skills correlate with better clinical outcomes and improved healthcare. Timely, holistic, and effective feedback provide a significant impetus to students acquiring skills with precision. In this paper, we analyze the activities performed by students while learning the central venous catheter installation procedure. We perform a holistic analysis, using trace alignment, declarative conformance checking, data visualization, and statistical analysis techniques, at different levels of abstraction on control-flow and time perspectives and provide insights at individual student level as well as across students. These insights can help students discover what they are doing right and where they are not and take corrective steps. Instructors can uncover common patterns and mistakes that students demonstrate and think of interventions in their teaching methodology.

[41]  arXiv:1908.07940 [pdf, ps, other]
Title: The Sounds of Music : Science of Musical Scales I -- Human Perception of Sound
Authors: Sushan Konar
Comments: First of a 3-article series on Musical Scales
Journal-ref: Resonance - Journal of Science Education, 24(8), 891 (2019)
Subjects: Popular Physics (physics.pop-ph); Physics Education (physics.ed-ph)

Both, human appreciation of music and musical genres, transcend time and space. The universality of musical genres and associated musical scales is intimately linked to the physics of sound and the special characteristics of human acoustic sensitivity. In this series of articles, we examine the science underlying the development of the heptatonic scale, one of the most prevalent scales of the modern musical genres, both western and Indian.

[42]  arXiv:1908.07959 [pdf]
Title: Inverse Structural Design of Graphene/Boron Nitride Hybrids by Regressional GAN
Subjects: Computational Physics (physics.comp-ph); Materials Science (cond-mat.mtrl-sci); Applied Physics (physics.app-ph)

Inverse design of materials with desired properties is currently laborious and heavily relies on intuition of researchers through a trial-and-error process. The massive combinational spaces due to the constituent elements and their structural configurations are too overwhelming to be all searched even by high-throughput computations. Herein, we demonstrated a novel regressional generative adversarial network (RGAN) for inverse design of representative two-dimensional materials, graphene and boron-nitride (BN) hybrids. A significant novelty of the proposed RGAN is that it combines the supervised and regressional convolutional neural network (CNN) with the traditional unsupervised GAN, thus overcoming the common technical barrier in the traditional GANs, which cannot generate data associated with given continuous quantitative labels. The proposed RGAN enables to autonomously generate graphene/BN hybrids with any given bandgaps. Moreover, the generated structures exhibit high fidelity, yielding bandgaps within ~ 10% MAEF of the desired bandgaps as cross-validated by density functional theory (DFT) calculations. Further analysis by principle component analysis (PCA) and modified locally linear embedding (MLLE) methods on the latent features encoded by the regressor reveals that the generator has successfully generated structures that followed the statistical distribution of the real structures. It implies the possibility of the RGAN in recognizing physical rules hidden in the high-dimensional data. This new inverse design methodology would speed up the discovery and development of other 2D materials and beyond.

[43]  arXiv:1908.07971 [pdf, other]
Title: Equilibrium and stability of two-dimensional pinned drops
Comments: 5 pages, 4 figures
Subjects: Fluid Dynamics (physics.flu-dyn); Soft Condensed Matter (cond-mat.soft)

Superhydrophobicity relies on the stability of drops's interfaces pinned on sharp edges to sustain non-wetting (Cassie-Baxter) equilibrium states. Gibbs already pointed out that equilibrium is possible as long as the pinning angle at the edge falls between the equilibrium contact angles corresponding to the flanks of the edge. However, the lack of stability can restrict further the realizable equilibrium configurations. To find these limits we analyze here the equilibrium and stability of two-dimensional drops bounded by interfaces pinned on mathematically sharp edges. We are specifically interested on how the drop's stability depends on its size, which is measured with the Bond number $Bo = (\mathcal{W}_d/\ell_c)^2$, defined as the ratio of the drop's characteristic length scale $\mathcal{W}_d$ to the capillary length $\ell_c = \sqrt{\sigma/\rho g}$. Drops with a fixed volume become more stable as they shrink in size. On the contrary, open drops, i.e. capable of exchanging mass with a reservoir, are less stable as their associated Bond number decreases.

[44]  arXiv:1908.07973 [pdf, other]
Title: Rotational Doppler cooling and heating
Comments: 8 pages, 3 figures
Subjects: Optics (physics.optics)

Doppler cooling is a widely used technique to laser cool atoms and nanoparticles exploiting the Doppler shift involved in translational transformations. The rotational Doppler effect arising from rotational coordinate transformations should similarly enable optical manipulations of the rotational degrees of freedom in rotating nanosystems. Here, we show that rotational Doppler cooling and heating (RDC and RDH) effects embody rich and unexplored physics, such as a strong dependence on particle morphology. For geometrically confined particles, such as a nanorod that can represent diatomic molecules, RDC and RDH follow similar rules as their translational Doppler counterpart, where cooling and heating are always observed at red- or blue-detuned laser frequencies, respectively. Surprisingly, nanosystems that can be modeled as a solid particle shows a strikingly different response, where RDH appears in a frequency regime close to their resonances, while a detuned frequency produces cooling of rotation. We also predict that the RDH effect can lead to unprecedented spontaneous chiral symmetry breaking, whereby an achiral particle under linearly polarized illumination starts spontaneously rotating, rendering it nontrivial compared to the translational Doppler effect. Our results open up new exciting possibilities to control the rotational motion of molecules and nanoparticles.

[45]  arXiv:1908.07975 [pdf, other]
Title: Thermodynamically Extended Symplectic Numerical Scheme with Half Space and Time Shift Applied for Rheological Waves in Solids
Comments: 20 pages, 38 figures
Subjects: Classical Physics (physics.class-ph)

On the example of the Poynting-Thomson-Zener rheological model for solids, which exhibits both dissipation and wave propagation - with nonlinear dispersion relation -, we introduce and investigate a finite difference numerical scheme for continuum thermodynamical problems. The key element is the positioning of the discretized quantities with shifts by half space and time steps with respect to each other. The arrangement is chosen according to the spacetime properties of the quantities and of the equations governing them. Numerical stability, dissipative error and dispersive error are analysed in detail. With the best settings found, the scheme is capable of making precise and fast predictions.

