Based on early phenomenological ideas about the operation of superconducting single-photon detectors (SSPD or SNSPD), it was expected that materials with a lower superconducting gap should perform better in the IR range. The plausibility of this concept could be checked using two popular SSPD materials - NbN and WSi films. However, these materials differ strongly in crystallographic structure (polycrystalline B1 versus amorphous), which makes their dependence on disorder different. In our work we present a study of the single-photon response of SSPDs made from two disordered B1 structure superconductors - vanadium nitride and niobium nitride thin films. We compare the intrinsic efficiency of devices made from films with different sheet resistance values. While both materials have a polycrystalline structure and comparable diffusion coefficient values, VN films show metallic behavior over a wide range of sheet resistance, in contrast toNbNfilms with an insulator-like temperature dependence of resistivity, which may be partially due to enhanced Coulomb interaction, leading to different starting points for the normal electron density of states. The results show that even though VN devices are more promising in terms of theoretical predictions, their optimal performance was not reached due to lower values of sheet resistance.
We investigate a well-known phenomenon of the appearance of the crossover points, corresponding to the intersections of the solubility isotherms of the solid compound in supercritical fluid. Opposed to the accepted understanding of the existence of two fixed crossover points, which confine the region of the inverse isobaric temperature dependence of the solubility, we have found that these points tend to shift with the change of the temperature and in the limit of the certain threshold value they converge to a single point. We demonstrate this analyzing the solubility data of a set of poorly soluble drug compounds, which have been computed in a wide area of the phase diagram via the approach, based on the classical density functional theory. Thorough analysis of the available in the literature experimental solubility data is found to be in an agreement with our conclusions, as one can find that the wider temperature region of the experimental study is, the more pronounced effect of the crossover points drift can be observed.
This article presents generalized model of collaborative actions, during which participants create, modify, and estimate digital objects. Such activities can be observed in numerous network communities. A prominent example is the repository of lesson scenarios of Moscow Electronic School (MES). The combination of methods of agent-based modeling and network analysis is used in the work. Using NetLogo environment in the frames of the model, an artificial community has been developed, where teachers-agents interact with scenarios-agents. Teacher-agent determines whether there are potential scenarios in his environment to be contacted with. If such scenarios are available, then the agent selects the nearest one and makes a step towards it. If the scenario has been opened by one of the teachers, then this is already an author’s scenario and the teacher-agent takes an action to reuse it. Variants of the reuse can be preset so that to correspond to the actions allowable in the environment of MES repository for learning scenarios: review, addition to bookmarks, running the scenario, downloading, using in home assignments. All these actions of teachers regarding scenarios are logged, then the log records are transformed into bipartite graph. The experiments demonstrate that while the area of participant scenarios is expanded, not only the general number of links among participants increases but also large networks of participants are subdivided into smaller and densely interconnected groups. One of the control trends of participant activities is in the use of multiagent-based modeling as a tool of collective reflection of teachers cooperating on the basis of MES.
We propose a novel algorithm for the construction of the sparse, nonetheless, the massive and rigid structure. The generated structures possess two significant properties reminiscent of the metallic foams. Firstly, the weight of the structures can be as low as the percent of the bulk one. Secondly, the structures are mechanically rigid. The structures are necessary for the simulation of the physical models of the foam properties.
The population annealing method is a promising approach for large-scale simulations because it is potentially scalable on any parallel architecture. We present an implementation of the algorithm on a hybrid program architecture combining CUDA and MPI. The problem is to keep all general-purpose graphics processing unit devices as busy as possible by efficiently redistributing replicas. We provide details of testing on hardware based the Intel Skylake/Nvidia V100 running more than two million replicas of the Ising model sample in parallel. The results are quite encouraging because the acceleration grows toward the perfect line as the complexity of the simulated system increases.
The process of poly(methyl methacrylate) (PMMA) matrix impregnation with mefenamic acid (MFA) in a supercritical carbon dioxide medium has been studied by the full atomistic classical molecular dynamics method. Simulations have been performed for two systems that differ in the polymer sample size (≈270 kDa and ≈1080 kDa) at 333 K and 40 MPa. The characteristics of the systems, such as the radius of gyration, end-to-end distance, mean squared displacement, radial distribution functions, average number of hydrogen bonds, and number of close contacts, have been analyzed and discussed. It has been found that by the end of the simulation (15 ns), the MFA loadings reach about 1.43 w/w % and 1.14 w/w % for the small and big PMMA samples, respectively. It was shown that the solute was distributed in the molecular form inside the polymer matrix. At the same time, when the CO2 molecules were removed from the systems and the simulation was performed in a canonical ensemble with the same cell length as in the previous isobaric-isothermal ensemble, the MFA molecules began to self-associate and get adsorbed on the polymer surface as hydrogen-bonded aggregates. In order to estimate the strength of the intermolecular interaction between the system components, ab initio calculations were performed. The calculated energies of the electron donor-acceptor (EDA) and hydrogen-bonded (HB) complexes can be arranged in the following order (in absolute value): ΔEEDA(PMMA-CO2)≈ΔEHB(MFA-CO2)<ΔEHB(PMMA-MFA)<ΔEHB(MFA-MFA).
