bluuebunny/crossref_metadata_2025 huggingface dataset:
The effect of size matching between anvils and the pressure transmitting medium on the pressure-generation efficiency and sealing performance for a large volume cubic pressure cell
Authors: Zhang Jiawei , He Duanwei , Fang Leiming , Hu Qiwei , Li Xin , Li Qiang , Wang Junpu , Wang Yipeng
Published: December 2020

Size matching between anvils and the pressure transmitting medium (PTM) is a key factor that affects pressure generation and sealing for a large volume cubic press. In this work, we studied the influence of PTM sizes from 30.5 mm to 34.5 mm at a fixed anvil geometry dimension (23.5 mm) on the pressure efficiency and sealing performance by measuring the pressure of the gasket and cell simultaneously at room temperature. Wires made of Bi, Tl, Ba, or Manganin were used for pressure calibration experiments within a pressure range of up to 6 GPa. It was found that a PTM with an edge length of 33.5 mm had the highest pressure-generation efficiency, but its sealing performance was the worst. Furthermore, it was confirmed that a PTM with an edge length of 32.5 mm had the best overall performance for a 23.5 mm anvil when both efficiency and sealing were considered. The results show that the pressure-generation efficiency and sealing performance are highly sensitive to PTM size. It is less rigorous to gauge the performance of the assembly only by the pressure-generation efficiency. This work provides practical guidelines and contributes to optimizing the design of the high-pressure assembly.
The Iris billiard: Critical geometries for global chaos
Authors: Page Gregory , Antoine Charles , Dettmann Carl P. , Talbot Julian
Published: December 2020

We introduce the Iris billiard that consists of a point particle enclosed by a unit circle around a central scattering ellipse of fixed elongation (defined as the ratio of the semi-major to the semi-minor axes). When the ellipse degenerates to a circle, the system is integrable; otherwise, it displays mixed dynamics. Poincaré sections are presented for different elongations. Recurrence plots are then applied to the long-term chaotic dynamics of trajectories launched from the unstable period-2 orbit along the semi-major axis, i.e., one that initially alternately collides with the ellipse and the circle. We obtain numerical evidence of a set of critical elongations at which the system undergoes a transition to global chaos. The transition is characterized by an endogenous escape event, E, which is the first time a trajectory launched from the unstable period-2 orbit misses the ellipse. The angle of escape, θesc, and the distance of the closest approach, dmin, of the escape event are studied and are shown to be exquisitely sensitive to the elongation. The survival probability that E has not occurred after n collisions is shown to follow an exponential distribution.
The balloon-borne cryogenic telescope testbed mission: Bulk cryogen transfer at 40 km altitude
Authors: Kogut A. , Essinger-Hileman T. , Denker S. , Bellis N. , Lowe L. , Mirel P.
Published: December 2020

The balloon-borne cryogenic telescope testbed is a stratospheric balloon payload intended to develop technology for a future cryogenic suborbital observatory. A series of flights are intended to establish ultra-light dewar performance and open-aperture observing techniques for large (3 m–5 m diameter) cryogenic telescopes at infrared wavelengths. An initial flight in 2019 demonstrated bulk transfer of liquid nitrogen and liquid helium at stratospheric altitudes. An 827 kg payload carried 14 l of liquid nitrogen (LN2) and 268 l of liquid helium (LHe) in pressurized storage dewars to an altitude of 39.7 km. Once at float altitude, liquid nitrogen transfer cooled a separate, unpressurized bucket dewar to a temperature of 65 K, followed by the transfer of 32 l of liquid helium from the storage dewar into the bucket dewar. Calorimetric tests measured the total heat leak to the LHe bath within the bucket dewar. A subsequent flight will replace the receiving bucket dewar with an ultra-light dewar of similar size to compare the performance of an ultra-light design dewar to that of conventional superinsulated dewars.
Operator growth bounds in a cartoon matrix model
Authors: Lucas Andrew , Osborne Andrew
Published: December 2020

We study operator growth in a model of N(N − 1)/2 interacting Majorana fermions that live on the edges of a complete graph of N vertices. Terms in the Hamiltonian are proportional to the product of q fermions that live on the edges of cycles of length q. This model is a cartoon “matrix model”: the interaction graph mimics that of a single-trace matrix model, which can be holographically dual to quantum gravity. We prove (non-perturbatively in 1/N and without averaging over any ensemble) that the scrambling time of this model is at least of order log N, consistent with the fast scrambling conjecture. We comment on apparent similarities and differences between operator growth in our “matrix model” and in the melonic models.
Vortex formation in a strongly coupled dusty plasma flow past an obstacle
Authors: Bailung Yoshiko , Chutia Bidyut , Deka T. , Boruah A. , Sharma Sumita K. , Kumar Sandeep , Chutia Joyanti , Nakamura Y. , Bailung H.
Published: December 2020