[46]  arXiv:1908.07988 [pdf, other]
Title: Large-Angle, Broadband and Multifunctional Gratings Based on Directively Radiating Waveguide Scatterers
Subjects: Optics (physics.optics); Classical Physics (physics.class-ph)

Conventional surface-relief gratings are inefficient at deflecting normally-incident light by large angles. This constrains their use in many applications and limits the overall efficiency of any optical instrument integrating gratings. Here, we demonstrate a simple approach for the design of diffraction gratings that can be highly efficient for large deflection angles, while also offering additional functionality. The gratings are composed of a unit cell comprising a vertically-oriented asymmetric slot-waveguide. The unit cell shows oscillating unidirectional scattering behavior that can be precisely tuned as a function of the waveguide length. This occurs due to interference between multiple modes excited by the incident light. In contrast to metasurface-based gratings with multiple resonant sub-elements, a periodic arrangement of such non-resonant diffracting elements allows for broadband operation and a strong tolerance for variations in angle of incidence. Full-wave simulations show that our grating designs can exhibit diffraction efficiencies ranging from 94% for a deflection angle of 47$^\circ$ to 80% for deflection angle of 80$^\circ$. To demonstrate the multifunctionality of our grating design technique, we have also proposed a flat polarization beamsplitter, which allows for the separation of the two orthogonal polarizations by 80$^\circ$, with an efficiency of 80%.

[47]  arXiv:1908.07991 [pdf, other]
Title: Prospects of designing gold-nanoparticles-based soft terahertz radiation sources and terahertz-to-infrared converters for concealed object detection technology
Comments: A manuscript for the SPIE Security+Defence 2019 conference, Strasbourg, Sep 9-12, 2019
Subjects: Applied Physics (physics.app-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Instrumentation and Detectors (physics.ins-det)

The two-phonon scheme of generation of terahertz (THz) photons by gold nanobars (GNBs) is considered. It is shown that in GNBs, by choosing their sizes, it is possible to provide conditions for converting the energy of longitudinal phonons with THz frequencies into the energy of THz photons. The prospects of designing GNBs-based soft THz radiation sources (frequencies: 0.14; 0.24; 0.41 and 0.70 THz) with a large flow cross-section (diameter ~40 cm) intended for detection of hidden objects under clothing to ensure security in public places (airports, railway stations, stadiums, etc.) are assessed. The choice of the above frequencies is a compromise between the requirements of low absorption of THz radiation by water vapor in air, good penetration through the fabric of clothing, favoring a sufficient resolution of the imaging system, and an abundance of corresponding longitudinal phonons, capable of exciting Fermi electrons in GNBs. Estimates of the characteristics of the terahertz-to-infrared converter based on gold nanospheres (GNSs), which could work in tandem with these sources of THz radiation -- as a means of visualization of hidden objects -- are also given.

[48]  arXiv:1908.08009 [pdf]
Title: Network Time Synchronization of the Readout Electronics for a New Radioactive Gas Detection System
Subjects: Instrumentation and Detectors (physics.ins-det); Nuclear Experiment (nucl-ex)

In systems with multiple radiation detectors, time synchronization of the data collected from different detectors is essential to reconstruct multi-detector events such as scattering and coincidences. In cases where the number of detectors exceeds the readout channels in a single data acquisition electronics module, multiple modules have to be synchronized, which is traditionally accomplished by distributing clocks and triggers via dedicated connections. To eliminate this added cabling complexity in the case of a new radioactive gas detection system prototype under development at the French Atomic Energy Commission, we implemented time synchronization between multiple XIA Pixie-Net detector readout modules through the existing Ethernet network, based on the IEEE 1588 precision time protocol. The detector system is dedicated to the measurement of radioactive gases at low activity and consists of eight large silicon pixels and two NaI(Tl) detectors, instrumented with a total of three 4-channel Pixie-Net modules. Detecting NaI(Tl)/silicon coincidences will make it possible to identify each radioisotope present in the sample. To allow these identifications at low activities, the Pixie-Net modules must be synchronized to a precision well below the targeted coincidence window of 500-1000 ns. Being equipped with an Ethernet PHY compatible with IEEE 1588 and synchronous Ethernet that outputs a locally generated but system-wide synchronized clock, the Pixie-Net can operate its analog to digital converters and digital processing circuitry with that clock and match time stamps for captured data across the three modules. Depending on the network configuration and synchronization method, the implementation is capable to achieve timing precisions between 300 ns and 200 ps.

Cross-lists for Thu, 22 Aug 19

[49]  arXiv:1908.07513 (cross-list from cond-mat.mes-hall) [pdf]
Title: Hetero-bilayers of 2D-materials as a platform for excitonic superfluidity
Comments: 5 figure
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci); Quantum Gases (cond-mat.quant-gas); Computational Physics (physics.comp-ph)

Excitonic condensate has been a phase long-sought within bulk indirect-gap semiconductors, quantum wells, and 2D material layers, all tried as carrying media. Here we propose intrinsically stable 2D semiconductor heterostructures with doubly-indirect overlapping bands as optimal material platforms for realizing excitonic condensation. After screening a list of exfoliable 2D materials, we identify several pairs as candidate heterostructures where spontaneous excitonic condensation mediated by purely electronic interaction should occur. Unlike other single-layered systems, where excitonic condensation may be hampered by the Peierls instability, these hetero-bilayers retain stable phonon spectra while offering near-optimal carrier density. Different parts of the electron-hole phase diagram, including BEC-BCS crossover, can be accessed through in-plane strain, external electric field, or changing interlayer distance, with exciting applications in superfluid transport, Josephson-like tunneling, and dissipationless charge counterflow.

