arXiv subjects

Jian-Sheng Wang

Publications and source records attributed to Jian-Sheng Wang.

At least 37 records · Page 2Linked to original sources

arXiv2021

Phonon Hall effect with first-principles calculations

Phonon Hall effect (PHE) has attracted a lot of attention in recent years with many theoretical and experimental explorations published. While experiments work on complicated materials, theoretical studies are still hovering around the phenomenon-based models. Moreover, previous microscopic theory was found unable to explain large thermal Hall conductivity obtained by experiments in strontium titanate (STO). Therefore, as a first attempt to bridge this gap, we implement first-principles calculations to explore the PHE in real materials. Our work provides a new benchmark of the PHE in sodium chloride (NaCl) under a large external magnetic field. Moreover, we demonstrate our results in barium titanate (BTO), and discuss the results in STO in detail about their deviation from experiments. As a possible future direction, we further propose that the inner electronic Berry curvature plays an important role in the PHE in STO.

cond-mat.mtrl-sci↗

arXiv2021

Erratum: Caroli formula in near-field heat transfer between parallel graphene sheets [PHYSICAL REVIEW B 96, 155437 (2017)]

This erratum aims to correct 1) the wrong expressions, 2) some typographical errors, 3) some erroneous points made in discussion of the disparity of heat flux ratios between our full RPA model and the local conductivity model, and 4) the lower bound of the characteristic distance scale comparable to the graphene thermal length, appearing in the original paper, which might confuse or mislead readers. Collectively, they don't change the key physics we sought to present.

cond-mat.mes-hall↗

arXiv2021

Dynamical Polarizability of Graphene with Spatial Dispersion

We perform a detailed analysis of electronic polarizability of graphene with different theoretical approaches. From Kubo's linear response formalism, we give a general expression of frequency and wave-vector dependent polarizability within the random phase approximation. Four theoretical approaches have been applied to the single-layer graphene and their differences are on the band-overlap of wavefunctions. By comparing with the \textit{ab initio} calculation, we discuss the validity of methods used in literature. Our results show that the tight-binding method is as good as the time-demanding \textit{ab initio} approach in calculating the polarizability of graphene. Moreover, due to the special Dirac-cone band structure of graphene, the Dirac model reproduces results of the tight-binding method for energy smaller than \SI{3}{\electronvolt}. For doped graphene, the intra-band transitions dominate at low energies and can be described by the Lindhard formula for two-dimensional electron gases. At zero temperature and long-wavelength limit, with the relaxation time approximation, all theoretical methods reduce to a long-wave analytical formula and the intra-band contributions agree to the Drude polarizability of graphene. Effects of electrical doping and temperature are also discussed. This work may provide a solid reference for researches and applications of the screening effect of graphene.

cond-mat.mes-hall↗

arXiv2021

Caroli formula in near-field heat transfer between parallel graphene sheets

In this work we conduct a close-up investigation into the nature of near-field heat transfer (NFHT) of two graphene sheets in parallel-plate geometry. We develop a fully microscopic and quantum approach using nonequilibrium Green's function method. A Caroli formula for heat flux is proposed and numerically verified. We show our near-field-to-black-body heat flux ratios generally exhibit $1/d^α$ dependence, with an effective exponent $α\approx 2.2$, at long distances exceeding 100 nm and up to one micron; in the opposite $d\rightarrow 0$ limit, the values converge to a range within an order of magnitude. We justify this feature by noting it is owing to the breakdown of local conductivity theory, which predicts a $1/d$ dependence. Furthermore, from the numerical result, we find in addition to thermal wavelength, $λ_{th}$, a shorter distance scale $\sim$ 10 - 100 nm, comparable to the graphene thermal length ($\hbar v_{F}/k_{B} T$) or Fermi wavelength ($k_{F}^{-1}$), marks the transition point between the short- and long-distance transfer behaviors; within that point, relatively large variation of heat flux in response to doping level becomes a typical characteristic. The emergence of such large variation is tied to relative NFHT contributions from the intra- and inter-band transitions. Beyond that point, scaling of thermal flux $\propto 1/d^α$ can be generally observed.