A pair of counter-rotating symmetric vortices has been observed in the wake behind a stationary obstacle (dust void) in a flowing dusty plasma. A strongly coupled dusty plasma flow with controllable velocity is generated and directed toward the void in a novel experiment. In the unsteady laminar flow regime, the curl of the fluid flow velocity along the boundary layer of the void generates the vortex pair behind the void. Particle image velocimetry analysis of high speed image data clearly depicts the flow pattern and the vorticities. The shear viscosity of the dusty plasma fluid along with the experimental parameters is considered to obtain the Reynolds number range for the evolution of the vortices. Molecular dynamics simulation is also performed to support the experimental observation.
Data-guided Multi-Map variables for ensemble refinement of molecular movies
Authors: Vant John W. , Sarkar Daipayan , Streitwieser Ellen , Fiorin Giacomo , Skeel Robert , Vermaas Josh V. , Singharoy Abhishek
Published: December 2020

Driving molecular dynamics simulations with data-guided collective variables offer a promising strategy to recover thermodynamic information from structure-centric experiments. Here, the three-dimensional electron density of a protein, as it would be determined by cryo-EM or x-ray crystallography, is used to achieve simultaneously free-energy costs of conformational transitions and refined atomic structures. Unlike previous density-driven molecular dynamics methodologies that determine only the best map-model fits, our work employs the recently developed Multi-Map methodology to monitor concerted movements within equilibrium, non-equilibrium, and enhanced sampling simulations. Construction of all-atom ensembles along the chosen values of the Multi-Map variable enables simultaneous estimation of average properties, as well as real-space refinement of the structures contributing to such averages. Using three proteins of increasing size, we demonstrate that biased simulation along the reaction coordinates derived from electron densities can capture conformational transitions between known intermediates. The simulated pathways appear reversible with minimal hysteresis and require only low-resolution density information to guide the transition. The induced transitions also produce estimates for free energy differences that can be directly compared to experimental observables and population distributions. The refined model quality is superior compared to those found in the Protein Data Bank. We find that the best quantitative agreement with experimental free-energy differences is obtained using medium resolution density information coupled to comparatively large structural transitions. Practical considerations for probing the transitions between multiple intermediate density states are also discussed.
Comparison of fixed charge and polarizable models for predicting the structural, thermodynamic, and transport properties of molten alkali chlorides
Authors: Wang Haimeng , DeFever Ryan S. , Zhang Yong , Wu Fei , Roy Santanu , Bryantsev Vyacheslav S. , Margulis Claudio J. , Maginn Edward J.
Published: December 2020

Results from extensive molecular dynamics simulations of molten LiCl, NaCl, KCl, and RbCl over a wide range of temperatures are reported. Comparison is made between the “Polarizable Ion Model” (PIM) and the non-polarizable “Rigid Ion Model” (RIM). Densities, self-diffusivities, shear viscosities, ionic conductivities, and thermal conductivities are computed and compared with experimental data. In addition, radial distribution functions are computed from ab initio molecular dynamics simulations and compared with the two sets of classical simulations as well as experimental data. The two classical models perform reasonably well at capturing structural and dynamic properties of the four molten alkali chlorides, both qualitatively and often quantitatively. With the singular exception of liquid density, for which the PIM is more accurate than the RIM, there are few clear trends to suggest that one model is more accurate than the other for the four alkali halide systems studied here.
Equipment design and performance characterization of full field x-ray fluorescence (FF-XRF) element distribution imaging system with combined collimating lens (CCL)
Authors: Xiong Genchao , Jia Wenbao , Shan Qing , Zhang Xinlei , Tang Xinru , Li Jun
Published: December 2020

This work describes a full field x-ray fluorescence element distribution imaging system with a combined collimating lens (CCL), which is more suitable for a higher x-ray energy range (12 keV–30 keV). The system consists of an optical-use charge-coupled device (CCD) camera coupled to a combined collimating lens (CCL), which includes pinhole collimator and x-ray window, x-ray tube, and sample room. The continuously variable magnification of 0.5–2 is achieved under a compact structure. The x-ray spectrum and two-dimensional element distribution mapping of the irradiated sample are obtained by processing a series of images acquired by using the CCD camera in a single photon counting mode. The energy resolution is 275 eV at the reference energy of 14.957 keV (yttrium Kα, Z = 39). The limit of detection is 46.41 ppm by measuring yttrium standard solution. The spatial resolution is 135 μm when using a 100 μm pinhole at the magnification of 1. Samples made by metal foils and mineral pigments are tested, and the results proved that the system was reliable when detecting elements of a high atomic number.
Correlation and spectrum of dust acoustic waves in a radio-frequency plasma using PK-4 on the International Space Station
Authors: Goree J. , Liu Bin , Pustylnik M. Y. , Thomas H. M. , Fortov V. E. , Lipaev A. M. , Molotkov V. I. , Usachev A. D. , Petrov O. F. , Thoma M. H. , Thomas E. , Konopka U. , Prokopiev S.
Published: December 2020