[50]  arXiv:1908.07516 (cross-list from eess.IV) [pdf, other]
Title: Direct Neural Network 3D Image Reconstruction of Radon Encoded Data
Comments: Submitted to the Journal of Medical Imaging
Subjects: Image and Video Processing (eess.IV); Computer Vision and Pattern Recognition (cs.CV); Medical Physics (physics.med-ph)

Neural network image reconstruction directly from measurement data is a growing field of research, but until now has been limited to producing small (e.g. 128x128) 2D images by the large memory requirements of the previously suggested networks. In order to facilitate further research with direct reconstruction, we developed a more efficient network capable of 3D reconstruction of Radon encoded data with a relatively large image matrix (e.g. 400x400). Our proposed network is able to produce image quality comparable to the benchmark Ordered Subsets Expectation Maximization (OSEM) algorithm. We address the most memory intensive aspect of transforming the data from sinogram space to image space through a specially designed Radon inversion layer. We insert this layer between an initial network segment designed to encode the sinogram input and an output segment designed to refine and scale the initial image estimate to produce the final image. We demonstrate 3D reconstructions comparable to OSEM for 1, 4, 8 and 16 slices with no modifications to the network's architecture, capacity or hyper-parameters on a data set of simulated PET whole-body scans. When batch operations are considered, this network can reconstruct an entire PET whole-body volume in a single pass or about one second. Although results in this paper are on PET data, the proposed methods would be equally applicable to X-ray CT or any other Radon encoded measurement data.

[51]  arXiv:1908.07564 (cross-list from cs.DL) [pdf, other]
Title: Predicting publication productivity for researchers: a piecewise Poisson model
Authors: Zheng Xie
Comments: 23 pages, 10 figures
Subjects: Digital Libraries (cs.DL); Physics and Society (physics.soc-ph)

Predicting the scientific productivity of researchers is a basic task for academic administrators and funding agencies. This study provided a model for the publication dynamics of researchers, inspired by the distribution feature of researchers' publications in quantity. It is a piecewise Poisson model, analyzing and predicting the publication productivity of researchers by regression. The principle of the model is built on the explanation for the distribution feature as a result of an inhomogeneous Poisson process that can be approximated as a piecewise Poisson process. The model's principle was validated by the high quality dblp dataset, and its effectiveness was testified in predicting the publication productivity for majority of researchers and the evolutionary trend of their publication productivity. Tests to confirm or disconfirm the model are also proposed. The model has the advantage of providing results in an unbiased way; thus is useful for funding agencies that evaluate a vast number of applications with a quantitative index on publications.

[52]  arXiv:1908.07565 (cross-list from cs.SI) [pdf, other]
Title: Finding the right scale of a network: Efficient identification of causal emergence through spectral clustering
Subjects: Social and Information Networks (cs.SI); Physics and Society (physics.soc-ph)

All networks can be analyzed at multiple scales. A higher scale of a network is made up of macro-nodes: subgraphs that have been grouped into individual nodes. Recasting a network at higher scales can have useful effects, such as decreasing the uncertainty in the movement of random walkers across the network while also decreasing the size of the network. However, the task of finding such a macroscale representation is computationally difficult, as the set of all possible scales of a network grows exponentially with the number of nodes. Here we compare various methods for finding the most informative scale of a network, discovering that an approach based on spectral analysis outperforms greedy and gradient descent-based methods. We then use this procedure to show how several structural properties of preferential attachment networks vary across scales. We describe how meso- and macroscale representations of networks can have significant benefits over their underlying microscale, which include properties such as increase in determinism, a decrease in degeneracy, a lower entropy rate of random walkers on the network, an increase in global network efficiency, and higher values for a variety of centrality measures than the microscale.

[53]  arXiv:1908.07570 (cross-list from q-bio.SC) [pdf, other]
Title: How kinesin waits for ATP affects the nucleotide and load dependence of the stepping kinetics
Subjects: Subcellular Processes (q-bio.SC); Soft Condensed Matter (cond-mat.soft); Biological Physics (physics.bio-ph)

Dimeric molecular motors walk on polar tracks by binding and hydrolyzing one ATP per step. Despite tremendous progress, the waiting state for ATP binding in the well-studied kinesin that walks on microtubule (MT), remains controversial. One experiment suggests that in the waiting state both heads are bound to the MT, while the other shows that ATP binds to the leading head after the partner head detaches. To discriminate between these two scenarios, we developed a theory to calculate accurately several experimentally measurable quantities as a function of ATP concentration and resistive force.
In particular, we predict that measurement of the randomness parameter could discriminate between the two scenarios for the waiting state of kinesin, thereby resolving this standing controversy.

[54]  arXiv:1908.07575 (cross-list from cond-mat.str-el) [pdf, other]
Title: Sparse sampling approach to efficient ab initio calculations at finite temperature
Comments: 13 pages, 8 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Computational Physics (physics.comp-ph)

Efficient ab initio calculations of correlated materials at finite temperature require compact representations of the Green's functions both in imaginary time and Matsubara frequency. In this paper, we introduce a general procedure which generates sparse sampling points in time and frequency from compact orthogonal basis representations, such as Chebyshev polynomials and intermediate representation (IR) basis functions. These sampling points accurately resolve the information contained in the Green's function, and efficient transforms between different representations are formulated with minimal loss of information. As a demonstration, we apply the sparse sampling scheme to diagrammatic $GW$ and GF2 calculations of a hydrogen chain, of noble gas atoms and of a silicon crystal.

[55]  arXiv:1908.07594 (cross-list from cond-mat.dis-nn) [pdf, other]
Title: Chaotic wave packet spreading in two-dimensional disordered nonlinear lattices
Comments: 15 pages, 5 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Statistical Mechanics (cond-mat.stat-mech); Chaotic Dynamics (nlin.CD); Computational Physics (physics.comp-ph)

We reveal the generic characteristics of wave packet delocalization in two-dimensional nonlinear disordered lattices by performing extensive numerical simulations in two basic disordered models: the Klein-Gordon system and the discrete nonlinear Schr\"{o}dinger equation. We find that in both models (a) the wave packet's second moment asymptotically evolves as $t^{a_m}$ with $a_m \approx 1/5$ ($1/3$) for the weak (strong) chaos dynamical regime, in agreement with previous theoretical predictions~\cite{F10}, (b) chaos persists, but its strength decreases in time $t$ since the finite time maximum Lyapunov exponent $\Lambda$ decays as $\Lambda \propto t^{\alpha_{\Lambda}}$, with $\alpha_{\Lambda} \approx -0.37$ ($-0.46$) for the weak (strong) chaos case, and (c) the deviation vector distributions show the wandering of localized chaotic seeds in the lattice's excited part, which induces the wave packet's thermalization. We also propose a dimension-independent scaling between the wave packet's spreading and chaoticity, which allows the prediction of the obtained $\alpha_{\Lambda}$ values.