cond-mat.mes-hall↗

arXiv2021

First-principles method to study near-field radiative heat transfer

We present a general and convenient first principle method to study near-field radiative heat transfer. We show that the Landauer-like expression of heat flux can be expressed in terms of a frequency and wave-vector dependent macroscopic dielectric function which can be obtained from the linear response density functional theory. A random phase approximation is used to calculate the response function. We computed the heat transfer in three systems -- graphene, molybdenum disulfide (MoS$_2$), and hexagonal boron nitride (h-BN). Our results show that the near-field heat flux exceeds the blackbody limit up to four orders of magnitude. With the increase of the distances between two parallel sheets, a $1/d^2$ dependence of heat flux is shown, consistent with Coulomb's law. The heat transfer capacity is sensitive to the dielectric properties of materials. Influences from chemical potential and temperature are also discussed. Our method can be applied to a wide range of materials including systems with inhomogeneities.

physics.comp-ph↗

arXiv2021

Angular momentum radiation from current-carrying molecular junctions

We consider the radiation of angular momentum (AM) from current-carrying molecular junctions. Using the nonequilibrium Green's function method, we derive a convenient formula for the AM radiation and apply it to a prototypical benzene molecule junction. We discuss the selection rules for inelastic transitions between the molecular angular momentum eigenstates due to a 6-fold rotational symmetry. Our study provides important insights into the generation of light with AM from DC-biased molecular junctions.

cond-mat.mes-hall↗

arXiv2020

Ab initio dipolar electron-phonon interactions in two-dimensional materials

We develop an ab initio formalism for dipolar electron-phonon interactions (EPI) in two-dimensional (2D) materials. Unlike purely longitudinal Fröhlich model, we show that the out-of-plane dipoles also contribute to the long-wavelength non-analytical behavior of EPI. And the 2D dipolar EPI plays an important role not only in the typical polar material MoS$_2$, but also in graphane and fluorinated graphene. By incorporating this formalism into Wannier-Fourier interpolation, we enable accurate EPI calculations for 2D materials and subsequent intrinsic carrier mobility prediction. The results show that Fröhlich model is inadequate for 2D materials and correct long-wavelength interaction must be included for the reliable prediction.

cond-mat.mtrl-sci↗

arXiv2020

Current-induced phonon Hall effect

Since the first experimental observation of the phonon Hall effect (PHE) in 2005, its physical origin and theoretical explanation have been extensively investigated. While spin-orbit interactions are believed to play important roles under external magnetic fields, nonmagnetic effects are also possible. Here, we propose a mechanism of PHE which is induced by electric current in a nonequilibrium system through electron-phonon interactions. The influence of the drift electrons to the phonon degrees of freedom, as a correction to the Born-Oppenheimer approximation, is represented by an antisymmetric matrix which has the same form as in a typical phonon Hall model. We demonstrate the idea with a graphene-like hexagonal lattice having a finite phonon Hall conductivity under a driven electric current.

cond-mat.mtrl-sci↗

arXiv2020

Phonon soft modes and para- to ferroelectric phase transitions

In this special issue of Physica A in memory of Professor Dietrich Stauffer, I first recall my impression on him while being his postdoc for a year at HLRZ. In the following scientific part, I discuss the theory of soft phonons with quartic nonlinear interactions. This is applied to the cubic crystal BaTiO$_3$ for phase transition from para- to ferroelectric phase.

cond-mat.stat-mech↗

arXiv2020

Highly Anisotropic Electronic and Mechanical Properties of Monolayer and Bilayer As2S3

Anisotropic materials, with orientation-dependent properties, have attracted more and more attention due to their compelling tunable and flexible performance in electronic and optomechanical devices. So far, two-dimensional (2D) black phosphorus shows the largest known anisotropic behavior, which is highly desired for synaptic and neuromorphic devices, multifunctional directional memories, and even polarization-sensitive photodetector, whereas it is unstable at ambient conditions. Recently, 2D few-layered As2S3 with superior chemical stability was successfully exfoliated in experiments. However, the electronic and mechanical properties of monolayer and bilayer As2S3 is still lacking. Here, we report the large anisotropic electronic and mechanical properties of As2S3 systems through first-principles calculations and general angle-dependent Hooke's law. Monolayer and bilayer As2S3 exhibit anisotropic factors of Young's modulus of 3.15 and 3.32, respectively, which are larger than the black phosphorous with experimentally confirmed and an anisotropic factor of 2. This study provides an effective route to flexible orientation-dependent nanoelectronics, nanomechanics, and offers implications in promoting related experimental investigations.