Dust acoustic waves were investigated in an experiment under microgravity conditions, using the European Space Agency-Roscosmos facility PK-4 on the International Space Station. A three-dimensional dust cloud was confined in a glow-discharge plasma powered by a radio-frequency coil in low-pressure neon gas. Low-frequency dust acoustic waves (DAWs) were spontaneously excited due to the flowing ions in the plasma. Dust motion was imaged using video cameras, yielding the position for individual dust particles. Besides these particle-level measurements, the dust was also measured as a fluid continuum to obtain its velocity and density fluctuations. A space–time diagram of the dust-fluid velocity reveals that the waves changed properties as they propagated across the dust cloud, in the direction of the ion flow. Density fluctuations were characterized by a wave spectrum, indicating the emergence of collective vibration modes in a broad range of wave numbers. The decay of the modes was characterized by density correlation functions. Fitting the correlation functions to an analytical model yields an experimental measurement of the dispersion relation for DAWs, which is compared to that of linear theory.
Multiplex recurrence networks from multi-lead ECG data
Authors: Kachhara Sneha , Ambika G.
Published: December 2020

We present an integrated approach to analyze the multi-lead electrocardiogram (ECG) data using the framework of multiplex recurrence networks (MRNs). We explore how their intralayer and interlayer topological features can capture the subtle variations in the recurrence patterns of the underlying spatio-temporal dynamics of the cardiac system. We find that MRNs from ECG data of healthy cases are significantly more coherent with high mutual information and less divergence between respective degree distributions. In cases of diseases, significant differences in specific measures of similarity between layers are seen. The coherence is affected most in the cases of diseases associated with localized abnormality such as bundle branch block. We note that it is important to do a comprehensive analysis using all the measures to arrive at disease-specific patterns. Our approach is very general and as such can be applied in any other domain where multivariate or multi-channel data are available from highly complex systems.
Temperature driven structural evolution of Ge-rich GeSbTe alloys and role of N-doping
Authors: Prazakova L. , Nolot E. , Martinez E. , Fillot F. , Rouchon D. , Rochat N. , Bernard M. , Sabbione C. , Morel D. , Bernier N. , Grenier A. , Papon A.-M. , Cyrille M.-C. , Navarro G.
Published: December 2020

Ge-rich GeSbTe alloys allowed overcoming temperature limitations of phase-change memory technology. In this paper, we present a thorough investigation of the structural evolution and the crystallization process of these alloys as a function of increasing temperature in annealing. We highlight the progressive rearrangement of the structure toward the demixing of Ge and GeSbTe phases. In particular, we show the stability of Sb–Te units and the development of Ge–Te bonds around these features. We observe the formation of a transient GeSbTe phase, which is driven by crystallization phenomena, leading to a gradual diffusion and expulsion of Ge. Therefore, the system moves toward the complete separation of Ge and Ge2Sb2Te5 stable phases. Furthermore, we investigate the effect of N-doping in Ge-rich GeSbTe, which induces the formation of Ge–N bonds. Such features are demonstrated to be responsible for a delayed structural reorganization to higher temperatures, thus affecting the entire process of crystallization and phase separation in the alloy.
Sn doping of (010) β-Ga2O3 films grown by plasma-assisted molecular beam epitaxy
Authors: Mauze Akhil , Zhang Yuewei , Itoh Takeki , Ahmadi Elaheh , Speck James S.
Published: December 2020

Sn doping of (010) β-Ga2O3 grown by conventional plasma-assisted molecular beam epitaxy (PAMBE) and via metal oxide catalyzed epitaxy (MOCATAXY) using a supplied indium flux during molecular beam epitaxy (MBE) growth was investigated. While high Sn concentrations were achievable over a range of growth conditions in conventional PAMBE, Sn doping less than 1019 cm−3 resulted in non-uniform doping profiles for constant Sn cell temperatures, as well as run-to-run variation in doping. Sn doping in MOCATAXY grown β-Ga2O3 allowed for sharp doping profiles and a wide range of donor concentrations from 3.9 × 1016 cm−3 to 2 × 1019 cm−3 and a maximum room temperature Hall mobility of 136 cm2/V s at 3.9 × 1016 cm−3. From temperature-dependent Hall measurements, Sn was found to have a relatively deep donor state at 77 meV below the conduction band edge. The samples showed low electron mobility at cryogenic temperatures, suggesting the existence of high background impurity levels in the MBE grown films and the need for impurity control in the oxide MBE growth environment.
Noise control of subsonic flow past open cavities based on porous floors
Authors: Li Bo , Ye Chuang-Chao , Wan Zhen-Hua , Liu Nan-Sheng , Sun De-Jun , Lu Xi-Yun
Published: December 2020

