arXiv subjects

Jian-Sheng Wang

Publications and source records attributed to Jian-Sheng Wang.

At least 19 recordsLinked to original sources

arXiv2026

Analytical Floquet Quantum Statistics from Nonequilibrium Green's Functions

We derive an analytical expression for the steady-state quantum statistics of periodically driven quantum systems coupled to a bath using the nonequilibrium Green's function (NEGF) formalism. By embedding Floquet theory into NEGF, we obtain closed expressions for the retarded, advanced, and lesser Green's functions in the Floquet representation, yielding the Floquet Fermi distribution in which the steady-state occupation is expressed as a weighted sum of Fermi functions shifted by integer multiples of the driving frequency. The weights are determined solely by the Fourier components of the micromotion operator, providing a transparent interpretation of Floquet sideband occupations. Our analysis extends beyond the diagonal commuting Hamiltonians treated in earlier work, and further shows that the robust Floquet distribution remains valid for a broad class of weakly coupled bath spectral functions beyond the ideal featureless-bath approximation. Finally, we establish a Floquet version of the Landauer formula for the DC part of the current, in which the equilibrium Fermi functions are replaced by their Floquet-modified counterparts. Together, these results provide a coherent description of Floquet quantum statistics and transport in periodically driven open quantum systems.

cond-mat.stat-mech↗

arXiv2026

The infinitesimal environmental dust as a photonic bath at infinity

In far-field thermal radiation, electromagnetic waves emitted by an object propagate to infinity, requiring the far region to be modeled as an effective thermal bath. This bath was proposed as infinitesimal environmental "dust", but explicit calculations with such distributed dust involve integrals over infinite space that are difficult to evaluate. In this work, we map this environmental dust to a photonic bath at infinity within the nonequilibrium photonic Green's function formalism. By explicitly evaluating the spatial integral over the dust, we show that its contribution reduces to a simple local self-energy, for which we derive analytical expressions for both three-dimensional objects and planar systems. We further demonstrate that the bath behaves as a black body and clarify its role in far-field thermal radiation. An alternative derivation based on the surface Green's function framework is also provided, demonstrating the theoretical consistency of the results without invoking the dust model. The photonic bath at infinity provides a convenient framework for both analytical and numerical calculations in far-field thermal radiation.

physics.optics↗

arXiv2026

Reply to "Comment on "Electric conductivity in graphene: Kubo model versus a nonlocal quantum field theory model"" (ArXiv:2506.10792v2)

In the Comment by Bordag et al. [Phys. Rev. B 113, 207401 (2026) and ArXiv:2506.10792], concerns are raised regarding the validity of the results presented in [Phys. Rev. B 111, 115428 (2025)], where the theoretical descriptions of the electric conductivity of graphene obtained from the Kubo formula and from quantum field theory via the polarization tensor are compared. In this Reply, we show that these concerns arise from misinterpretations of Phys. Rev. B 111, 115428 (2025), in which the results are either inaccurately represented or applied outside the domain of validity of the model. We address the comments concerning the derivation of the Luttinger formula for the electric conductivity from the Kubo formula and clarify why the results of Phys. Rev. B 111, 115428 (2025) cannot be arbitrarily extended to make claims on the gauge invariance. We further demonstrate that our findings are fully consistent with the established and widely accepted literature cited in the Comment. We confirm that the model for electric conductivity discussed in Phys. Rev. B 111, 115428 (2025) correctly predicts a vanishing electric current in the absence of an external electric field, as physically required, and in contrast with the model advocated by the Authors of the Comment. We also show that the electric permittivity does not exhibit a double pole in $ω$, contrary to the claim made in the Comment. Finally, we emphasize that the inclusion of losses is a standard and well-established approach in the study of transport properties of materials, including graphene, and we take the opportunity to correct a few minor typographical errors in Phys. Rev. B 111, 115428 (2025). We show and maintain that all results derived in Phys. Rev. B 111, 115428 (2025) are fully valid and correct.

