Large nonreciprocal absorption and emission of radiation in type-I Weyl semimetals with time reversal symmetry breaking

Abstract

The equality between the spectral, directional emittance and absorptance of an object under local thermal equilibrium is known as Kirchhoff's law of radiation. The breakdown of Kirchhoff's law of radiation is physically allowed by breaking time-reversal symmetry and can open new opportunities for novel non-reciprocal light emitters and absorbers. Large anomalous Hall conductivity and angle recently observed in topological Weyl semimetals, particularly type-I magnetic Weyl semimetals and type-II Weyl semimetals, are expected to create large nonreciprocal electromagnetic wave propagation. In this work, we focus on type-I magnetic Weyl semimetals and show via modeling and simulation that non-reciprocal surface plasmons polaritons can result in pronounced non-reciprocity without an external magnetic field. The modeling in this work begins with a single pair of Weyl nodes, followed by a more realistic model with multiple paired Weyl nodes. Fermi-arc surface states are also taken into account through the surface conductivity. This work points to the promising applicability of topological Weyl semimetals for magneto-optical and energy applications. . These authors contributed equally to this work. Main textKirchhoff's law of radiation establishes the equality between the spectral, directional absorptance ( , ) and the spectral, directional emittance ( , ) of an object in local thermal equilibrium, i.e., ( , ) = ( , ) , where and are the wavelength and the direction of incoming and outgoing radiation, respectively. Fundamentally, Kirchhoff's law of radiation underlies the theoretical efficiency limit in radiative energy conversion since converting absorbed incoming radiation into another form of energy, such as electricity or heat, always entails the outgoing emission at the same wavelength in the same direction from the object, which causes an intrinsic loss 1-3 . It has been argued 2,4,5 that Kirchhoff's law of radiation is not a required condition for the validity of the second law of thermodynamics in systems that exchange radiative energy, but rather a result of the Lorentz reciprocity theorem in which the only assumptions are a linear constitutive relation and symmetric permittivity and permeability tensors 6,7 . Thus, the violation of Kirchhoff's law of radiation, i.e., non-reciprocity in the spectral, directional absorptance and emittance, is physically allowed, and its realization can open new opportunities for novel light emitters and absorbers for a wide range of radiative applications including solar photovoltaics, thermo-photovoltaics, and antennas 1,8 .Non-reciprocity in a medium often arises due to non-zero anti-symmetric off-diagonal elements of the dielectric tensor of the medium, which creates non-reciprocal electromagnetic modes 9 . One way to create the anti-symmetric off-diagonal elements is by inducing magnetic responses either by the Hall response under an external magnetic field or by spontaneous magnetization in materials, namely the anomalous Hall effect 10 . The anomalous Hall effect can origin...

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“…This is known as Kirchhoff's law of radiation (henceforth referred to as "Kirchhoff's law") and is usually thought of as a consequence of the second law of thermodynamics. In reality, Kirchhoff's law is derived assuming Lorentz reciprocity holds [1,[3][4][5], which requires the reflectivity of the surface to be reciprocal; i.e., ρ(ω, θ, φ) = ρ(ω, θ, φ + 180 • ) for specular surfaces [2,6]. However, Lorentz reciprocity does not always hold, such as in magnetic systems [3,4].…”

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