[56]  arXiv:1908.07608 (cross-list from astro-ph.HE) [pdf, other]
Title: Light diffusion in birefringent polycrystals and the IceCube ice anisotropy
Authors: Dmitry Chirkin, Martin Rongen (for the IceCube Collaboration)
Comments: Presented at the 36th International Cosmic Ray Conference (ICRC 2019). See arXiv:1907.11699 for all IceCube contributions
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Instrumentation and Methods for Astrophysics (astro-ph.IM); Optics (physics.optics)

The IceCube Neutrino Observatory instruments about 1 km^3 of deep, glacial ice at the geographic South Pole with 5160 photomultipliers to detect Cherenkov light from charged relativistic particles. The experiment pursues a wide range of scientific questions ranging from particle physics such as neutrino oscillations to high-energy neutrino astronomy. Most of these efforts rely heavily on an ever more precise understanding of the optical properties of the instrumented ice. An unexpected light propagation effect, observed by the experiment, is an anisotropic attenuation, which is aligned with the local flow of the ice. The exact cause is still under investigation. In this contribution, the micro-structure of ice as a birefringent polycrystal is explored as the cause for this anisotropy.

[57]  arXiv:1908.07609 (cross-list from cond-mat.mes-hall) [pdf]
Title: Persistent, rewritable frequency tuning of a nanoelectromechanical resonator using photoinduced doping
Comments: 19 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (physics.app-ph)

Tuning the frequency of a resonant element is of vital importance in both the macroscopic world, such as when tuning a musical instrument, as well as at the nanoscale. In particular, precisely controlling the resonance frequency of isolated nanoelectromechanical resonators (NEMS) has enabled innovations such as tunable mechanical filtering and mixing as well as commercial technologies such as robust timing oscillators. Much like their electronic device counterparts, the potential of NEMS grows when they are built up into large-scale arrays. Such arrays have enabled neutral-particle mass spectroscopy and have been proposed for ultralow-power alternatives to traditional analog electronics as well as nanomechanical information technologies like memory, logic, and computing. A fundamental challenge to these applications is to precisely tune the vibrational frequency and coupling of all resonators in the array, since traditional tuning methods, like patterned electrostatic gating or dielectric tuning, become intractable when devices are densely packed. Here, we demonstrate a persistent, rewritable, scalable, and high-speed frequency tuning method for graphene-based NEMS. Our method uses a focused laser and two shared electrical contacts to photodope individual resonators by simultaneously applying optical and electrostatic fields. After the fields are removed, the trapped charge created by this process persists and applies a local electrostatic tension to the resonators, tuning their frequencies. By providing a facile means to locally address the strain of a NEMS resonator, this approach lays the groundwork for fully programmable large-scale NEMS lattices and networks.

[58]  arXiv:1908.07628 (cross-list from cond-mat.mtrl-sci) [pdf, other]
Title: A linear-scaling algorithm for rapid computation of inelastic transitions in the presence of multiple electron scattering
Comments: 10 pages, 5 figures, 2 tables
Subjects: Materials Science (cond-mat.mtrl-sci); Computational Physics (physics.comp-ph)

Strong multiple scattering of the probe in scanning transmission electron microscopy (STEM) means image simulations are usually required for quantitative interpretation and analysis of elemental maps produced by electron energy-loss spectroscopy (EELS). These simulations require a full quantum-mechanical treatment of multiple scattering of the electron beam, both before and after a core-level inelastic transition. Current algorithms scale quadratically and can take up to a week to calculate on desktop machines even for simple crystal unit cells and do not scale well to the nano-scale heterogeneous systems that are often of interest to materials science researchers. We introduce an algorithm with linear scaling that typically results in an order of magnitude reduction in compute time for these calculations without introducing additional error and discuss approximations that further improve computational scaling for larger scale objects with modest penalties in calculation error. We demonstrate these speed-ups by calculating the atomic resolution STEM-EELS map using the L-edge transition of Fe, for of a nanoparticle 80 \AA\ in diameter in 16 hours, a calculation that would have taken at least 80 days using a conventional multislice approach.

[59]  arXiv:1908.07642 (cross-list from astro-ph.IM) [pdf, other]
Title: Recent Advances in Frequency-Multiplexed TES Readout: Vastly Reduced Parasitics and an Increase in Multiplexing Factor with sub-Kelvin SQUIDs
Comments: 7 pages, 6 figures, Submitted to the JLTP for LTD-18
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Instrumentation and Detectors (physics.ins-det)

Cosmic microwave background (CMB) measurements are fundamentally limited by photon statistics. Therefore, ground-based CMB observatories have been increasing the number of detectors that are simultaneously observing the sky. Thanks to the advent of monolithically fabricated transition edge sensor (TES) arrays, the number of on-sky detectors has been increasing exponentially for over a decade. The next-generation experiment CMB-S4 will increase this detector count by more than an order of magnitude from the current state-of-the-art to ~500,000. The readout of such a huge number of exquisitely precise sub-Kelvin sensors is feasible using an existing technology: frequency-domain multiplexing (fMux). To further optimize this system and reduce complexity and cost, we have recently made significant advances including the elimination of 4 K electronics, a massive decrease of parasitic in-series impedances, and a significant increase in multiplexing factor.

[60]  arXiv:1908.07648 (cross-list from cond-mat.mes-hall) [pdf]
Title: Hot exciton transport in WSe2 monolayers
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (physics.app-ph); Optics (physics.optics)

We experimentally demonstrate hot exciton transport in h-BN encapsulated WSe2 monolayers via spatially and temporally resolved photoluminescence measurements at room temperature. We show that the nonlinear evolution of the mean squared displacement of the non-resonantly excited hot exciton gas is primarily due to the relaxation of its excess kinetic energy and is characterized by a density-dependent fast expansion that converges to a slower, constant rate expansion. We also observe saturation of the hot exciton gas' expansion rate at high excitation densities due to the balance between Auger-assisted hot exciton generation and the phonon-assisted hot exciton relaxation processes.