cond-mat.mtrl-sci↗

arXiv2020

Geometric effect on near-field heat transfer analysis using efficient graphene and nanotube models

Following the recent research enthusiasm on the effect of geometry on near-field heat transfer (NFHT) enhancement, we present an analysis based on simplified yet highly efficient graphene and nanotube models. Two geometries are considered: that of two parallel infinite "graphene" surfaces and that of a one-dimensional infinite "nanotube" line in parallel with an infinite surface. Due to its symmetry, the former is in principal simpler to analyze and even so, earlier works suggested that the application of a full model in this problem still demands heavy computations. Among other findings, our simplified computation - having successfully replicated the results of relevant earlier works - suggests a sharper NFHT enhancement dependence on distance for the line-surface system, namely $J\sim d^{-5.1}$ as compared to $J\sim d^{-2.2}$ for the parallel surface. Such comparisons together with applications of our efficient approach would be the important first steps in the attempt to find a general rule describing geometric dependence of NFHT.

physics.comp-ph↗

arXiv2020

Far-field heat and angular momentum radiation of the Haldane model

We investigate the radiation of energy and angular momentum from 2D topological systems with broken inversion symmetry and time reversal symmetry. A general theory of far-field radiation is developed using the linear response of 2D materials to the fluctuational electromagnetic field. Applying the theory to the Haldane model, we verify that the heat radiation complies with Planck's law only at low temperature and deviates from it as temperature becomes high. Angular momentum radiation is possible for this system and exhibits saturation as temperature increases. Parameters crucial to the radiation are investigated and optimized. This research enlightens the possibility of transposing the quantum information to the angular momentum degree of freedom.

cond-mat.mes-hall↗

arXiv2020

Thermoelectric Penta-Silicene with a High Room-Temperature Figure of Merit

Silicon is one of the most frequently used chemical elements of the periodic table in nanotechnology. Two-dimensional (2D) silicene, a silicon analog of graphene, has been readily obtained to make field-effect transistors since 2015. Recently, as new members of the silicene family, penta-silicene and its nanoribbon have been experimentally grown on Ag(110) surface with exotic electronic properties. However, the thermoelectric performance of penta-silicene has not been so far studied that would hinder its potential applications of electric generation from waste heat and solid-state Peltier coolers. Based on the Boltzmann transport theory and ab initio calculations, we find that penta-silicene shows a remarkable room temperature figure of merit ZT of 3.4 and 3.0 at the reachable hole and electron concentrations, respectively. We attribute this high ZT to the superior pudding-mold electronic band structure and ultralow lattice thermal conductivity. The discovery provides new insight into the transport property of pentagonal nanostructures and highlights the potential applications of thermoelectric materials at room temperature.

cond-mat.mtrl-sci↗

arXiv2020

Nonadiabatic particle and energy pump at strong system-reservoir coupling

We study the dynamics of electron and energy currents in a nonadiabatic pump. The pump is a quantum dot nanojunction with time-varying gate potential and tunnel couplings to the leads. The leads are unbiased and maintained at the same temperature and chemical potential. We find that synchronized variations of the gate and tunnel couplings can pump electrons and energy from the left to the right lead. Inspired by quantum heat engines, we devise a four-stroke operating protocol that can optimally pump energy and hence, we investigate energy transfer and the coefficient of performance of the device. We compare our device to a two-stroke pump and find that the latter's lower performance is due to the bi-directional flow of energy currents resulting in low net energy currents. The performance of our four-stroke pump can be improved, up to a point, by increasing the net energy carried by the pumped electrons through energy charging via the gate potential. This is achieved by increasing the durations of energy charging and discharging strokes in the pump's protocol. However, despite the large energy output for long charging and discharging strokes, the energy required to maintain the strokes become large too resulting in a stagnant pump performance. Our pump operates effectively only in the strong lead coupling regime and becomes a dud in the weak coupling regime due to the net output energy flowing in the reverse direction. We use nonequilibirum Green's functions techniques to calculate the currents and capture the effects of strong lead-channel coupling exactly while simultaneously incorporating three time-varying parameters. Results from our work could aid in the design of high-performance quantum pumps.