To achieve higher throughputs in cellular networks, 3GPP proposes to use unlicensed frequency bands and develops technologies - the latest one is NR-U - allowing a cellular base station to transmit in unlicensed bands, which are already occupied by Wi-Fi networks. To enable fair channel sharing between two technologies, the base station uses a sort of CSMA/CA with binary exponential backoff similar to Wi-Fi. However, the base station can start data transmission only at strictly periodic time moments. Many papers propose sending a reservation signal between the end of the backoff procedure and such a moment to prevent nearby devices from accessing the channel. However, this approach significantly reduces Wi-Fi performance. The paper proposes a novel method CR-LBT of transmitting a reservation signal that greatly decreases channel resource waste caused by collisions and improves channel resource sharing fairness. With developed analytical models and simulations, it is shown that CR-LBT may simultaneously increase the throughput of both NR-U and Wi-Fi networks. The effect is more noticeable for the Wi-Fi network, the throughput of which may rise three times compared with the traditional method of sending the reservation signal. Finally, the influence of various factors on CR-LBT performance is studied.
Inelastic interactions of quantum systems with the environment usually wash coherent effects out. In the case of Friedel oscillations, the presence of disorder leads to a fast decay of the oscillation amplitude. Here we show both experimentally and theoretically that in three-dimensional topological insulator Bi2Te3 there is a nesting-induced splitting of coherent scattering vectors which follows a peculiar evolution in energy. The effect becomes experimentally observable when the lifetime of quasiparticles shortens due to disorder. The amplitude of the splitting allows an evaluation of the lifetime of the electrons. A similar phenomenon should be observed in any system with a well-defined scattering vector regardless of its topological properties.
Based on the quantum-mechanical theory of electron transfer (ET), the parameter was proposed to describe the electrochemical activity of doped graphenes. The parameter is calculated using the density of states (DOS), local density of state (LDOS) values, which are in turn obtained from the density functional theory (DFT) calculations and reorganization energies of redox system. DOS describes the contribution of the electronic structure of the electrode to the ET process, while the LDOS describes the electron density contribution of the atoms at some distance from the surface electrode. Reorganization energy corresponds to the restriction of solvation shell and bonds in redox system due to ET process. The overall contribution of these parameters enables a comprehensive assessment of the activity that is acceptable for semi-quantitative analysis. Calculations have shown that the proposed activity parameter correlates well with the calculated ET rate constants. Theoretical study of the oxygen reduction reaction (ORR) on graphene doped with p-elements in the framework of quantum-mechanical theory showed that ET activity decreases in the series P-Gr > S-Gr > N-Gr > B-Gr > O-Gr > Gr. According to our estimates, the mixed or adiabatic regime of ET is probably observed on doped graphenes for all steps of ORR. Using N- and B-graphenes as an example and activity parameter, the influence of the applied potential and the atomic fraction of the doped element on the ET activity are studied.
The stability of the quasi-two-dimensional droplet flow is of great importance in microfluidic devices. We check the drop's stability in the square box using the immersed boundary and lattice Boltzmann methods. We implement two-dimensional equations within the immersed boundary approach in the Palabos programming platform. We check the influence of the boundaries on the drop movement. We estimate fluctuations in the quantities while applying different initial conditions of the linear and angular velocities. We found that the level of fluctuations depends on the symmetrical displacement of drop at the initial state. The effect is connected with the hydrodynamic interaction of drop with the walls.
We investigate the local time evolution in the Personal Communication Service (PCS) model simulated with the parallel discrete event simulation method's optimistic algorithm. We propose a model for the optimistic local virtual time evolution (OLVT) in PCS, which is reminiscent of statistical physics's surface growth. We use Rensselaer's optimistic simulation system with the Time Warp implementation. We compare the results of the simulations of both PCS and OLVT models and found good agreement. We discuss the highlights of our approach in the analysis of scalability and synchronization using the OLVT model.
We develop a new quantitative molecular theory of liquid-phase dipolar polymer gels. We model monomer units of the polymer network as a couple of charged sites separated by a fluctuating distance. For the first time, within the random phase approximation, we have obtained an analytical expression for the electrostatic free energy of the dipolar gel. Depending on the coupling parameter of dipole–dipole interactions and the ratio of the dipole length to the subchain Kuhn length, we describe the gel collapse induced by electrostatic interactions in the good solvent regime as a first-order phase transition. This transition can be realized at reasonable physical parameters of the system (temperature, solvent dielectric constant, and dipole moment of monomer units). The obtained results could be potentially used in modern applications of stimuliresponsive polymer gels and microgels, such as drug delivery, nanoreactors, molecular uptake, coatings, superabsorbents, etc.