[61]  arXiv:1908.07712 (cross-list from quant-ph) [pdf, other]
Title: Probing non-Hermitian Skin Effect and non-Bloch Phase Transitions
Authors: Stefano Longhi
Comments: 14 pages, 8 figures, to appear in Physical Review Research
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)

In non-Hermitian crystals showing the non-Hermitian skin effect, ordinary Bloch band theory and Bloch topological invariants fail to correctly predict energy spectra, topological boundary states, and symmetry breaking phase transitions in systems with open boundaries. Recently, it has been shown that a correct description requires to extend Bloch band theory into complex plane. A still open question is whether non-Hermitian skin effect and non-Bloch symmetry-breaking phase transitions can be probed by real-space wave dynamics far from edges, which is entirely governed by ordinary Bloch bands. Here it is shown that the Lyapunov exponent in the long-time behavior of bulk wave dynamics can reveal rather generally non-Bloch symmetry breaking phase transitions and the existence of the non-Hermitian skin effect.

[62]  arXiv:1908.07714 (cross-list from cond-mat.dis-nn) [pdf, other]
Title: Effective transport properties of conformal Voronoi-bounded columns via recurrent boundary element expansions
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Computational Physics (physics.comp-ph)

Effective transport properties of heterogeneous structures are predicted by geometric microstructural parameters, but these can be difficult to calculate. Here, a boundary element code with a recurrent series method accurately and efficiently determines the high order parameters of polygonal and conformal prisms in regular two-dimensional lattices and Voronoi tessellations (VT). This reveals that proximity to simpler estimates is associated with: centroidal VT (cf random VT), compactness, and VT structures (cf similarly compact semi-regular lattices). An error in previously reported values for triangular lattices is noted.

[63]  arXiv:1908.07725 (cross-list from math.NA) [pdf, other]
Title: Data-driven model reduction, Wiener projections, and the Mori-Zwanzig formalism
Authors: Kevin K. Lin, Fei Lu
Subjects: Numerical Analysis (math.NA); Computational Physics (physics.comp-ph); Machine Learning (stat.ML)

First-principles models of complex dynamic phenomena often have many degrees of freedom, only a small fraction of which may be scientifically relevant or observable. Reduced models distill such phenomena to their essence by modeling only relevant variables, thus decreasing computational cost and clarifying dynamical mechanisms. Here, we consider data-driven model reduction for nonlinear dynamical systems without sharp scale separation. Motivated by a discrete-time version of the Mori-Zwanzig projection operator formalism and the Wiener filter, we propose a simple and flexible mathematical formulation based on Wiener projection, which decomposes a nonlinear dynamical system into a component predictable by past values of relevant variables and its orthogonal complement. Wiener projection is equally applicable to deterministic chaotic dynamics and randomly-forced systems, and provides a natural starting point for systematic approximations. In particular, we use it to derive NARMAX models from an underlying dynamical system, thereby clarifying the scope of these widely-used tools in time series analysis. We illustrate its versatility on the Kuramoto-Sivashinsky model of spatiotemporal chaos and a stochastic Burgers equation.

[64]  arXiv:1908.07768 (cross-list from cond-mat.soft) [pdf, other]
Title: Non-affinity and fluid-coupled viscoelastic plateau for immersed fiber networks
Comments: 5 pages, 5 figures. Supplementary information provided in submission ("suppInf.pdf")
Subjects: Soft Condensed Matter (cond-mat.soft); Biological Physics (physics.bio-ph)

We employ a matrix-based solver for the linear rheology of fluid-immersed disordered spring networks to reveal four distinct dynamic response regimes. One regime - completely absent in the known vacuum response - exhibits coupled fluid flow and network deformation, with both components responding non-affinely. This regime contains an additional plateau (peak) in the frequency-dependent storage (loss) modulus - features which vanish without full hydrodynamic interactions. The mechanical response of immersed networks such as biopolymers and hydrogels is thus richer than previously established, and offers additional modalities for design and control through fluid interactions.

[65]  arXiv:1908.07787 (cross-list from astro-ph.GA) [pdf, other]
Title: Graphene Oxide Nanoparticles in the Interstellar Medium
Authors: P.J. Sarre
Comments: 5 pages, 3 figures
Subjects: Astrophysics of Galaxies (astro-ph.GA); Chemical Physics (physics.chem-ph)

Dust particles play a major role in the formation, evolution and chemistry of interstellar clouds, stars and planetary systems. Commonly identified forms include amorphous and crystalline carbon-rich particles and silicates. Also present in many astrophysical environments are polycyclic aromatic hydrocarbons (PAHs), detected through their infrared emission, and which are essentially small flakes of graphene. Astronomical observations over the past four decades have revealed a widespread unassigned Extended Red Emission (ERE) feature which is attributed to luminescence of dust grains. Numerous potential carriers for ERE have been proposed but none has gained general acceptance. In this Letter it is shown that there is a strong similarity between laboratory optical emission spectra of graphene oxide and ERE, leading to this proposal that emission from graphene oxide nanoparticles is the origin of ERE and that these are a significant component of interstellar dust. The proposal is supported by infrared emission features detected by the Infrared Space Observatory (ISO) and the Spitzer Space Telescope.

[66]  arXiv:1908.07790 (cross-list from astro-ph.IM) [pdf, other]
Title: Rotational spectroscopy of methyl mercaptan CH$_3$$^{32}$SH at millimeter and submillimeter wavelengths
Comments: 12 pages; Astron. Astrophys., in press
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Solar and Stellar Astrophysics (astro-ph.SR); Atomic and Molecular Clusters (physics.atm-clus); Chemical Physics (physics.chem-ph)

We present a new global study of the millimeter (mm) wave, submillimeter (sub-mm) wave, and terahertz (THz) spectra of the lowest three torsional states of methyl mercaptan (CH$_3$SH). New measurements have been carried out between 50 and 510 GHz using the Kharkiv mm wave and the Cologne sub-mm wave spectrometers whereas THz spectra records were used from our previous study. The new data, involving torsion-rotation transitions with $J$ up to 61 and $K_a$ up to 18, were combined with previously published measurements and fit using the rho-axis-method torsion-rotation Hamiltonian. The final fit used 124 parameters to give an overall weighted root-mean-square deviation of 0.72 for the dataset consisting of 6965 microwave (MW) and 16345 far-infrared line frequencies sampling transitions within and between the ground, first, and second excited torsional states. This investigation presents a two-fold expansion in the $J$ quantum numbers and a significant improvement in the fit quality, especially for the MW part of the data, thus allowing us to provide more reliable predictions to support astronomical observations.

