Nonzero Flux Research Articles

Spin current and internal Zeeman field in spin-orbit coupled rings

We investigate the one-dimensional quantum ring constructed by the spin-orbit coupled material, in which the quantum spin-Hall Bernevig-Zhang (BZ) Hamiltonian and Rashba-Dirac (RD) type spin-orbit coupling are taken into account (called RD-BZ Hamiltonian in this paper). It is known that the curvature of the ring generates an out-of-plane effective magnetic field, acting as an internal Zeeman field. We find that only the BZ coupling can change the strength of the internal Zeeman field, which enables us to detect the effect of the internal Zeeman field. Furthermore, we find that the total angular momentum is conserved in the RD-BZ Hamiltonian, and the energy eigenvalue and wave function must be modified to fit the conserved quantity, which are ignored in the previous studies. The conductance without leads is discussed. Different from the previous results, we find that the conductance behaves like a beat phenomenon resulting from the interplay between the magnetic flux and Aharonov-Casher (AC) phase, and thus, it can oscillate without passing through the insulating state in some regimes of magnetic flux or AC phase. Importantly, we find that the conductance with integer and half-integer magnetic flux provides us a method to measure the AC phase. The cancellation of the internal Zeeman field due to the BZ coupling can be detected by using specific fractional magnetic flux. In the ring with nonvanishing RD and BZ couplings, the conductance can exhibit a quasiplateau near the small RD coupling. The increase in the strength of BZ coupling would result in wider quasiplateau in conductance, which implies that the ring could remain insulating state (or conducting state) regardless of the small change in RD coupling. The thermal average of spin and charge currents are calculated at low temperatures without impurities. Importantly, we find that the persistent spin and charge currents as a function of the magnetic flux exhibit nodelike lines, which amazingly are perpendicular to each other. As a consequence, the persistent spin current could be nonzero even when the charge current vanishes at nonzero magnetic flux. The result suggests a pure spin current in quantum rings, and its direction can be reversed by changing the magnetic flux. The increase in the BZ coupling would exhibit the plateaulike pure spin current.

An adaptive immersed finite element method for linear parabolic interface problems with nonzero flux jump

An adaptive immersed finite element method for linear parabolic interface problems with nonzero flux jump

Resolving the mystery of electron perpendicular temperature spike in the plasma sheath

A large family of plasmas has collisional mean-free-path much longer than the non-neutral sheath width, which scales with the plasma Debye length. The plasmas, particularly the electrons, assume strong temperature anisotropy in the sheath. The temperature in the sheath flow direction (Te∥) is lower and drops toward the wall as a result of the decompressional cooling by the accelerating sheath flow. The electron temperature in the transverse direction of the flow field (Te⊥) not only is higher but also spikes up in the sheath. This abnormal behavior of Te⊥ spike is found to be the result of a negative gradient of the parallel heat flux of transverse degrees of freedom (qes) in the sheath. The non-zero heat flux qes is induced by pitch-angle scattering of electrons via either their interaction with self-excited electromagnetic waves in a nearly collisionless plasma or Coulomb collision in a collisional plasma, or both in the intermediate regime of plasma collisionality.

Mechanisms for magnetic skyrmion catalysis and topological superconductivity

We propose an alternative route to stabilize magnetic skyrmion textures which does not require Dzyaloshinkii-Moriya interactions, magnetic anisotropy, or an external Zeeman field. Instead, it solely relies on the emergence of flux in the system's ground state. We discuss scenarios that lead to a nonzero flux and identify the magnetic skyrmion ground states which become accessible in its presence. Moreover, we explore the chiral superconductors obtained for the surface states of a topological crystalline insulator when two types of magnetic skyrmion crystals coexist with a pairing gap. Our work opens perspectives for engineering topological superconductivity in a minimal fashion and promises to unearth functional topological materials and devices which may be more compatible with electrostatic control than the currently explored skyrmion-Majorana platforms.