In implicit large eddy simulations, a porous material was used to suppress the noise produced by a flow past an open cavity. The base case has a Mach number and Reynolds number based on the cavity depth ReD = 105. Strong pressure fluctuations were produced. The solid cavity floor was replaced by a porous material to suppress high-pressure oscillations. With a porous floor, both the pressure fluctuations inside the cavity and noise in the near field were substantially suppressed. Four controlled cases with different porosities were considered. For low porosities, the control was better with increasing porosity. The control was best when the porosity was about 11.2%, and the maximum noise reduction was more than 10 dB. As the porosity was further increased from 11.2% to 19.27%, the control effect was decreased slightly. A porous floor can produce effects of suction and injection, which alter the structures of the recirculation and the shear layer. The control is mainly influenced by the strength of the suction effect. With control, the shear layer has less energetic smaller structures, and the interactions between the shear layer and recirculation inside the cavity are weakened. The vortex–edge impingements are mitigated, and thus, the acoustic feedback is lower, which decreases the self-sustained oscillations and the noise. Basically, the noise reduction mechanisms of the four controlled cases are similar. Our results suggest that a porous cavity floor is an effective method of noise control. However, an appropriate porosity must be chosen.
Adjusting the electromechanical coupling behaviors of piezoelectric semiconductor nanowires via strain gradient and flexoelectric effects
Authors: Ren C. , Wang K. F. , Wang B. L.
Published: December 2020

In this paper, the strain gradient is introduced to tune the semiconducting performance of piezoelectric semiconductor (PSC) nanowires by changing their cross sections. A one-dimensional model of the PSC nanowire with a non-homogeneous cross section under axial extension is established. The combined influences of piezoelectricity and flexoelectricity resulting from the strain gradient are taken into account. Approximate closed-form solutions for the distribution of carriers and electromechanical fields of the PSC nanowire are given. The effects of strain gradient, flexoelectricity, and initial carrier concentration on the distribution of carriers, electric field, electric potential, and displacement are discussed. It is observed that non-homogeneous PSC nanowires show strong size-dependent behaviors in connection with their cross-sectional diameters. The strain gradient and flexoelectric effect enhance the electromechanical coupling effect. This research provides a new way to tune inner carrier distributions and electromechanical characteristics for piezoelectric semiconductor devices.
A hybrid method for investigating the detailed emission spectra of mid-Z plasmas in non-local thermodynamic equilibrium: Ge as an example
Authors: Gao Cheng , He Xin , Wu Jianhua , Liu Yanpeng
Published: December 2020

The detailed radiative properties of plasmas in non-local thermodynamic equilibrium (NLTE) are important for determining experimental plasma states. However, a complete detailed-level-accounting approach calculation is impractical for mid- and high-Z elements. Herein, we propose a hybrid method for obtaining the detailed radiative properties of mid-Z NLTE plasmas. First, a large-scale rate equation within the framework of a detailed-configuration-accounting method is established using atomic data in a configuration–configuration formalism. Second, we assume that the population distributions in fine-structure levels belonging to a particular configuration are in equilibrium. Thus, the populations at fine-structure levels are obtained through the populations in the corresponding configurations. Finally, detailed radiative properties are calculated using the populations in fine-structure levels and radiative data in level–level formalism. Such a method can balance computation costs and accuracy. Examples utilizing Ge plasmas demonstrate that the proposed method can better predict detailed structures in emission spectra than the detailed-configuration-accounting method.
Space–time characteristics of turbulence in minimal flow units
Authors: Wang Hai-Ning , Huang Wei-Xi , Xu Chun-Xiao
Published: December 2020

Turbulent flows in minimal flow units (MFUs) provide intrinsic information on near-wall physics of turbulence and help to develop useful models for numerical simulations. In the present study, MFUs at friction Reynolds numbers ranging from 1000 to 4000 are simulated. The space–time spectra and correlations of velocity and pressure fluctuations are computed. Two models, i.e., the elliptic approximation and the local amplitude modulated wave method are tested against the MFU data, and their performances in representing and reconstructing space–time spectra and correlations are evaluated. The models are then utilized to analyze the space–time characteristics of MFUs. An important feature of MFUs – the Reynolds number independence – holds for space–time statistics as well as the energy spectra. Comparisons are further carried out among the space–time statistics of velocity components and pressure, at different heights and scales. It is shown that despite the anisotropy in space, the space–time distributions of the three components of velocity fluctuations are quite similar. The scale dependence of convection speed is weak even near the wall, but that of sweeping velocity is always strong. The spanwise-temporal spectra and correlations are also presented, of which the proper modeling requires future work.
Thermionic emission or tunneling? The universal transition electric field for ideal Schottky reverse leakage current: A case study in
β
-Ga2O3
Authors: Li Wenshen , Nomoto Kazuki , Jena Debdeep , Xing Huili Grace
Published: December 2020

The reverse leakage current through a Schottky barrier transitions from a thermionic emission-dominated regime to a barrier tunneling-dominated regime as the surface electric field increases. In this study, we evaluate such a transition electric field (ET) in β-Ga2O3 using a numerical reverse leakage model. ET is found to depend on temperature but has an extremely weak dependence on the doping concentration and the barrier height; as a result, a simple empirical expression can be derived to capture this near-universal dependence of ET on temperature. With the help of a field-plate design, we observed experimentally in lightly doped Ga2O3 Schottky barrier diodes near-ideal bulk reverse leakage characteristics, which match well with our numerical model and that confirm the presence of the transition region. Near the transition electric field, both thermionic emission and barrier tunneling should be considered. This study provides important guidance toward accurate design and modeling of Schottky barrier diodes, which can be readily extended to other semiconductors.
Linear stability analysis of one-dimensional detonation coupled with vibrational relaxation
Authors: Uy Ken Chun Kit , Shi Lisong , Hao Jiaao , Wen Chih-Yung
Published: December 2020