[67]  arXiv:1908.07868 (cross-list from cond-mat.mes-hall) [pdf, other]
Title: Tunable room-temperature spin galvanic and spin Hall effects in van der Waals heterostructures
Comments: 13 pages, 4 figures
Subjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Applied Physics (physics.app-ph)

Spin-orbit coupling stands as a powerful tool to interconvert charge and spin currents and to manipulate the magnetization of magnetic materials through the spin torque phenomena. However, despite the diversity of existing bulk materials and the recent advent of interfacial and low-dimensional effects, control of the interconvertion at room-temperature remains elusive. Here, we unequivocally demonstrate strongly enhanced room-temperature spin-to-charge (StC) conversion in graphene driven by the proximity of a semiconducting transition metal dichalcogenide(WS2). By performing spin precession experiments in properly designed Hall bars, we separate the contributions of the spin Hall and the spin galvanic effects. Remarkably, their corresponding conversion effiencies can be tailored by electrostatic gating in magnitude and sign, peaking nearby the charge neutrality point with a magnitude that is comparable to the largest efficiencies reported to date. Such an unprecedented electric-field tunability provides a new building block for spin generation free from magnetic materials and for ultra-compact magnetic memory technologies.

[68]  arXiv:1908.07881 (cross-list from cond-mat.soft) [pdf, other]
Title: Apollonian Packing in Polydisperse Emulsions
Comments: 5 pages, 4 figures
Subjects: Soft Condensed Matter (cond-mat.soft); Materials Science (cond-mat.mtrl-sci); Chemical Physics (physics.chem-ph)

We have discovered the existence of polydisperse High Internal-Phase-Ratio Emulsions (HIPE) in which the internal-phase droplets, present at 95% volume fraction, remain spherical and organize themselves in the available space according to Apollonian packing rules. These polydisperse HIPE are formed during emulsification of surfactant-poor compositions of oil-surfactant-water two-phase systems. Their droplet size-distributions evolve spontaneously towards power laws with the Apollonian exponent. Small-Angle X-Ray Scattering performed on aged HIPEs demonstrated that the droplet packing structure coincided with that of a numerically simulated Random Apollonian Packing. We argue that these peculiar, space-filling assemblies are a result of coalescence and fragmentation processes obeying simple geometrical rules of conserving total volume and total interface area.

[69]  arXiv:1908.07884 (cross-list from q-bio.PE) [pdf, other]
Title: Evolution of specialization in dynamic fluids
Comments: 20 pages, 5 figures
Subjects: Populations and Evolution (q-bio.PE); Biological Physics (physics.bio-ph)

Previously we found mechanical factors involving diffusion and fluid shear promote evolution of social behavior in microbial populations (Uppal and Vural 2018). Here, we extend this model to study the evolution of specialization using realistic physical simulations of bacteria that secrete two public goods in a dynamic fluid. Through this first principles approach, we find physical factors such as diffusion, flow patterns, and decay rates are as influential as fitness economics in governing the evolution of community structure, to the extent that when mechanical factors are taken into account, (1) Generalist communities can resist becoming specialists, despite the invasion fitness of specialization (2) Generalist and specialists can both resist cheaters despite the invasion fitness of free-riding. (3) Multiple community structures can coexist despite the opposing force of competitive exclusion. Our results emphasize the role of spatial assortment and physical forces on niche partitioning and the evolution of diverse community structures.

[70]  arXiv:1908.07921 (cross-list from hep-ex) [pdf, other]
Title: Dark Matter at the Intensity Frontier: the new MESA electron accelerator facility
Comments: Proceedings of ALPS 2019, April 22 - 27, 2019, Obergurgl, Austria. arXiv admin note: substantial text overlap with arXiv:1809.07168
Subjects: High Energy Physics - Experiment (hep-ex); Nuclear Experiment (nucl-ex); Instrumentation and Detectors (physics.ins-det)

Dark Matter is being searched with a variety of methods, each of which tackles this challenge focusing on different kinds of particles, masses and couplings. Here we describe Dark Matter searches conducted with accelerators and fixed targets. In particular, we highlight the contribution of the experiments being built at the new Mainz Energy-recovery Superconducting Accelerator (MESA) facility. MESA will provide intense electron beams for hadron and nuclear physics, as well as for Dark Matter searches with competitive sensitivities.

[71]  arXiv:1908.07945 (cross-list from cond-mat.soft) [pdf, ps, other]
Title: Inertial migration of neutrally-buoyant particles in superhydrophobic channels
Comments: 10 pages, 10 figures
Subjects: Soft Condensed Matter (cond-mat.soft); Fluid Dynamics (physics.flu-dyn)

At finite Reynolds numbers particles migrate across flow streamlines to their equilibrium positions in microchannels. Such a migration is attributed to an inertial lift force, and it is well-known that the equilibrium location of neutrally-buoyant particles is determined only by their size and the channel Reynolds number. Here we demonstrate that the decoration of a bottom wall of the channel by superhydrophobic grooves provides additional possibilities for manipulation of neutrally-buoyant particles. It is shown that the effective anisotropic hydrodynamic slip of such a bottom wall can be readily used to alter the equilibrium positions of particles and to generate their motion transverse to the pressure gradient. These results may guide the design of novel inertial microfluidic devices for efficient sorting of neutrally-buoyant microparticles by their size.