cond-mat.mes-hall↗

arXiv2026

Analytical formulas for far-field radiated energy and angular momentum of metallic thin films

We investigate far-field radiation of energy, linear momentum, and angular momentum from two-dimensional electron systems, focusing on metallic thin films described by the Drude conductivity. Using the Keldysh formalism within the non-equilibrium Green's function framework, we derive analytical expressions for radiative power, force, and torque. To enable angular momentum radiation, an out-of-plane magnetic field is applied to break reciprocity, resulting in gyrotropic terms in the permittivity tensor. By approximating the emitter as a thin film, the photon Green's functions can be solved analytically. Expressions for the Poynting vector and Maxwell's stress tensor can subsequently be extracted from the lesser Green's function, which governs the field correlations. The final radiation formulas can be expressed in terms of Fresnel coefficients, revealing an insightful connection to energy conservation via Kirchhoff's law. Using the Wigner transform, the analytical expression for the radiative torque can also be related to the generalized Fresnel coefficients. Numerical calculations based on the optical conductivity of bismuth are presented to corroborate the analytical results. These results provide a unified framework for energy, momentum, and angular momentum radiation in gyrotropic thin films.

cond-mat.mes-hall↗

arXiv2025

Clarification of Floquet--Enhanced Thermal Emission Through the Nonequilibrium Green's Function Formalism

Floquet engineering offers a powerful route to enhance emission in time-modulated media. Here, we investigate the influence of time-modulated permittivity in silicon carbide on its intensity spectrum. We consider both the nonequilibrium Green's function approach and the macroscopic quantum electrodynamics approach, and establish their formal compatibility by deriving the Lippmann-Schwinger equation in both cases. To analyze spectral features, we propose several methods for decomposing the electric field into positive- and negative-frequency components, along with the criteria required for physical consistency. Our analytical and numerical results show that, when defined appropriately, the intensity spectrum avoids divergence, though the resulting enhancement remains modest. These findings provide a unified theoretical foundation for modeling time-dependent media, and reinforce the utility of Floquet engineering as a versatile platform for tailoring emission dynamics.

cond-mat.mes-hall↗

arXiv2025

Enhancing far-field thermal radiation by Floquet engineering

Time modulation introduces a dynamic degree of freedom for tailoring thermal radiation beyond the limits of static materials. Here we investigate far-field thermal radiation from a periodically time-modulated SiC film under the Floquet nonequilibrium Green's function framework. We show that time modulation enables radiative energy transfer into the far field that surpasses the limit imposed by the equilibrium thermal fluctuations. This enhancement originates from the modulation-induced coupling between evanescent surface phonon polaritons and propagating modes, effectively bridging the energy and momentum mismatch through frequency conversion. Notably, even at zero temperature, the film emits a finite radiative heat flux due to nonequilibrium photon occupation generated by the modulation. The radiative output grows with increasing modulation strength, highlighting the role of external work in driving far-field emission. These results establish time modulation as an effective mechanism for bridging near-field and far-field regimes, opening new pathways for active thermal radiation control.

cond-mat.mes-hall↗

arXiv2025

Asymmetry-induced radiative heat transfer in Floquet systems

Time modulation opens new avenues for light, heat control, and energy harvesting, yet the impact of nonequilibrium dynamics of microscopic particles remains largely unexplored. We develop a microscopic theory to describe radiative heat transfer in such Floquet systems. Significant heat transfer occurs due to differences in electronic properties between parallel metal plates, despite identical driving protocols and temperatures. This arises from a unique exponential-staircase distribution of radiative photons, induced by nonequilibrium electronic fluctuations, and can be tuned via both microscopic properties and driving parameters. Our work highlights the importance of nonequilibrium microscopic details, unlocking new opportunities for active cooling, thermophotovoltaics, thermal imaging and manipulation, and carrier dynamics probing.