Kinetics of Precipitation Processes at Non-Zero Input Fluxes of Segregating Particles

We consider the process of formation and growth of clusters of a new phase in segregation processes in solid or liquid solutions in an open system when segregating particles are added continuously to it with a given rate of input fluxes, Φ. As shown here, the value of the input flux significantly affects the number of supercritical clusters formed, their growth kinetics, and, in particular, the coarsening behavior in the late stages of the process. The detailed specification of the respective dependencies is the aim of the present analysis, which combines numerical computations with an analytical treatment of the obtained results. In particular, a treatment of the coarsening kinetics is developed, allowing a description of the development of the number of clusters and their average sizes in the late stages of the segregation processes in open systems, which goes beyond the scope of the classical Lifshitz, Slezov and Wagner theory. As is also shown, in its basic ingredients, this approach supplies us with a general tool for the theoretical description of Ostwald ripening in open systems, or systems where the boundary conditions, like temperature or pressure, vary with time. Having this method at one's disposal supplies us with the possibility that conditions can be theoretically tested, leading to cluster size distributions that are most appropriate for desired applications.

Thermo-diffusion properties and active and passive control of migrated nanoparticles with zero and non-zero mass fluxes

Recently, the research in nanoparticles is widely focused by researchers owing to the role of such nanomaterials is various industrial and thermal systems. However, the control of thermal enhancement in heat transfer with interaction of nanoparticles is quite necessary in engineering processes to maintain the energy level. Following to such motivations, current research model communicates the thermal control of nanoparticles with applications of active and passive constraints. The flow is generated due to bidirectional nonlinear stretching surface. The heat transfer is studied with radiated chemical reaction effects subject to convective condition. The nanoparticles concentration distribution is assumed through Brownian motion and thermophoresis effects. The three-dimensional surface with nonlinear flow velocity originates the flow. The impulsion of tiny particles from the surface has been presented with zero and non-zero mass fluxes. The convergent simulation via homotopic analytical algorithm is performed against the formulated set of system. The heat and mass transfer effects are carried out through numerically and graphically for the various parameter of interest.

Dispersive higher harmonic generation and enhancement in mechanical metamaterials

Higher harmonic generation (HHG) of elastic waves is beneficial for various applications, including engineering nondestructive testing, imaging, therapy, etc. Due to the ubiquity of dispersion in most materials and structures, however, it is usually difficult to simultaneously satisfy the conditions of phase-matching and non-zero energy flux that are needed for high-efficient HHG. Here we propose a mechanical metamaterial with coupled translation-rotation motion and explore its rich dispersion property for the enhancement of two HHGs in arbitrary regions of the acoustic band, i.e., the dispersive second harmonic generation (SHG) and the dispersive third harmonic generation (THG). Besides the phase-matching condition for enhancing SHG, we analytically obtain two phase-matching conditions for THG and reveal that THG is a complex process involving the interaction of multiple waves. By tailoring the dispersion of the metamaterial, the enhanced dispersive SHG and HHG are shown to be able to propagate at an arbitrarily slow group velocity, or even zero velocity, giving rise to localization. The analytical predictions are validated by numerical simulations and experimental tests. This work shows that by combining nonlinearity, mechanical metamaterials can be designed to have advanced capabilities of wave manipulation.

A unified framework for Navier–Stokes Cahn–Hilliard models with non-matching densities

Over the last decades, many diffuse-interface Navier–Stokes Cahn–Hilliard (NSCH) models with non-matching densities have appeared in the literature. These models claim to describe the same physical phenomena, yet they are distinct from one another. The overarching objective of this work is to bring all of these models together by laying down a unified framework of NSCH models with non-zero mass fluxes. Our development is based on three unifying principles: (1) there is only one system of balance laws based on continuum mixture theory that describes the physical model, (2) there is only one natural energy-dissipation law that leads to quasi-incompressible NSCH models, (3) variations between the models only appear in the constitutive choices. The framework presented in this work now completes the fundamental exploration of alternate non-matching density NSCH models that utilize a single momentum equation for the mixture velocity, but leaves open room for further sophistication in the energy functional and constitutive dependence.