[72]  arXiv:1908.08008 (cross-list from cond-mat.mtrl-sci) [pdf, other]
Title: Small polarons and the Janus nature of $\text{TiO}_\text{2}(110)$
Subjects: Materials Science (cond-mat.mtrl-sci); Computational Physics (physics.comp-ph)

Polarons are ubiquitous in many semiconductors and have been linked with conductivity and optical response of materials for photovoltaics and heterogeneous catalysis, yet how surface polarons influence adsorption remains unclear. Here, by modelling the surface of rutile titania using density functional theory, we reveal the effect of small surface polarons on water adsorption, dissociation, and hydrogen bonding. On the one hand the presence of such polarons significantly suppresses dissociation of water molecules that are bonded directly to polaronic sites. On the other hand, polarons facilitate water dissociation at certain non-polaronic sites. Furthermore, polarons strengthen hydrogen bonds, which in turn affects water dissociation in hydrogen bonded overlayer structures. This study reveals that polarons at the rutile surface have complex, multi-faceted, effects on water adsorption, dissociation and hydrogen bonding, highlighting the importance of polarons on water structure and dynamics on such surfaces. We expect that many of the physical properties of surface polarons identified here will apply more generally to surfaces and interfaces that can host small polarons, beyond titania.

[73]  arXiv:1908.08012 (cross-list from cs.NE) [pdf]
Title: Efficient training and design of photonic neural network through neuroevolution
Comments: 11 pages, 4 figures
Subjects: Neural and Evolutionary Computing (cs.NE); Optics (physics.optics)

Recently, optical neural networks (ONNs) integrated in photonic chips has received extensive attention because they are expected to implement the same pattern recognition tasks in the electronic platforms with high efficiency and low power consumption. However, the current lack of various learning algorithms to train the ONNs obstructs their further development. In this article, we propose a novel learning strategy based on neuroevolution to design and train the ONNs. Two typical neuroevolution algorithms are used to determine the hyper-parameters of the ONNs and to optimize the weights (phase shifters) in the connections. In order to demonstrate the effectiveness of the training algorithms, the trained ONNs are applied in the classification tasks for iris plants dataset, wine recognition dataset and modulation formats recognition. The calculated results exhibit that the training algorithms based on neuroevolution are competitive with other traditional learning algorithms on both accuracy and stability. Compared with previous works, we introduce an efficient training method for the ONNs and demonstrate their broad application prospects in pattern recognition, reinforcement learning and so on.

[74]  arXiv:1908.08032 (cross-list from astro-ph.HE) [pdf, other]
Title: Kinetic turbulence in shining pair plasma: intermittent beaming and thermalization by radiative cooling
Comments: 25 pages, 30 figures, submitted for publication
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Plasma Physics (physics.plasm-ph)

High-energy astrophysical systems frequently contain collisionless relativistic plasmas that are heated by turbulent cascades and cooled by emission of radiation. Understanding the nature of this radiative turbulence is a frontier of extreme plasma astrophysics. In this paper, we use particle-in-cell simulations to study the effects of external inverse Compton radiation on turbulence driven in an optically thin, relativistic pair plasma. We focus on the statistical steady state (where injected energy is balanced by radiated energy) and perform a parameter scan spanning from low magnetization to high magnetization ($0.04 \lesssim \sigma \lesssim 11$). We demonstrate that the global particle energy distributions are quasi-thermal in all simulations, with only a modest population of nonthermal energetic particles (extending the tail by a factor of $\sim 2$). This indicates that nonthermal particle acceleration (observed in similar non-radiative simulations) is quenched by strong radiative cooling. The quasi-thermal energy distributions are well fit by analytic models in which stochastic particle acceleration (due to, e.g., second-order Fermi mechanism or gyroresonant interactions) is balanced by the radiation reaction force. Despite the efficient thermalization of the plasma, nonthermal energetic particles do make a conspicuous appearance in the anisotropy of the global momentum distribution as highly variable, intermittent beams (for high magnetization cases). The beamed high-energy particles are spatially coincident with intermittent current sheets, suggesting that localized magnetic reconnection may be a mechanism for kinetic beaming. This beaming phenomenon may explain rapid flares observed in various astrophysical systems (such as blazar jets, the Crab nebula, and Sagittarius A*).