The linear stability of one-dimensional detonations with one-reaction chemistry coupled with molecular vibration nonequilibrium is investigated using the normal mode approach. The chemical kinetics in the Arrhenius form depend on an averaged temperature model that consists of translational–rotational mode and vibrational mode. The Landau–Teller model is applied to specify the vibrational relaxation. A time ratio is introduced to denote the ratio between the chemical time scale and the vibrational time scale in this study, which governs the vibrational relaxation rate in this coupling kinetics. The stability spectrum of disturbance eigenmodes is obtained by varying the bifurcation parameters independently at a different time ratio. These parameters include the activation energy, the degree of overdrive, the characteristic vibrational temperature, and the heat release. The results indicate that the neutral stability limit shifts to higher activation energy on the vibrational nonequilibrium side with a smaller time ratio, implying that the detonation is stabilized. A similar observation is seen at a lower degree of overdrive. Compared with the above two parameters, the characteristic vibrational temperature plays a minor role in the stabilization of detonation, and no change in the number of eigenmodes is identified throughout the selected range. By plotting the neutral stability curves relating the heat release to the above parameters, the decreases in instability ranges are obviously seen under vibrational nonequilibrium. The thermal nonequilibrium effect on detonation stability is clearly demonstrated. The analysis presented in this paper is ultimately justified by comparing the results with numerical simulation.
Two-dimensional convection–diffusion in multipolar flows with applications in microfluidics and groundwater flow
Authors: Boulais Etienne , Gervais Thomas
Published: December 2020

Advection–diffusion in two-dimensional plane flows plays a key role in numerous transport problems in physics, including groundwater flow, micro-scale sensing, heat dissipation, and, in general, microfluidics. However, transport profiles are usually only known in a purely convective approximation or for the simplest geometries, such as for quasi one-dimensional planar microchannels. This situation greatly limits the use of these models as design tools for fully 2D planar flows. We present a complete analysis of the problem of convection–diffusion in low Reynolds number 2D flows with distributions of singularities, such as those found in open-space microfluidics and in groundwater flows. Using Boussinesq transformations and solving the problem in streamline coordinates, we obtain concentration profiles in flows with complex arrangements of sources and sinks for both high and low Peclet numbers. These yield the complete analytical concentration profile at every point in applications such as microfluidic probes, groundwater heat pumps, or diffusive flows in porous media, which previously relied on material surface tracking, local lump models, or numerical analysis. Using conformal transforms, we generate families of symmetrical solutions from simple ones and provide a general methodology that can be used to analyze any arrangement of source and sinks. The solutions obtained include explicit dependence on the various parameters of the problems, such as Pe, the spacing of the apertures, and their relative injection and aspiration rates. We then show how these same models can be used to model diffusion in confined geometries, such as channel junctions and chambers, and give examples for classic microfluidic devices such as T-mixers and hydrodynamic focusing. The high Pe models can model problems with Pe as low as 1 with a maximum error committed of under 10%, and this error decreases approximately as Pe−1.5.
Emergence of multistability and strongly asymmetric collective modes in two quorum sensing coupled identical ring oscillators
Authors: Hellen Edward H. , Volkov Evgeny
Published: December 2020

The simplest ring oscillator is made from three strongly nonlinear elements repressing each other unidirectionally, resulting in the emergence of a limit cycle. A popular implementation of this scheme uses repressor genes in bacteria, creating the synthetic genetic oscillator known as the Repressilator. We consider the main collective modes produced when two identical Repressilators are mean-field-coupled via the quorum-sensing mechanism. In-phase and anti-phase oscillations of the coupled oscillators emerge from two Andronov–Hopf bifurcations of the homogeneous steady state. Using the rate of the repressor's production and the value of coupling strength as the bifurcation parameters, we performed one-parameter continuations of limit cycles and two-parameter continuations of their bifurcations to show how bifurcations of the in-phase and anti-phase oscillations influence the dynamical behaviors for this system. Pitchfork bifurcation of the unstable in-phase cycle leads to the creation of novel inhomogeneous limit cycles with very different amplitudes, in contrast to the well-known asymmetrical limit cycles arising from oscillation death. The Neimark–Sacker bifurcation of the anti-phase cycle determines the border of an island in two-parameter space containing almost all the interesting regimes including the set of resonant limit cycles, the area with stable inhomogeneous cycle, and very large areas with chaotic regimes resulting from torus destruction and period doubling of resonant cycles and inhomogeneous cycles. We discuss the structure of the chaos skeleton to show the role of inhomogeneous cycles in its formation. Many regions of multistability and transitions between regimes are presented. These results provide new insights into the coupling-dependent mechanisms of multistability and collective regime symmetry breaking in populations of identical multidimensional oscillators.
State-to-state quantum dynamics of H2O/HOD scattering from Cu(111): Mode- and bond-selective vibrational energy transfer
Authors: Zhang Liang , Jiang Bin
Published: December 2020