cond-mat.mes-hall↗

arXiv2025

Anomalous Scaling Laws of Dispersion Interactions in Anisotropic Nanostructures

The van der Waals (vdW) dispersion interaction between two finite neutral objects typically follows the standard nonretarded $d^{-6}$ law. Here, we reveal an anomalous $d^{-10}$ scaling law between nanostructures with strong geometric or electric anisotropy, driven intrinsically by symmetry-restricted plasmon interactions. At finite anisotropy ratios, a scaling crossover from $d^{-10}$ to $d^{-6}$ occurs due to plasmon mode competition, marked by a finite critical separation. Furthermore, we demonstrate tunability of interlayer vdW forces in two-dimensional materials with strong in-plane electronic anisotropy. By pushing the conventional lower bound of vdW scaling laws, these findings open new opportunities for tailoring nanoscale forces, with potential applications in low-stiction nanomechanical devices, vdW superstructure assembly, metamaterials, and molecular simulations.

cond-mat.mes-hall↗

arXiv2025

Electric conductivity in graphene: Kubo model versus a nonlocal quantum field theory model

We compare three models of graphene electric conductivity: a non-local Kubo model, a local model derived by Falkovsky, and finally, a non-local quantum field theory (QFT) polarization-based model. These models are supposed to provide consistent results since they are derived from the same Hamiltonian. While we confirm that the local model is a proper $\textbf{q}\to\textbf{0}$ limit of both the non-local Kubo and the non-local QFT model (once losses are added to this last model), we find hard inconsistencies in the non-local QFT model as derived and currently used in literature. In particular, in the genuine non-local region ($\textbf{q}\neq\textbf{0}$), the available QFT model shows an intrinsic non-physical plasma-like behavior for the interband transversal electric conductivity at low frequencies (even after introducing the unavoidable losses). The Kubo model, instead, shows the expected behavior, i.e., an almost constant electric conductivity as a function of frequency $ω$ with a gap for frequencies $\hbarω<\sqrt{(\hbar v_F q)^{2}+4m^{2}}$. We show that the Kubo and QFT models can be expressed using an identical Polarization operator $Π_{μν}(ω,\textbf{q})$, but they employ different expressions for the electric conductivity $σ_{μν}(ω,\textbf{q})$. In particular, the Kubo model uses a standard regularized expression, a direct consequence of Ohm's Law and causality, as we rigorously re-derive. We show that, once the standard regularized expression for $σ_{μν}(ω,\textbf{q})$ is used in the QFT model, and losses are included, the Kubo and QFT model coincide, and all its anomalies naturally disappear. Our findings show the necessity to appropriately define and regularize the electric conductivity to connect it with the available QFT model.

cond-mat.mes-hall↗

arXiv2025

Beyond the Drude model: surface and non-local effects in near-field radiative heat transfer and the Casimir puzzle

We study the charge and current response functions $P$ and $Π$ in a semi-infinite metal block using the electron surface Green's functions. The surface electrons behave similarly to a two-dimensional Fermi gas but are strongly damped due to coupling to the bulk. This substantially reduces the region of validity of the Drude model for $P$, which requires the frequency $ω\gg \max( v_F q, 1/τ)$, here $v_F$ is the Fermi velocity, $q$ is the wavevector and $τ$ is an effective relaxation time. As a consequence, for typical metal in near-field heat transfer, the Coulomb interaction goes as $1/d^4$ with the distance of the vacuum gap instead of the well-known $1/d^2$ of Drude model result. The current response $Π$ is shown to be highly anisotropic. The Drude model describes well the transverse directions parallel to the surface but is very different in the normal direction up to about 100 lattice sites away from the surface. These ideas and the residue diamagnetic effect of a nonzero $Π$ on the surface at zero frequency still cannot resolve the Casimir puzzle.

