Momentum Relaxation Research Articles

Infinite families of fracton fluids with momentum conservation

We construct infinite families of new universality classes of fracton hydrodynamics with momentum conservation, both with multipole conservation laws and/or subsystem symmetry. We explore the effects of broken inversion and/or time-reversal symmetry at the ideal fluid level, along with momentum relaxation. In the case of one-dimensional multipole-conserving models, we write down explicit microscopic Hamiltonian systems realizing these new universality classes. All of these hydrodynamic universality classes exhibit instabilities and will flow to new nonequilibrium fixed points. Such fixed points are predicted to exist in arbitrarily large spatial dimensions.

Numerical study of simultaneous transport of heat and mass transfer in Maxwell hybrid nanofluid in the presence of Soret and Dufour effects

The article aims to investigate the influence of copper (Cu) and a combination of copper (Cu) and aluminum oxide (Al2O3) on the simultaneous transfer of heat and mass in sodium alginate liquid moving over a circular pipe. This transport activity is modeled by the use of conservation laws with correlations for physical quantities of Cu, Al2 O3, and sodium alginate. Through cylindrical coordinates formulation, the set of partial differential equations is obtained. These models are solved numerically by the finite element method (FEM). The relaxation time associated with momentum diffusion in Maxwell fluid plays role in controlling the viscous region. Moreover, momentum relaxation time in Cu/sodium alginate is strong than that in Cu–Al2O3/sodium alginate. It is noticed from simulations that particles of Cu/ sodium alginate have a greater velocity than the velocity of Cu–Al2O3/sodium alginate. Therefore, distortion of magnetic lines by the flow of Cu/sodium alginate is more than the distortion of magnetic lines by the flow of Cu–Al2O3/sodium alginate. The rise in the thermal conductivity of sodium alginate due to simultaneous dispersion of Cu and Al2O3 is more than the rise in thermal conductivity of sodium alginate. Thus for maximum HT, the simultaneous dispersion of Cu and Al2O3 in sodium alginate is recommended.

Spin-dependent analysis of homogeneous and inhomogeneous exciton decoherence in magnetic fields

This paper discusses the combined effects of optical excitation power, interface roughness, lattice temperature, and applied magnetic fields on the spin coherence of excitonic states in GaAs/AlGaAs multiple quantum wells. For low optical powers, at lattice temperatures between 4 and 50 K, the scattering with acoustic phonons and short-range interactions appear as the main decoherence mechanisms. Statistical fluctuations of the band gap, however, become also relevant in this regime and we were able to deconvolute them from the decoherence contributions. The circularly polarized magneto-photoluminescence unveils a nonmonotonic tuning of the coherence for one of the spin components at low magnetic fields. This effect has been ascribed to the competition between short-range interactions and spin-flip scattering, modulated by the momentum relaxation time.

Thermodynamic geometry and Joule-Thomson expansion of black holes in modified theories of gravity

The implication of Ruppeiner thermodynamic geometry theory to spherically symmetric black holes in anti--de Sitter (AdS) spacetime produces an unavoidable singularity in the line element of thermodynamic geometry. The singularity occurs due to the nonindependence of volume and entropy of black holes (BH) in AdS spacetime. In this work, we present an efficient and easy approach to deal the thermodynamic geometry of AdS BH in Einstein-Maxwell-scalars theory. We show that enthalpy instead of internal energy is central thermodynamic quantity. We develop the particular forms of the line element of thermodynamic geometry in different phase spaces. We find out that the curvatures in different phase spaces are identical and positive which leads to the repulsive interaction information between black hole molecules. We also analyze the thermal stability of AdS BH in Einstein-Maxwell-scalars theory and regularized Lovelock theory in the presence of thermal fluctuations. We observed that momentum relaxation parameter and coupling constants of Lovelock theory increase the thermal stability of the BHs. Finally, we also study the Joule-Thomson expansion for black hole in regularized Lovelock theory.

The Effect of phonon focusing on the mutual drag of electrons and phonons and the electrical resistance of potassium

The effect influence of elastic energy anisotropy on the mutual drag of electrons and phonons and the electrical resistance of potassium crystals at low temperatures have investigated. We have analyzed the momentum exchange between the electron and three phonon flows corresponding to three branches of the vibrational spectrum in the hydrodynamic approximation. The actual mechanisms of phonon momentum relaxation have taken into account: scattering at sample boundaries, dislocations, and in the processes of phonon-phonon transfer. It have shown that in the limiting case of strong mutual drag of electrons and phonons, the electrical resistance will be much lower than that given by the Bloch--Gruneisen theory, and the phonon and electron drift velocities are close and they are determined by the total phonon relaxation rate in resistive scattering processes. In the opposite case, when resistive scattering processes dominate for phonons and the phonon system remains in equilibrium, then the electrical resistance follows the Bloch--Gruneisen theory. In this case, the drift velocities of all modes are different and much less than the electron drift velocity. Keywords: electrical resistance, elastic anisotropy, electron-phonon relaxation.