NS three-form flux deformation for the critical non-Abelian vortex string

It has been shown that the non-Abelian solitonic vortex string supported in four-dimensional (4D) $\mathcal{N}=2$ supersymmetric QCD (SQCD) with the U(2) gauge group and ${N}_{f}=4$ quark flavors becomes a critical superstring. This string propagates in the ten-dimensional space formed by a product of the flat 4D space and an internal space given by a Calabi-Yau noncompact threefold, namely, the conifold. The spectrum of low-lying closed string states in the associated type-IIA string theory was found and interpreted as a spectrum of hadrons in 4D $\mathcal{N}=2$ SQCD. In particular, the lowest string state appears to be a massless BPS baryon associated with the deformation of the complex structure modulus $b$ of the conifold. In the previous work the deformation of the ten-dimensional background with nonzero Neveu-Schwarz 3-form flux was considered and interpreted as a switching on a particular choice of quark masses in 4D SQCD. This deformation was studied to leading order at small 3-form flux. In this paper we study the back reaction of the nonzero 3-form flux on the metric and the dilaton introducing the ansatz with several warp factors and solving the gravity equations of motion. We show that 3-form flux produces a potential for the conifold complex structure modulus $b$, which leads to the runaway vacuum. At the runaway vacuum warp factors disappear, while the conifold degenerates. In 4D SQCD we relate this to the flow to the U(1) gauge theory upon switching on quark masses and decoupling of two flavors.

Flux vacua with approximate flat directions

We present a novel method to obtain type IIB flux vacua with flat directions at tree level. We perform appropriate choices of flux quanta that induce relations between the flux superpotential and its derivatives. This method is implemented in toroidal and Calabi-Yau compactifications in the large complex structure limit. Explicit solutions are obtained and classified on the basis of duality equivalences. In the toroidal case we present solutions with N = 1 and N = 2 supersymmetry and arbitrarily weak coupling. In Calabi-Yaus we find novel perturbatively flat vacua, as well as solutions with non-zero flux superpotential and an axionic flat direction which represent a promising starting point for de Sitter constructions from non-zero F-terms in the complex structure sector. The higher order (perturbative and non-perturbative) effects that can lift these flat directions are discussed. We also outline applications in a wide variety of settings involving the classical Regge growth conjecture, inflation and quintessence, supersymmetry breaking and F-term de Sitter uplifting.

Optical Properties of Cylindrical Quantum Dots with Hyperbolic-Type Axial Potential under Applied Electric Field.

In this paper, we have researched the electronic and optical properties of cylindrical quantum dot structures by selecting four different hyperbolic-type potentials in the axial direction under an axially-applied electric field. We have considered a position-dependent effective mass model in which both the smooth variation of the effective mass in the axial direction adjusted to the way the confining potentials change and its abrupt change in the radial direction have been considered in solving the eigenvalue differential equation. The calculations of the eigenvalue equation have been implemented considering both the Dirichlet conditions (zero flux) and the open boundary conditions (non-zero flux) in the planes perpendicular to the direction of the applied electric field, which guarantees the validity of the results presented in this study for quasi-steady states with extremely high lifetimes. We have used the diagonalization method combined with the finite element method to find the eigenvalues and eigenfunction of the confined electron in the cylindrical quantum dots. The numerical strategies that have been used for the solution of the differential equations allowed us to overcome the multiple problems that the boundary conditions present in the region of intersection of the flat and cylindrical faces that form the boundary of the heterostructure. To calculate the linear and third-order nonlinear optical absorption coefficients and relative changes in the refractive index, a two-level approach in the density matrix expansion is used. Our results show that the electronic and, therefore, optical properties of the structures focused on can be adjusted to obtain a suitable response for specific studies or goals by changing structural parameters such as the widths and depths of the potentials in the axial direction, as well as the electric field intensity.

Local Volume Conservation in Concentrated Electrolytes Is Governing Charge Transport in Electric Fields.