Molecular scattering at solid surfaces has been a sensitive probe of the molecule–surface interaction. Existing theoretical studies have primarily focused on diatomic molecules scattering from metal surfaces. Here, we investigate the vibrational state-to-state scattering dynamics of H2O/HOD from Cu(111) by a fully coupled six-dimensional quantum dynamical model based on a first-principles determined potential energy surface. Specifically, state-to-state scattering probabilities of H2O(1ν1) and HOD with its O–H or O–D excitation are obtained in a wide range of incidence energies. We find very efficient ν1-to-ν3 vibrational energy redistribution of H2O, with a similar efficiency to what we found previously for ν3-to-ν1 energy flow in H2O(1ν3) scattering. In comparison, we find that the energy transfer from the more localized 1νOH or 1νOD state to the other bond is much more difficult, in line with the strong bond selectivity observed in the dissociation of HOD on Cu(111). These results suggest that vibrational energy transfer in H2O/HOD scattering from Cu(111) is mode- and bond-selective, which is better described in the sudden limit via a local mode picture. Implications of these results on the mode-specific vibrational energy transfer of other polyatomic molecules scattering from metal surfaces, such as methane and ammonia, have been discussed. We hope that our study will inspire more quantum state-resolved experiments on state-to-state scattering of polyatomic molecules at metal surfaces.
Deterministically fabricated strain-tunable quantum dot single-photon sources emitting in the telecom O-band
Authors: Srocka N. , Mrowiński P. , Große J. , Schmidt M. , Rodt S. , Reitzenstein S.
Published: December 2020

Most quantum communication schemes aim at the long-distance transmission of quantum information. In the quantum repeater concept, the transmission line is subdivided into shorter links interconnected by entanglement distribution via Bell-state measurements to overcome inherent channel losses. This concept requires on-demand single-photon sources with a high degree of multi-photon suppression and high indistinguishability within each repeater node. For a successful operation of the repeater, a spectral matching of remote quantum light sources is essential. We present a spectrally tunable single-photon source emitting in the telecom O-band with the potential to function as a building block of a quantum communication network based on optical fibers. A thin membrane of GaAs embedding InGaAs quantum dots (QDs) is attached onto a piezoelectric actuator via gold thermocompression bonding. Here, the thin gold layer acts simultaneously as an electrical contact, strain transmission medium, and broadband backside mirror for the QD-micromesa. The nanofabrication of the QD-micromesa is based on in situ electron-beam lithography, which makes it possible to integrate pre-selected single QDs deterministically into the center of monolithic micromesa structures. The QD pre-selection is based on distinct single-QD properties, signal intensity, and emission energy. In combination with strain-induced fine tuning, this offers a robust method to achieve spectral resonance in the emission of remote QDs. We show that the spectral tuning has no detectable influence on the multi-photon suppression with g(2)(0) as low as 2%–4% and that the emission can be stabilized to an accuracy of 4 μeV using a closed-loop optical feedback.
Quantifying leakage fields at ionic grain boundaries using off-axis electron holography
Authors: Xu Xin , Barrows Frank , Dravid Vinayak P. , Haile Sossina M. , Phatak Charudatta
Published: December 2020

The electrical properties of interfaces in semiconductors and ionic conductors are immensely important in a wide range of applications. Electron holography is ideally suited for the direct measurement of the electrostatic potential of such interfaces. A key challenge with this approach is the contribution of the leakage field from the sample to the observed electron phase shift. This leakage field cannot be a priori independently determined and can cause an overestimation of the phase shift. In this work, we use finite element simulations to compute the three-dimensional electrostatic potential in the vicinity of an interface associated with a given interfacial charge density distribution. We then evaluate the predicted phase shift and demonstrate that the leakage field strongly affects the recovery of the projected interface potential. From the difference between the true potential and uncorrected, recovered potential, we propose a method to correct for this effect. We then demonstrate the application of this methodology to the analysis of experimental off-axis electron holography data acquired from the grain boundaries in lightly doped ceria.
Conversions of a stacking fault to threading dislocations in homoepitaxial diamond growth studied by transmission electron microscopy
Authors: Tsubouchi Nobuteru
Published: December 2020

In this paper, we present how configurations of stacking faults (SFs) in (001)-oriented single crystal diamond substrates changed and extended in the film during homoepitaxial diamond growth by chemical vapor deposition, which was investigated using transmission electron microscopy. In the early stages of the film growth, the SFs in the substrate converted to threading dislocation (TD) rows along the end of the (111) SF planes appearing on the substrate surface, where the direction of the end of the SFs on the surface was [1-10]-[-110]. These TDs then traveled nearly in the same direction tilted by 10° to [-110] from the [001] direction without divergence, while extending with slight divergence within ∼7° in the [110]-[-1-10] direction. The diffraction contrasts observed under several two-beam conditions demonstrated that the SF in the substrate was intrinsic type and had a (111) slip plane with a displacement vector R=-a/3[111], where a is the lattice constant of diamond. At the same time, all the TDs converted from the SFs were edge type and their Burgers vector was b = a/2[1-10], perpendicular to the [001] direction.
Mixtures of octanol and an ionic liquid: Structure and transport
Authors: Zhao Man , Wu Boning , Castner Edward W.
Published: December 2020