cond-mat.mes-hall↗

arXiv2024

Super-Planckian radiative heat transfer between coplanar two-dimensional metals

Using the nonequilibrium Green's function formalism, we propose a general microscopic framework to investigate the radiative heat transfer (RHT) between coplanar objects with a square lattice. We employ the obtained formulas to two-dimensional (2D) metal configurations with a tight-binding model and the Drude model. Our results reveal that the RHT between coplanar 2D metals is significantly larger than black-body radiation in both the near and far fields, leading to a global super-Planckian RHT. As the separation distance increases, the heat flux density exhibits a rapid decrease in the near field, followed by a slower decrease and eventual $1/d$ dependence in the far field, while maintaining a much higher magnitude than black-body radiation. Evanescent waves dominate the heat transfer in the near field, while propagating waves dominate the far field. Surprisingly, the propagating heat flux remains almost constant over a wide range of distances, resulting in a super-Planckian behavior in the far field. The dispersion relation of the spectrum function reveals distinct contributions from propagating and evanescent waves, with possible origins from surface plasmon resonance. These findings provide insights into the unique characteristics of RHT between coplanar 2D metals and highlight the potential for achieving enhanced heat transfer beyond the black-body limit. Our method is applicable to any coplanar objects with square lattices, paves the way for expanded investigations into various lattice geometries.

cond-mat.mes-hall↗

arXiv2024

Photon mediated energy, linear and angular momentum transport in fullerene and graphene systems beyond local equilibrium

Based on a tight-binding model for the electron system, we investigate the transfer of energy, momentum, and angular momentum mediated by electromagnetic fields among buckminsterfullerene (C$_{60}$) and graphene nano-strips. Our nonequilibrium Green's function approach enables calculations away from local thermal equilibrium where the fluctuation-dissipation theorem breaks down. For example, the forces between C$_{60}$ and current-carrying nano-strips are predicted. It is found that the presence of current usually enhances the van der Waals attractive forces. For two current-carrying graphene strips rotated at some angle, the fluctuational force and torque are much stronger at the nanoscale compared to that of the static Biot-Savart law.

cond-mat.mes-hall↗

arXiv2024

Modulating near-field thermal transfer through temporal drivings: a quantum many-body theory

The traditional approach to studying near-field thermal transfer is based on fluctuational electrodynamics. However, this approach may not be suitable for nonequilibrium states due to dynamic drivings. In our work, we introduce a theoretical framework to describe the phenomenon of near-field heat transfer between two objects when subjected to periodic time modulations. We utilize the machinery of nonequilibrium Green's function to derive general expressions for the DC energy current in Floquet space. Furthermore, we also obtain the energy current under the condition of small driving amplitude. The external drivings create a nonequilibrium state, which gives rise to various effects such as heat-transfer enhancement, heat-transfer suppression, and cooling. To illustrate these phenomena, we conduct numerical calculations on a system of Coulomb-coupled quantum dots, and specifically investigate the scenario of periodically driving electronic reservoir. In our calculations, we employ the $G_0W_0$ approximation, which does not require self-consistent iteration and is suitable for weak Coulomb interaction. Our theoretical formalism can be applied to study near-field energy transfer between two metallic plates under periodic time modulations.

cond-mat.mes-hall↗

arXiv2022

Transport in electron-photon systems

We review the description and modeling of transport phenomena among the electron systems coupled via scalar or vector photons. It consists of three parts. The first part is about scalar photons, i.e., Coulomb interactions. The second part is with transverse photons described by vector potentials. The third part is on $ϕ=0$ or temporal gauge, which is a full theory of the electrodynamics. We use the nonequilibrium Green's function (NEGF) formalism as a basic tool to study steady-state transport. Although with local equilibrium it is equivalent to the fluctuational electrodynamics (FE), the advantage of NEGF is that it can go beyond FE due to its generality. We have given a few examples in the review, such as transfer of heat between graphene sheets driven by potential bias, emission of light by a double quantum dot, and emission of energy, momentum, and angular momentum from a graphene nanoribbon. All of these calculations are based on a generalization of the Meir-Wingreen formula commonly used in electronic transport in mesoscopic systems, with materials properties represented by photon self-energy, coupled with the Keldysh equation and the solution to the Dyson equation.

