arXiv subjects

Yong-Mei Zhang

Publications and source records attributed to Yong-Mei Zhang.

6 recordsLinked to original sources

arXiv2025

Modulation of energy and angular momentum radiation of two-dimensional altermagnets

This paper investigates the energy and angular momentum radiation of altermagnets under Rashba spin-orbit coupling (RSOC) and external magnetic fields. Using an effective low-energy Hamiltonian, we derive electronic energy bands and calculate optical conductivity via the Kubo formula. Results show that RSOC strength, altermagnet interactions strength, and Neel vector direction notably affect the optical conductivity of altermagnet metals. Energy radiation is highly sensitive to Rashba spin-orbit coupling, with a saturation effect beyond a particular value, and its peak emission rate is lower than that of graphene due to reduced conductivity. Different from usual semi-conductor, semi-metal or Dirac materials, eg. graphene or silicene, altermagnets generate angular momentum radiation with specific Rashba spin-orbit coupling and altermagnet interaction strength. Angular momentum radiation is minimally reactive to Rashba spin-orbit coupling at low altermagnet interactions strength values but exhibits drastic oscillations between extreme values as altermagnet interactions strength reaches a critical point, showing high sensitivity. These findings suggest that adjusting these parameters can tailor altermagnet applications in spintronics and quantum technologies, potentially leading to innovative devices with customized radiation attributes.

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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.

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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.

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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.

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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.

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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.

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