Ionic liquids (ILs) with long alkyl substituents are amphiphilic, which leads to a bicontinuous liquid structure. The strongly interacting anionic and cationic head groups form a long range charge network, with the hydrocarbon tails forming a nonpolar domain. Such nonpolar domains have been shown to dissolve a variety of neutral organic solvents. In mixtures of ILs with solvents the neutral organic molecules residing in the nonpolar domains experience different environments and friction from the charged cations and anions. Thus, the neutral molecules diffuse much faster than predicted by hydrodynamic scaling using the average viscosity of the mixture. In this work, we report studies on the structure and transport properties of mixtures of 1-octanol with the IL trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (P6,6,6,14+/NTf2−). The majority of the atom fraction in the P6,6,6,14+ cation comprises four hydrocarbon substituents. The unique amphiphilic nature of ILs with the P6,6,6,14+ cation makes 1-octanol fully miscible with the IL at ambient temperatures. X-ray scattering experiments show that the IL structure persists in the mixtures for 1-octanol mole fractions as large as xoct = 0.90. The self-diffusion coefficients of the three molecular species in the mixtures were measured by NMR experiments. The self-diffusion of the P6,6,6,14+ cation is well described by the Stokes–Einstein equation, while the diffusivity of the NTf2− anion is slightly lower than the hydrodynamic prediction. The measured diffusivities of octanol in these mixtures are 1.3–4 times higher than the hydrodynamic predictions.
Life of a droplet: Buoyant vortex dynamics drives the fate of micro-particle expiratory ejecta
Authors: Renzi E. , Clarke A.
Published: December 2020

We show that the dynamics of the expiratory cloud ejected during human respiratory events can be modeled by extending the theory of buoyant vortex rings with an initial momentum. We embed the integral conservation laws that govern the cloud’s motion into the model of an expanding vortex to determine the velocity field inside and outside the cloud. We then apply a Lagrangian particle-tracking model to calculate the trajectories of the mucosalivary droplets suspended within the cloud. Our results show very good agreement with the available experimental data. The vortex is shown to have a significant effect on suspending the droplets present in the cloud, increasing the time they remain airborne and extending their range further than predicted by the existing models. We also study the role that initial conditions have on the maximum streamwise range of the droplets, finding that decreasing the angle of projection can reduce the spread of the droplets by an order of meters. Finally, we discuss the importance of these findings in the context of informing public health policies and global information campaigns to slow down the spread of respiratory viruses.
A generalized forward fit for neutron detectors with energy-dependent response functions
Authors: Mohamed Z. L. , Mannion O. M. , Hartouni E. P. , Knauer J. P. , Forrest C. J.
Published: December 2020

To date, most of the analysis of neutron time-of-flight data from inertial confinement fusion experiments has focused on the relatively small range of energies corresponding to the primary neutrons from deuterium–deuterium and deuterium–tritium fusion and has, therefore, employed instrument response functions (IRFs) corresponding to monoenergetic 2.45-MeV or 14.03-MeV neutrons. For the analysis of time-of-flight signals corresponding to broader ranges of neutron energies, accurate treatment of the data requires the use of an energy-dependent IRF. This work describes interpolation of the IRF for neutrons of arbitrary energy, construction of an energy-dependent IRF, and application of this IRF in a forward fit via matrix multiplication. As an example of the application of this method, an analysis of synthetic data relevant to tritium–tritium fusion experiments at the Omega Laser Facility is discussed. This example is used to illustrate the differences between a forward fit that uses an energy-dependent IRF and a forward fit that uses a monoenergetic IRF. Use of the energy-dependent IRF is shown to result in accurate inference of the fit parameters of interest.
The motion of respiratory droplets produced by coughing
Authors: Wang Hongping , Li Zhaobin , Zhang Xinlei , Zhu Lixing , Liu Yi , Wang Shizhao
Published: December 2020

Coronavirus disease 2019 has become a global pandemic infectious respiratory disease with high mortality and infectiousness. This paper investigates respiratory droplet transmission, which is critical to understanding, modeling, and controlling epidemics. In the present work, we implemented flow visualization, particle image velocimetry, and particle shadow tracking velocimetry to measure the velocity of the airflow and droplets involved in coughing and then constructed a physical model considering the evaporation effect to predict the motion of droplets under different weather conditions. The experimental results indicate that the convection velocity of cough airflow presents the relationship t−0.7 with time; hence, the distance from the cougher increases by t0.3 in the range of our measurement domain. Substituting these experimental results into the physical model reveals that small droplets (initial diameter D ≤ 100 μm) evaporate to droplet nuclei and that large droplets with D ≥ 500 μm and an initial velocity u0 ≥ 5 m/s travel more than 2 m. Winter conditions of low temperature and high relative humidity can cause more droplets to settle to the ground, which may be a possible driver of a second pandemic wave in the autumn and winter seasons.
The theory of black hole formation shares the Nobel Prize in Physics
Authors: Hill Heather M.
Published: December 2020