While ion transport processes in concentrated electrolytes, e.g., based on ionic liquids (IL), are a subject of intense research, the role of conservation laws and reference frames is still a matter of debate. Employing electrophoretic NMR, we show that momentum conservation, a typical prerequisite in molecular dynamics (MD) simulations, is not governing ion transport. Involving density measurements to determine molar volumes of distinct ion species, we propose that conservation of local molar species volumes is the governing constraint for ion transport. The experimentally quantified net volume flux is found to be zero, implying a nonzero local momentum flux, as tested in pure ILs and IL-based electrolytes for a broad variety of concentrations and chemical compositions. This constraint is consistent with incompressibility, but not with a local application of momentum conservation. The constraint affects the calculation of transference numbers as well as comparisons of MD results to experimental findings.

Magnetic warping in topological insulators

We analyze the electronic structure of topological surface states in the family of magnetic topological insulators MnBi$_{2n}$Te$_{3n+1}$. We show that, at natural-cleavage surfaces, the Dirac cone warping changes its symmetry from hexagonal to trigonal at the magnetic ordering temperature. In particular, an energy splitting develops between the surface states of same band index but opposite surface momenta upon formation of the long-range magnetic order. As a consequence, measurements of such energy splittings constitute a simple protocol to detect the magnetic ordering via the surface electronic structure, alternative to the detection of the surface magnetic gap. Interestingly, while the latter signals a nonzero surface magnetic flux, the trigonal warping predicted here is, in addition, sensitive to the direction of the surface magnetic flux.

Realizability conditions for relativistic gases with a non-zero heat flux

This work introduces a limitation on the minimum value that can be assumed by the energy of a relativistic gas in the presence of a non-zero heat flux. Such a limitation arises from the non-negativity of the particle distribution function and is found by solving the Hamburger moment problem. The resulting limitation is seen to recover the Taub inequality in the case of a zero heat flux but is more strict if a non-zero heat flux is considered. These results imply that, in order for the distribution function to be non-negative, (i) the energy of a gas must be larger than a minimum threshold; (ii) the heat flux, on the other hand, has a maximum value determined by the energy and the pressure tensor; and (iii) there exists an upper limit for the adiabatic index Γ of the relativistic equation of state and that limit decreases in the presence of a heat flux and pressure anisotropy, asymptoting to a value Γ = 1. The latter point implies that the Synge equation of state is formally incompatible with a relativistic gas showing a heat flux, except in certain gas states.

Effect of stars on the dark matter spike around a black hole: A tale of two treatments

We revisit the role that gravitational scattering off stars plays in establishing the steady-state distribution of collisionless dark matter (DM) around a massive black hole (BH). This is a physically interesting problem that has potentially observable signatures, such as $\ensuremath{\gamma}$ rays from DM annihilation in a density spike. The system serves as a laboratory for comparing two different dynamical approaches, both of which have been widely used: a Fokker-Planck treatment and a two-component conduction fluid treatment. In our Fokker-Planck analysis we extend a previous analytic model to account for a nonzero flux of DM particles into the BH, as well as a cutoff in the distribution function near the BH due to relativistic effects or, further out, possible DM annihilation. In our two-fluid analysis, following an approximate analytic treatment, we recast the equations as a ``heated Bondi accretion'' problem and solve the equations numerically without approximation. While both the Fokker-Planck and two-fluid methods yield basically the same DM density and velocity dispersion profiles away from the boundaries in the spike interior, there are other differences, especially the determination of the DM accretion rate. We discuss limitations of the two treatments, including the assumption of an isotropic velocity dispersion.

Reduction of a Lithium-Ion Solid Electrolyte Model Under Potentiostatic Hold

Solid state electrolytes are becomingly increasingly attractive due to their improved safety, lower self-discharge, and higher power densities over the more commonly used liquid electrolyte [1,2]. Solid electrolytes are a relatively new material in the field of lithium-ion batteries, therefore there is considerably less mathematical investigation and analysis into these materials, compared with their liquid counterparts.We review a model for a solid electrolyte from thermodynamics principles [3]. We non-dimensionalise and scale the model to identify small parameters, where we identify a scaling that widens the boundary layer in the electrolyte compared to previous literature. We consider an Li2O solid electrolyte which is attached on each end by blocks of solid lithium. We fix a potential difference across the electrolyte and consider different charge flux conditions. We present asymptotic analysis and numerical solutions for both the zero-charge flux equilibrium and for the non-zero charge flux equilibrium problems. We consider numerical simulations of the full-time dependent problem and show that there are two important time scales in the problem, an early transient timescale where the boundary layers first approach their equilibrium followed by a longer timescale where we observe the transience in the bulk of the solution.We briefly talk about the application of a solid electrolyte to cylindrical nanowire electrodes. Recent advances in electrode chemistry have allowed for cylindrical nanowire geometries exhibiting low-capacity fade. However, the performance of these electrodes is very sensitive to experimental conditions and early charge behavior and thus it is important to understand the lithium transport process in these systems. Following from this work, we will combine the solid electrolyte model with a nanowire electrode in order to investigate these processes.