We discuss an international conference, \Computer simulations in Physics and beyond" organized in October 2020 during a pandemic. We pay a particular account for the advantages and disadvantages of holding an international conference online.
High sensitivity imaging at the level of single photons is an invaluable tool in many areas, ranging from microscopy to astronomy. However, development of single-photon sensitive detectors with high spatial resolution is very non-trivial. Here we employ the singlepixel imaging approach and demonstrate a proof-of-principle single-pixel single-photon imaging setup. We overcome the problem of low light gathering efficiency by developing a large-area microstrip superconducting single photon detector coupled to a multi-mode optical fiber interface. We show that the setup operates well in the visible and near infrared spectrum, and is able to capture images at the singlephoton level.
We discuss the applicability of multiphase lattice Boltzmann method for the simulation of the drop oscillation. We demonstrate that the simulation of the single drop excited to the first eigenmode does follow Rayleigh formula. Simulations show no sensitivity to the number of the discrete velocities with D3Q19 and D3Q27 representations of the distribution function in three dimensions. The boundaries do influent the motion of the drop—division of the computational area by the even and the odd number of cells comes out important and leads to symmetry violence. The second part of the chapter describes the oscillations of the ensemble of three drops due to the excitation of the central drop in the first eigenmode. The motion of the backdrops does strongly depend on the viscosity of the fluid. We provide future details of simulations.
We consider a spatially distributed evolutionary game based on the Prisoner’s Dilemma with agents arranged on a three-dimensional simple cubic lattice. Comparing to two- dimensional arrangements, we find that the larger number of neighbors favors the formation of spatial chaos: the steady state of the game is chaotic unless the payoff parameter is small.
The magnetization in a superconductor induced due to the inverse proximity effect is investigated in hybrid bilayers containing a superconductor and a ferromagnetic insulator or a strongly spin-polarized ferromagnetic metal. The study is performed within a quasiclassical Green function framework, wherein Usadel equations are solved with boundary conditions appropriate for strongly spin-polarized ferromagnetic materials. A comparison with recent experimental data is presented. The singlet to triplet conversion of the superconducting correlations as a result of the proximity effect with a ferromagnet is studied.
MgPd2 is an intermetallic compound with a reversible hydrogen uptake near ambient conditions. Hydrogenation occurs near room temperature at pressures below 0.1 MPa to form a hydrogen-rich MgPd2H0.88 phase. In this work, hydrogen sorption isotherms were measured at 283 K ≤ T ≤ 328 K as well as at a cryogenic temperature of 77 K and pressure values up to 0.1 MPa by manometric and gravimetric methods. In addition, the gravimetric hydrogen sorption uptake under isobaric conditions was determined and compared with the structural information based on the previous work by Götze et al. At lower pressures we suggest the formation of the MgPd2H0.14 phase at isobaric conditions of p = 2.5 MPa and T > 437 K. We propose a model combining the hydrogen sorption properties and the volume expansion during the hydrogenation process for describing the phase diagram of the MgPd2-H system. The model was used to describe the experimental data in the temperature range of 283 K ≤ T ≤ 328 K and extrapolate them to higher temperatures. The critical point of MgPd2H0.88 was calculated to be at Tc = 358 K and pc = 0.23 MPa. Below this critical point, two phases – the hydrogen-poor and the hydrogen-rich ones – coexist. However, the critical temperature is much lower than in PdH-systems (563 K), which makes it more attractive for potential applications at moderate temperatures and pressures than PdHx. The calculated enthalpy of the H2 absorption into MgPd2H0.88 is ΔHabs = -38.7 (mol-1 H2), whereas the calculated enthalpy of desorption is ΔHdes = 42.4 kJ (mol-1 H2). The resulting Gibbs free energy of nearly ΔG ≈ -3.7 kJ mol-1 indicates a reversible absorption and desorption process at temperature and pressure values close to ambient conditions.
The rectification of electromagnetic waves to direct currents is a crucial process for energy harvesting, beyond-5G wireless communications, ultra-fast science, and observational astronomy. As the radiation frequency is raised to the sub-terahertz (THz) domain, ac-to-dc conversion by conventional electronics becomes challenging and requires alternative rectification protocols. Here, we address this challenge by tunnel field-effect transistors made of bilayer graphene (BLG). Taking advantage of BLG’s electrically tunable band structure, we create a lateral tunnel junction and couple it to an antenna exposed to THz radiation. The incoming radiation is then down-converted by the tunnel junction nonlinearity, resulting in high responsivity (>4 kV/W) and low-noise (0.2 pW/Hz−−−√Hz) detection. We demonstrate how switching from intraband Ohmic to interband tunneling regime can raise detectors’ responsivity by few orders of magnitude, in agreement with the developed theory. Our work demonstrates a potential application of tunnel transistors for THz detection and reveals BLG as a promising platform therefor.