Replacements for Thu, 22 Aug 19

[75]  arXiv:1609.03862 (replaced) [pdf, ps, other]
Title: Induction and physical theory formation as well as universal computation by machine learning
Comments: 5 pages; section on simulation of universal Turing machines by deep forward networks added
Subjects: General Physics (physics.gen-ph)
[76]  arXiv:1703.01128 (replaced) [pdf, other]
Title: Statistical biases in measurements with multiple candidates
Comments: This is the 2019 update of the original 2017 preprint
Subjects: High Energy Physics - Experiment (hep-ex); Data Analysis, Statistics and Probability (physics.data-an)
[77]  arXiv:1711.00104 (replaced) [pdf]
Title: A Multiple Data Source Framework for the Identification of Activities of Daily Living Based on Mobile Device Data
Subjects: Computers and Society (cs.CY); Data Analysis, Statistics and Probability (physics.data-an)
[78]  arXiv:1802.06727 (replaced) [pdf, other]
Title: Photonic mid-infrared nulling for exoplanet detection on a planar chalcogenide platform
Comments: Was published in SPIE: Optical and Infrared Interferometry and Imaging VI, Mike Ireland presented
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Astrophysics of Galaxies (astro-ph.GA); Optics (physics.optics)
[79]  arXiv:1901.05943 (replaced) [pdf]
Title: Multi-plane, Multi-band image projection via Broadband Diffractive Optics
Subjects: Optics (physics.optics)
[80]  arXiv:1902.06083 (replaced) [pdf, other]
[81]  arXiv:1903.02328 (replaced) [pdf, ps, other]
Title: Evolution equation for multi-photon states in turbulence
Authors: Filippus S. Roux
Comments: 22 pages, no figures, to appear in J. Phys. A
Subjects: Quantum Physics (quant-ph); Optics (physics.optics)
[82]  arXiv:1903.02945 (replaced) [pdf, ps, other]
Title: Compact multi-fringe interferometry with sub-picometer precision
Comments: 6 pages, 4 figures, accepted in Physical Review Applied
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM); Optics (physics.optics)
[83]  arXiv:1904.02042 (replaced) [pdf, other]
Title: On-the-Fly Bayesian Active Learning of Interpretable Force-Fields for Atomistic Rare Events
Subjects: Computational Physics (physics.comp-ph); Materials Science (cond-mat.mtrl-sci)
[84]  arXiv:1904.08609 (replaced) [pdf, ps, other]
Title: Precise computation of rovibronic resonances of molecular hydrogen: $EF\ ^1Σ_\mathrm{g}^+$ inner-well rotational states
Journal-ref: Phys. Rev. A (2019)
Subjects: Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)
[85]  arXiv:1905.03437 (replaced) [pdf, other]
Title: Intervention Threshold for Epidemic Control in Susceptible-Infected-Recovered Metapopulation Models
Comments: 11pages, 8 figures
Journal-ref: Phys. Rev. E 100, 022302 (2019)
Subjects: Physics and Society (physics.soc-ph); Statistical Mechanics (cond-mat.stat-mech)
[86]  arXiv:1905.03898 (replaced) [pdf, other]
Title: Prediction of thermal conductivity in dielectrics using fast, spectrally-resolved phonon transport simulations
Comments: 36 pages, 13 figures
Subjects: Materials Science (cond-mat.mtrl-sci); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Computational Physics (physics.comp-ph)
[87]  arXiv:1905.05525 (replaced) [pdf, ps, other]
Title: Non-adiabatic mass correction for excited states of molecular hydrogen: improvement for the outer-well $H\bar{H}\ ^1Σ_\mathrm{g}^+$ term values
Journal-ref: J. Chem. Phys. (2019)
Subjects: Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)
[88]  arXiv:1905.07229 (replaced) [pdf, ps, other]
Title: Charged (A)dS black hole solutions in conformal teleparallel equivalent of general relativity
Comments: 11 pages, no figure, manuscript revised overall
Subjects: General Physics (physics.gen-ph)
[89]  arXiv:1905.07392 (replaced) [pdf, other]
Title: An embedded boundary approach for efficient simulations of viscoplastic fluids in three dimensions
Subjects: Fluid Dynamics (physics.flu-dyn); Computational Physics (physics.comp-ph)
[90]  arXiv:1905.08569 (replaced) [pdf, other]
Title: Limits of III-V nanowire growth based on particle dynamics
Subjects: Applied Physics (physics.app-ph); Chemical Physics (physics.chem-ph)
[91]  arXiv:1905.08903 (replaced) [pdf, other]
Title: Topology optimization on two-dimensional manifolds
Subjects: Computational Physics (physics.comp-ph); Computational Engineering, Finance, and Science (cs.CE); Optimization and Control (math.OC)
[92]  arXiv:1905.11696 (replaced) [pdf, other]
Title: Computing three-dimensional densities from force densities improves statistical efficiency
Comments: Main paper 13 pages 8 figures. Supporting information 2 pages 4 figures. Published in the Journal of Chemical Physics
Journal-ref: J. Chem. Phys. 151 (2019) 064124
Subjects: Chemical Physics (physics.chem-ph); Statistical Mechanics (cond-mat.stat-mech)
[93]  arXiv:1906.02115 (replaced) [pdf, ps, other]
Title: Cosmological Constant $Λ$ vs. Massive Gravitons: A Case Study in General Relativity Exceptionalism vs. Particle Physics Egalitarianism
Authors: J. Brian Pitts
Comments: Part of forthcoming Einstein Studies volume with Birkh\"auser, provisionally entitlted _Back with a Flourish: Revisiting the Renaissance of General Relativity_, edited by Alexander Blum, Roberto Lalli, and J\"urgen Renn. Minor additions, reference and more discussion of Feynman added
Subjects: History and Philosophy of Physics (physics.hist-ph)
[94]  arXiv:1906.04154 (replaced) [pdf, other]
Title: Zeeman-tunable Modulation Transfer Spectroscopy
Comments: 5 pages, 5 figures
Subjects: Atomic Physics (physics.atom-ph); Instrumentation and Detectors (physics.ins-det); Optics (physics.optics); Quantum Physics (quant-ph)
[95]  arXiv:1907.03157 (replaced) [pdf, ps, other]
Title: Attosecond helical pulses
Authors: Miguel A. Porras
Comments: 7 pages, 5 figures, errors corrected and comments added
Subjects: Optics (physics.optics)
[96]  arXiv:1907.04075 (replaced) [pdf, ps, other]
Title: On mechanisms of electromechanophysiological interactions between the components of nerve signals in axons
Comments: Submission updated to correct few typos and to remove one reference being present twice
Subjects: Biological Physics (physics.bio-ph)
[97]  arXiv:1907.08928 (replaced) [pdf]
Title: Study of muon tomographic imaging for high Z material detection with a Micromegas-based tracking system
Subjects: Instrumentation and Detectors (physics.ins-det)
[98]  arXiv:1908.03908 (replaced) [pdf, other]
Title: Negative refraction in time-varying, strongly-coupled plasmonic antenna-ENZ systems
Subjects: Optics (physics.optics)
[99]  arXiv:1908.06578 (replaced) [pdf, other]
Title: Ultraviolet to Near-infrared Single Photon Emitters in hBN
Comments: 5 Figures
Subjects: Applied Physics (physics.app-ph); Materials Science (cond-mat.mtrl-sci); Optics (physics.optics)
[100]  arXiv:1908.07113 (replaced) [pdf, other]
Title: Neutrino Detectors as Tools for Nuclear Security
Comments: 20 pages, 5 figures, comments welcome, submitted to Rev. Mod. Phys; v2 minor editing correction
Subjects: Physics and Society (physics.soc-ph); High Energy Physics - Experiment (hep-ex); High Energy Physics - Phenomenology (hep-ph); Nuclear Experiment (nucl-ex); Instrumentation and Detectors (physics.ins-det)
[101]  arXiv:1908.07207 (replaced) [pdf, other]
Title: A Brief Review of Plasma Wakefield Acceleration
Comments: 11 pages, 3 figures, 2 tables, 64 references, submitted to Instruments
Subjects: Accelerator Physics (physics.acc-ph); Plasma Physics (physics.plasm-ph)
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