Axion-Field-Enabled Nonreciprocal Thermal Radiation in Weyl Semimetals

TLDR

Thermal radiation is governed by emissivity and absorptivity, which are equal in reciprocal systems by Kirchhoff's law, limiting control and causing losses, and existing methods to break this symmetry rely on strong or narrow‑band magneto‑optical effects. The study aims to demonstrate that axion electrodynamics in magnetic Weyl semimetals can produce strongly nonreciprocal thermal emitters. By exploiting axion electrodynamics in magnetic Weyl semimetals, the authors design emitters that violate Kirchhoff's law without external magnetic fields. The resulting emitters nearly completely violate Kirchhoff's law over broad angular and frequency ranges, achieving strong nonreciprocity without requiring an external magnetic field.

Abstract

Objects around us constantly emit and absorb thermal radiation. The emission and absorption processes are governed by two fundamental radiative properties: emissivity and absorptivity. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation. This restriction limits the degree of freedom to control thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff's law typically utilize magneto-optical effects with an external magnetic field. However, these approaches require either a strong magnetic field (∼3T) or narrow-band resonances under a moderate magnetic field (∼0.3T), because the nonreciprocity in conventional magneto-optical effects is weak in the thermal wavelength range. Here, we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters that nearly completely violate Kirchhoff's law over broad angular and frequency ranges without requiring any external magnetic field.