cond-mat.mes-hall↗

arXiv2022

Microscopic theory of photon-induced energy, momentum, and angular momentum transport in the nonequilibrium regime

We set up a general microscopic theory for the transfer of energy, momentum, and angular momentum mediated by photons. Using the nonequilibrium Green's function method, we propose a unified Meir-Wingreen formalism for the energy emitted, force experienced, and torque experienced by the objects due to the fluctuating electromagnetic field. Our theory does not require the local thermal equilibrium that is the central assumption of the conventional theory of fluctuational electrodynamics (FE). The obtained formulas are valid for arbitrary objects as well as the environment without the requirement of reciprocity. To show the capability of our microscopic theory, we apply the general formulas to transport problems of graphene edges in both equilibrium and nonequilibrium situations. We show the local equilibrium energy radiation of graphene obeys the well-known $T^4$ law with a converged theoretical emissivity of 2.058$\%$. In the ballistic nonequilibrium situation driven by chemical potential biases, we observe nonzero results for force and torque from the graphene edges, which go beyond the predictive ability of the FE theory. Our method is general and efficient for large systems, which paves the way for studying more complex transport phenomena in the nonequilibrium regime.

cond-mat.mes-hall↗

arXiv2022

Anharmonic quantum thermal transport across a van der Waals interface

We investigate the anharmonic phonon scattering across a weakly interacting interface by developing a quantum mechanics-based theory. We find that the contribution from anharmonic three-phonon scatterings to interfacial thermal conductance can be cast into Landauer formula with transmission function being temperature-dependent. Surprisingly, in the weak coupling limit, the transmission due to anharmonic phonon scattering is unbounded with increasing temperature, which is physically impossible for two-phonon processes. We further reveal that the anharmonic contribution in a real heterogeneous interface (e.g., between graphene and monolayer molybdenum disulfide) can dominate over the harmonic process even at room temperature, highlighting the important role of anharmonicity in weakly interacting heterogeneous systems.

cond-mat.mes-hall↗

arXiv2022

Controllable thermal radiation from twisted bilayer graphen

The presence of interlayer interactions in twisted bilayer graphene (TBG) enhances several characteristics, including the optical and electrical properties. We theoretically investigate the magic angle of TBG according to the vanishing of Fermi velocity and find double magic angles in a series. The thermal radiation from TBG can be tuned to the far infrared range by changing twist angles. The peculiar radiation spectrum is out of atmospheric window, which can be of great use in invisibility and keeping warm. The total radiation of TBG is slightly more than twice of a single layer graphene.

cond-mat.mes-hall↗

arXiv2021

Generalized first-principles method to study near-field heat transfer mediated by Coulomb interaction

We present a general microscopic first-principles method to study the Coulomb-interaction-mediated heat transfer in the near field. Using the nonequilibrium Green's function formalism, we derive Caroli formulas for heat transfers between materials with translational invariance. The central physical quantities are the screened Coulomb potential and the spectrum function of polarizability. Within the random phase approximation, we calculate the polarizability using the linear response density functional theory and obtain the screened Coulomb potential from a retarded Dyson equation. We show that the heat transfer mediated by the Coulomb interaction is consistent with that of the $p$-polarized evanescent waves which dominate the heat transfer in the near field. We adopt single-layer graphene as an example to calculate heat transfers between two parallel sheets separated by a vacuum gap $d$. Our results show a saturation of heat flux at the extreme near field which is different from the reported $1/d$ dependence for local response functions. The calculated heat flux is up to $5\times10^4$ times more than the black-body limit, and a $1/d^2$ dependence is shown at large separations. From the spectrum of energy current density, we infer that the near-field enhancement of heat transfer stems from electron transitions around the Fermi energy. With a uniform strain, the heat flux increases for most of the distances while a negative correlation is shown at the moderate field. Our method is valid for inhomogeneous materials in which the macroscopic response function used in conventional theory of fluctuational electrodynamics would fail at the subnanometer scale.

cond-mat.mes-hall↗