Roger Penrose proved that black holes could and should form in the universe. The topological picture he used in that proof has become foundational to general relativity.
Electromagnetic waves in an optical photonic lattice
Authors: Korovay O.V.
Published: December 2020

Laser light propagation in a photonic lattice consisting of two parallel waveguide arrays is theoretically studied using the coupled mode method, with the interaction of each waveguide with the nearest neighbours and between the waveguides of the arrays being taken into account. Analytical expressions are obtained that make it possible to accurately predict the presence of localisation of light depending on the coupling constants. Particular solutions of a system of coupled waves are found, which describe strongly localised light propagating without transverse diffraction along the entire lattice at certain values of the coupling constants. The emergence of spatially limited transverse diffraction of light is predicted.
Production of two-colour radiation with polarisations of components rotating in opposite directions
Authors: Kotov V.M.
Published: December 2020

A method is proposed for the production of two-colour radiation with monochromatic components, the polarisation vectors of which rotate in opposite directions, and the rotation speed is controlled by the sound frequency. The method is based on acousto-optic (AO) interaction upon double passage of two-colour radiation through an AO cell made of a gyrotropic crystal; the method efficiency is demonstrated by the example of radiation from an Ar laser generating at wavelengths of 0.488 and 0.514 μm and an AO cell made of TeO2, operating at a sound frequency of 60 MHz.
Effect of CO2-laser power density on the absorption coefficient of polycrystalline CVD diamonds
Authors: Andreeva M.S. , Artyushkin N.V. , Krymskii M.I. , Laptev A.I. , Polushin N.I. , Rogalin V.E. , Rogozhin M.V.
Published: December 2020

The effect of 1-s irradiation of an uncooled polycrystalline CVD diamond plate by a focused cw CO2 laser beam with a power density of 300 – 800 kW cm−2 is investigated. The absorption coefficient of the sample at a power density of 800 kW cm−2 is found to be 0.035 cm−1 larger as compared with that at 300 kW cm−2, which is related to the temperature dependence of its phonon-induced absorption with a change in temperature from 44 to 100 °C. It is shown that polycrystalline diamond, in contrast to other optical materials, does not exhibit nonlinear (avalanche-like) rise in absorption at high CO2-laser power densities, at least up to 800 kW cm−2.
Numerical simulation of the beam self-cleaning process in a multimode graded-index fibre during propagation of a pump wave and a Stokes component
Authors: Sidelnikov O.S. , Podivilov E.V. , Babin S.A. , Wabnitz S. , Fedoruk M.P.
Published: December 2020

A model of coupled modes is proposed, which makes it possible to describe the propagation of a pump wave and a Stokes component in a multimode graded-index fibre. A decrease in the depression in the centre of the profile of the pump wave intensity distribution as a result of the influence of Kerr effects and random linear coupling of modes is demonstrated.
Nonlinear waves in a thin dielectric film on the surface of a topological insulator
Authors: Maimistov A.I. , Lyashko E.I. , Elyutin S.O.
Published: December 2020

Based on the dispersion relation for a wave localised in a thin film of a nonlinear dielectric, which is located on the surface of a topological insulator, we have derived a system of equations that describes the propagation of a surface wave. It is shown that the longitudinal and transverse tangential components of the electric field vector are related due to the nonlinearity of the film and change periodically during propagation. It is found that the rotation period of this vector is determined by the axion charge of the topological dielectric and the nonlinear susceptibility of the thin film.
Seven-core fibre Raman laser with intercore coupling
Authors: Dostovalov A.V. , Skvortsov M.I. , Wolf A.A. , Labuntsov V.I. , Egorova O.N. , Semjonov S.L. , Babin S.A.
Published: December 2020

This paper reports results of a study aimed at producing a Raman fibre laser based on a multicore fibre with high-reflectivity fibre Bragg gratings inscribed in its peripheral cores using femtosecond laser pulses. We present spectral and power characteristics of a Raman laser whose output power at a wavelength of 1090 nm is 2.5 W. The laser emission linewidth has been shown to decrease as a result of the reduction in nonlinear effects owing to the larger effective mode area in the multicore fibre in comparison with a standard single-mode fibre Raman laser.
Application of neural networks to determine the discrete spectrum of the direct Zakharov – Shabat problem
Authors: Sedov E.V. , Chekhovskoy I.S. , Prilepsky J.E. , Fedoruk M.P.
Published: December 2020

A neural network architecture is proposed to determine the number of solitons generated by random processes in optical wavelength-division multiplexed telecommunication systems with QPSK, 16-QAM, 64-QAM, and 1024-QAM modulation. The dependence of the prediction quality of a neural network with a special architecture on the number of soliton modes in the signal and the parameters of this signal is studied.
Output power saturation effect in Yb – Er fibre lasers
Authors: Surin A.A. , Kovalenko N.V.
Published: December 2020

The output power saturation effect in Yb – Er fibre lasers is experimentally observed. A formula for estimating the saturation power is derived. A method is proposed for measuring the 4I11/2 level lifetime or the concentration of Er ions based on the measurements of the saturation power.