cond-mat.mes-hall↗

arXiv2019

Insight into Two-Dimensional Borophene: Five-Center Bond and Phonon-Mediated Superconductivity

We report a previously unknown monolayer borophene allotrope and we call it super-B with a flat structure based on the ab initio calculations. It has good thermal, dynamical, and mechanical stability compared with many other typical borophenes. We find that super-B has a fascinating chemical bond environment consisting of standard sp, sp2 hybridizations, and delocalized five-center three-electron $π$ bond, called $π$(5c-3e). This particular electronic structure plays a pivotal role in stabilizing the super-B chemically. By extra doping, super-B can be transformed into a Dirac material from pristine metal. Like graphene, it can also sustain tensile strain smaller than 24%, indicating superior flexibility. Moreover, due to the small atomic mass and large density of states at the Fermi level, super-B has the highest critical temperature Tc of 25.3 K in single-element superconductors at ambient conditions. We attribute this high Tc of super-B to the giant anharmonicity of two linear acoustic phonon branches and an unusually low optic phonon mode. These predictions provide new insight into the chemical nature of low dimensional boron nanostructures and highlight the potential applications of designing flexible devices and high Tc superconductors.

cond-mat.mtrl-sci↗

arXiv2019

Ultralow lattice thermal conductivity of monolayer penta-silicene and penta-germanene

We study the lattice thermal conductivity of two-dimensional (2D) pentagonal systems, such as penta-silicene and penta-germanene. Penta-silicene has been recently reported, while the stable penta-germanene, belonging to the same group IV element, is revealed firstly by our ab initio calculations. We find that both penta-silicene and penta-germanene at room temperature have ultralow lattice thermal conductivities $κ$ of 1.29 W/mK and 0.30 W/mK respectively. To the best of our knowledge, penta-germanene may have the lowest $κ$ in 2D crystal materials. We attribute ultralow $κ$ to the weak phonon harmonic interaction and strong anharmonic scattering. A small phonon group velocity, a small Debye frequency, a large Gruneisen parameter, and a large number of modes available for phonon-phonon interplay together lead to the ultralow $κ$ of penta-silicene and penta-germanene. These discoveries provide new insight into the manipulation of ultralow $κ$ in 2D materials and highlight the potential applications of designing silicon and germanium based high thermoelectric materials.

cond-mat.mtrl-sci↗

arXiv2019

Three-terminal interface as a thermoelectric generator beyond Seebeck effect

We investigate thermoelectric transport through interfaces with inelastic scatterings by developing a quantum theory, which has been extensively validated by existing theories. We find that under temperature bias, while a two-terminal conductor-insulator interface behaves only as a thermal resistor, a three-terminal conductor-insulator-conductor interface can function as an electricity generator caused by phonon-mediated electron scatterings with heat-charge current separation. Unlike conventional thermoelectrics which is a property of a bulk caused by the Seebeck effect, this thermoelectric behavior is a property of an interface driven by electron-phonon scatterings.

cond-mat.mes-hall↗

arXiv2019

Current-Induced Heat Transfer in Double-Layer Graphene

Using the fluctuational electrodynamics and nonequilibrium Green's function methods, we demonstrate the existence of a current-induced heat transfer in double-layer graphene even when the temperatures of the two sheets are the same. The heat flux is quadratically dependent on the current. When temperatures are different, external voltage bias can reverse the direction of heat flow. The drift effect can exist in both macroscopic and nanosized double-layer graphene and extend to any other 2D electron systems. These results pave the way for a different approach to the thermal management through radiation in nonequilibrium systems.

cond-mat.mes-hall↗