arXiv subjects

Zu-Quan Zhang

Publications and source records attributed to Zu-Quan Zhang.

8 recordsLinked to original sources

arXiv2024

Chiral-induced angular momentum radiation in single molecular junctions

We study angular momentum radiation from electrically-biased chiral single molecular junctions using the nonequilibrium Green's function method. Using single helical chains as examples, we make connections between the ability of a chiral molecule to emit photons with angular momentum to the geometrical factors of the molecule. We point out that the mechanism studied here does not involve the magnetic moment. Rather, it relies on inelastic transitions between scattering states originated from two electrodes with different chiral properties and chemical potentials. The required time-reversal symmetry breaking is provided by nonequilibrium electron transport. Our work sheds light on the relationship between geometrical and optoelectrical chiral properties at single molecular limit.

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↗

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↗

arXiv2018

Coulomb force mediated heat transfer in the near field - geometric effect

It has been shown recently that the Coulomb part of electromagnetic interactions is more important than transverse propagation waves for the near-field enhancement of heat transfer between metal objects at a distance of order nanometers. Here we present a theory focusing solely on the Coulomb potential between electrons hopping among tight-binding sites. When the relevant systems are reduced to very small geometry, for example, a single site, the enhancement is much higher compared to a collection of them packed within a distance of a few angstroms. We credit this to the screening effect. This result may be useful in designing metal-based meta-materials to enhance heat transfer much higher.

cond-mat.mes-hall↗

arXiv2018

Energy transfer between two vacuum-gapped metal plates: Coulomb fluctuation and electron tunneling

Recent experimental measurements for near-field radiative heat transfer between two bodies have been able to approach the gap distance within $2 \; \textrm{nm}$, where the contributions of Coulomb fluctuation and electron's tunneling are comparable. Using the nonequilibrium Green's function method in the $G_{0}W_{0}$ approximation, based on a tight-binding model, we obtain for the energy current a Caroli formula from the Meir-Wingreen formula in the local equilibrium approximation. Also, the Caroli formula is consistent with the evanescent part of the heat transfer from the theory of fluctuational electrodynamics. We go beyond the local equilibrium approximation to study the energy transfer in the crossover region from electron tunneling to Coulomb fluctuation based on a numerical calculation.

cond-mat.mes-hall↗

arXiv2017

Fano-shaped impurity spectral density, electric-field-induced in-gap state and local magnetic moment of an adatom on trilayer graphene

Recently, the existence of local magnetic moment in a hydrogen adatom on graphene has been confirmed experimentally [González-Herrero et al., Science, 2016, 352, 437]. Inspired by this breakthrough, we theoretically investigate the top-site adatom on trilayer graphene (TLG) by solving the Anderson impurity model via self-consistent mean field method. The influence of the stacking order, the adsorption site and external electric field are carefully considered. We find that, due to its unique electronic structure, the situation of the TLG is drastically different from that of the monolayer graphene. Firstly, the adatom on rhombohedral stacked TLG (r-TLG) can have a Fano-shaped impurity spectral density, instead of the normal Lorentzian-like one, when the impurity level is around the Fermi level. Secondly, the impurity level of the adatom on r-TLG can be tuned into an in-gap state by an external electric field, which strongly depends on the direction of the applied electric field and can significantly affect the local magnetic moment formation. Finally, we systematically calculate the impurity magnetic phase diagrams, considering various stacking orders, adsorption sites, doping and electric field. We show that, because of the in-gap state, the impurity magnetic phase of r-TLG will obviously depend on the direction of the applied electric field as well. All our theoretical results can be readily tested in experiment, and may give a comprehensive understanding about the local magnetic moment of adatom on TLG.

cond-mat.mes-hall↗