Влияние фокусировки на взаимное увлечение электронов и фононов и электросопротивление кристаллов калия

The effect influence of elastic energy anisotropy on the mutual drag of electrons and phonons and the electrical resistance of potassium crystals at low temperatures have investigated. We have analyzed the momentum exchange between the electron and three phonon flows corresponding to three branches of the vibrational spectrum in the hydrodynamic approximation. The actual mechanisms of phonon momentum relaxation have taken into account: scattering at sample boundaries, dislocations, and in the processes of phonon-phonon transfer. It have shown that in the limiting case of strong mutual drag of electrons and phonons, the electrical resistance will be much lower than that given by the Bloch–Grüneisen theory, and the phonon and electron drift velocities are close and they are determined by the total phonon relaxation rate in resistive scattering processes. In the opposite case, when resistive scattering processes dominate for phonons and the phonon system remains in equilibrium, then the electrical resistance follows the Bloch–Grüneisen theory. In this case, the drift velocities of all modes are different and much less than the electron drift velocity.

Anisotropic photoconductivity excited in a semiconductor by two-frequency optical radiation

Anisotropic photoconductivity at a difference frequency, excited in a semiconductor by linearly polarized two-frequency optical radiation, is considered. The anisotropy of the photoconductivity arises due to the optical alignment of photoexcited electrons momenta and dependence of their effective mass and momentum relaxation time on energy. It is shown that the contribution of the anisotropic photoconductivity to the photocurrent at the difference frequency lying in the terahertz frequency range can be comparable with that of the isotropic photoconductivity. This effect can manifest itself in photoconductive antennae, devices used to generate terahertz radiation. Keywords: Anisotropic photoconductivity, photomixing, terahertz radiation, photoconductive antenna.

Анизотропная фотопроводимость, возбуждаемая в полупроводнике двухчастотным оптическим излучением

Anisotropic photoconductivity at a difference frequency, excited in a semiconductor by linearly polarized two-frequency optical radiation, is considered. The anisotropy of the photoconductivity arises due to the optical alignment of photoexcited electrons momenta and dependence of their effective mass and momentum relaxation time on energy. It is shown that the contribution of the anisotropic photoconductivity to the photocurrent at the difference frequency lying in the terahertz frequency range can be comparable with that of the isotropic photoconductivity. This effect can manifest itself in photoconductive antennaе – devices used to generate terahertz radiation.

Implementation and verification of a conservative,multi‐species,gyro‐averaged, full‐f,Lenard‐Bernstein/Dougherty collision operator in the gyrokinetic codeGENE‐X

AbstractGyrokinetic simulations are among the main tools to predict turbulent transport in the core of fusion devices. Unlike the core, the plasma edge and scrape‐off layer are characterized by lower temperature and higher collisionality. To allow for realistic gyrokinetic modelling of edge and scrape‐off layer turbulence, it is crucial to include collisional effects into the simulations. In this work, we present a full‐f, gyro‐averaged, multi‐species, Lenard‐Bernstein/Dougherty (LBD) collision operator, for the use in the gyrokinetic turbulence codeGENE‐X. The operator accounts for exact particle density, momentum, and energy conservation, with collision frequencies chosen such that the momentum or temperature relaxation rates of the Boltzmann collision operator are recovered. We provide a conservative second‐order finite‐volume implementation of the operator and present a thorough verification. Due to the excellent conservation properties of the finite‐volume implementation, it is possible to use a coarser velocity space grid, save computational resources and, as a consequence, perform simulations of larger fusion devices.

Computational simulation of Scott-Blair model to fractional hybrid nanofluid with Darcy medium

The primary and foremost objective of the recent assessment is to offer a comprehensive analysis regarding the heat transfer of fractional unsteady magnetohydrodynamic flow of viscoelastic hybrid nanofluids (blend of MWCNTs and SWCNTs) over an inclined surface fixed in a Darcy porous medium where the impact of mixed convection is also taken into account. We have also presented the Scott Blair model and fractional Fourier's law in the constitutive relations. Furthermore, we have employed Caputo fractional derivatives (CFD) (α and β) together with fractional momentum and thermal relaxation times λ1 and λ2 to model the flow problem under consideration to attain flow and heat transfer control. Since the principal FPDEs are nonlinear and hence numerical solved by utilizing FD discretization combined with the L1 method. Simulated outcomes of velocity and temperature are elaborated in both cases, i.e., the single and hybrid nanofluids. The obtained results are of significant value due to FD, CNTs. The same may be utilized to proficiently tackle thermal management challenges like the achievement of temperature control in automobile engines, boilers, grinders, heat exchangers, and refrigerators, etc.