String integrability of the ABJM defect

ABJM theory in the presence of a half-BPS domain wall is dual to the D2-D4 probe brane system with nonzero worldvolume flux. The ABJM domain wall was recently shown to be integrable to lowest order in perturbation theory and bond dimension. In the present paper we show that the string theory dual of this system is integrable, namely that the string boundary conditions on the probe D4-brane preserve the integrability of the Green-Schwarz sigma model. Our result suggests that the ABJM domain wall is integrable to all loop orders and for any value of the bond dimension.

Chiral anomaly in noncentrosymmetric systems induced by spin-orbit coupling

The chiral anomaly may be realized in condensed matter systems with pairs of Weyl points. Here we show that the chiral anomaly can be realized in diverse noncentrosymmetric systems even without Weyl point pairs when spin-orbit coupling induces nonzero Berry curvature flux through Fermi surfaces. This motivates the condensed matter chiral anomaly to be interpreted as a Fermi surface property rather than a Weyl point property. The spin-orbit-coupling-induced anomaly reproduces the well-known charge transport properties of the chiral anomaly such as the negative longitudinal magnetoresistance and the planar Hall effect in Weyl semimetals. Since it is of spin-orbit coupling origin, it also affects the spin transport and gives rise to anomaly-induced longitudinal spin currents and the magnetic spin Hall effect, which are absent in conventional Weyl semimetals.

Thermal and flow characteristics of nonequilibrium monatomic, diatomic, and polyatomic gases in cylindrical Couette flow based on second-order non-Navier–Fourier constitutive model

The thermal and flow characteristics of nonequilibrium monatomic, diatomic, and polyatomic gases in cylindrical Couette flow are investigated using first- and second-order Boltzmann–Curtiss-based constitutive models. The mixed modal discontinuous Galerkin scheme is used for solving the conservation laws in conjunction with the Maxwell velocity-slip and Smoluchowski temperature-jump boundary conditions. Also derived are new analytic solutions for compressible cylindrical Couette gas flow including the temperature profile, and they are used to verify the numerical scheme. Further, the second-order non-Navier–Fourier constitutive relations are derived for the cylindrical coordinates. Various abnormal behaviour is found in the second-order constitutive model, such as non-zero normal stress and excess normal stress, non-zero tangential heat flux, and flattened pressure and density profiles. The physical mechanisms behind this abnormal behaviour are found to be similar to the Knudsen layer in planar Couette gas flow, and the curvature of the cylindrical geometry does not affect the fundamental second-order physics. Moreover, two new abnormal mechanisms are found in diatomic and polyatomic gases: (i) the subtle interplay of excess normal stress (and bulk viscosity) with the nonlinear coupled constitutive relation, and (ii) the combined role of the bulk viscosity ratio and the specific heat ratio.

Effect of inflow boundary conditions on phonon transport in suspended graphene

Phonon hydrodynamics originated from the Guyer-Krumhansl heat conduction model. However, there is still no reasonable explanation for the high thermal conductivity of suspended graphene by using the Guyer-Krumhansl heat conduction model. In this article, based on the Guyer-Krumhansl equation we find that the inflow boundary condition specifying the non-zero heat flux at the inlet of the rectangular two-dimensional nonmetallic materials can induce phonon transport. Results show that the combined action of the phonon flow caused by the inflow boundary condition and the Poiseuille phonon flow induced by the temperature difference can make the effective thermal conductivity of two-dimensional nonmetallic materials (such as suspended graphene) much higher than the bulk thermal conductivity.