Terahertz scattering-type near-field microscopy quantitatively determines the conductivity and charge carrier density of optically doped and impurity-doped silicon

A terahertz scattering-type scanning near-field optical microscope is used for nano-scale non-invasive conductivity measurements on bulk silicon samples. We first investigate the case where the density of charge carriers is determined by optical interband excitation. We show that the amplitude and phase of the near-field signal are reproduced by simulations based on an established near-field interaction model, which takes the Drude conductivity, ambipolar carrier diffusion, and known recombination properties of photo-excited carrier pairs in Si into account. This study is then extended to impurity-doped Si. We demonstrate that the phase of the near-field signal, which can easily be measured in absolute terms, allows us to quantitatively determine the conductivity of the specimens, from which the carrier density is derived based on the known carrier momentum relaxation time. A measurement at a single properly chosen terahertz frequency is sufficient. The technique proposed here holds promise for the spatially resolved quantitative characterization of micro- and nanoelectronic materials and devices.

Momentum and energy relaxation in femtosecond-scale energy transport in metals

In the current report we investigate the processes of heat transport and energy relaxation in metals using Boltzmann transport equation for the electron distribution function perturbation, simultaneously accounting both energy and momentum relaxation. Mutual influence of electron-electron and electron-phonon energy relaxation on the electron distribution function is considered using linearized integro-differential collision integrals. These energy exchange effects provide main contribution to the difference between results obtained using Boltzmann equation solution and widely considered two-temperature model.

Momentum relaxation effects in 2D-Xene field effect device structures

We analyze the electric field driven topological field effect transition on 2D-xene materials with the addition of momentum relaxation effects, in order to account for dephasing processes. The topological field effect transition between the quantum spin Hall phase and the quantum valley Hall phase is analyzed in detail using the Keldysh non-equilibrium Green’s function technique with the inclusion of momentum and phase relaxation, within the self-consistent Born approximation. Details of the transition with applied electric field are elucidated for the ON–OFF characteristics with emphasis on the transport properties along with the tomography of the current carrying edge states. We note that for moderate momentum relaxation, the current carrying quantum spin Hall edge states are still pristine and show moderate decay with propagation. To facilitate our analysis, we introduce two metrics in our calculations, the coherent transmission and the effective transmission. In elucidating the physics clearly, we show that the effective transmission, which is derived rigorously from the quantum mechanical current operator is indeed the right quantity to analyze topological stability against dephasing. Exploring further, we show that the insulating quantum valley Hall phase, as a result of dephasing carries band-tails which potentially activates parasitic OFF currents, thereby degrading the ON–OFF ratios. Our analysis sets the stage for realistic modeling of topological field effect devices for various applications, with the inclusion of scattering effects and analyzing their role in the optimization of the device performance.

Impurity effect on hysteric magnetoconductance: holographic approach

In this paper we study a hysteric phase transition from weak localization phase to hysteric magnetoconductance phase using gauge/gravity duality. This hysteric phase is triggered by a spontaneous magnetization related to ℤ2 symmetry and time reversal symmetry in a 2+1 dimensional system with momentum relaxation. We derive thermoelectric conductivity formulas describing non-hysteric and hysteric phases. At low temperatures, this magnetoconductance shows similar phase transitions of topological insulator surface states. We also obtain hysteresis curves of Seebeck coefficient and Nernst signal. It turns out that our impurity parameter changes magnetic properties of the dual system. This is justified by showing increasing susceptibility and the spontaneous magnetization with increasing impurity parameter.

Holographic complexity for black branes with momentum relaxation

We employ the "complexity equals action" conjecture to investigate the action growth rate for the charged and neutral AdS black branes of a holographic toy model consisting of Einstein-Maxwell theory in $d + 1$-dimensional bulk spacetime with $d - 1$ massless scalar fields which is called Einstein-Maxwell-Axion (EMA) theory. From the holographic point of view, the scalar fields source a spatially dependent field theory with momentum relaxation on the boundary, which is dual to the homogeneous and isotropic black branes. We find that the growth rate of the holographic complexity within the Wheeler-DeWitt (WDW) patch saturates the corresponding Lloyd's bound at the late time limit. Especially for the neutral AdS black branes, it will be shown that the complexity growth rate at late time vanishes for a particular value of relaxation parameter $\beta_{max}$ where the temperature of the black hole is minimal. Then, we investigate the transport properties of the holographic dual theory in the minimum temperature. A non-linear contribution of the axion field kinetic term in the context of k-essence model in the four-dimensional spacetime is considered as well. We also study the time evolution of the holographic complexity for the dyonic AdS black branes in this model.

Conformal maps of viscous electron flow in the Gurzhi crossover

We investigate the impact of geometric constriction on the viscous flow of electron liquid through quantum point contacts. We provide analysis on the electric potential distribution given the setup of a slit configuration and use the method of conformal mapping to obtain analytical results. The potential profile can be tested and contrasted experimentally with the scanning tunneling potentiometry technique. We discuss intricate physics that underlies the Gurzhi effect, i.e., the enhancement of conductivity in the viscous flow, and compare results for different boundary conditions. In addition, we calculate the temperature dependence of the momentum relaxation time as a result of impurity assisted quasiballistic interference effects and discuss various correlational corrections that lead to the violation of Matthiessen's rule in the hydrodynamic regime. We caution that spatially inhomogeneous profiles of current in the Gurzhi crossover between Ohmic and Stokes flows might also appear in the nonhydrodynamic limit where nonlocality plays an important role. This conclusion is corroborated by calculation of dispersive conductivity in the weakly impure limit.

Strong suppression of electron convection in Dirac and Weyl semimetals

It is shown that the convective instability in electron fluids in three- and two-dimensional (3D and 2D) Dirac and Weyl semimetals is strongly inhibited. The major obstacles for electron convection are the effects of the Coulomb forces and the momentum relaxation related to the interaction with impurities and phonons. The effect of the Coulomb forces is less pronounced in 2D materials, such as graphene. However, momentum relaxation still noticeably inhibits convection making it very difficult to achieve in practice.

Enhancement of electron-impact ionization induced by warm dense environments.

Studies have shown significant discrepancies between the recent experiment [Berg etal., Phys. Rev. Lett. 120, 055002 (2018)PRLTAO10.1103/PhysRevLett.120.055002] and current theoretical calculations on the electron-impact ionization cross section of ions in warm dense magnesium. Here, we present a systematic study the effects of the ionic correlations and free-electron screening on the electron-impact ionization of ions in warm dense matter. The ionic correlation and the free-electron screening effects yield additional Hermitian terms to the calculation of the ionic central-force-field potential, which significantly change the electronic structure compared with that of the isolated ion. In calculating the electron-impact ionization, we describe the impact and ionized electrons using a damped-distorted wave function, which considers the momentum relaxation of free electrons due to collisions with other free electrons and ions. We reproduce the electron-impact ionization process for Mg^{7+} in the solid-density plasma and increase the ionization cross section by one order of magnitude compared with that of the isolated ion, which excellently agrees with the experimental result of Berg etal.

Simulation of Z-Shaped Graphene Geometric Diodes Using Particle-in-Cell Monte Carlo Method in the Quasi-Ballistic Regime.

Geometric diodes are planar conductors patterned asymmetrically to provide electrical asymmetry, and they have exhibited high-frequency rectification in infrared rectennas. These devices function by ballistic or quasi-ballistic transport in which the transport characteristics are sensitive to the device geometry. Common methods for predicting device performance rely on the assumption of totally ballistic transport and neglect the effects of electron momentum relaxation. We present a particle-in-cell Monte Carlo simulation method that allows the prediction of the current–voltage characteristics of geometric diodes operating quasi-ballistically, with the mean-free-path length shorter than the critical device dimensions. With this simulation method, we analyze a new diode geometry made from graphene that shows an improvement in rectification capability over previous geometries. We find that the current rectification capability of a given geometry is optimized for a specific mean-free-path length, such that arbitrarily large mean-free-path lengths are not desirable. These results present a new avenue for understanding geometric effects in the quasi-ballistic regime and show that the relationship between device dimensions and the carrier mean-free-path length can be adjusted to optimize device performance.

On a One-Dimensional Hydrodynamic Model for Semiconductors with Field-Dependent Mobility

We consider a one-dimensional, isentropic, hydrodynamical model for a unipolar semiconductor, with the mobility depending on the electric field. The mobility is related to the momentum relaxation time, and field-dependent mobility models are commonly used to describe the occurrence of saturation velocity, that is, a limit value for the electron mean velocity as the electric field increases. For the steady state system, we prove the existence of smooth solutions in the subsonic case, with a suitable assumption on the mobility function. Furthermore, we prove uniqueness of subsonic solutions for sufficiently small currents.