First-principles study of large gyrotropy in MnBi for infrared thermal photonics

Md Roknuzzaman, Sathwik Bharadwaj, Yifan Wang, Chinmay Khandekar, Dan Jiao, Rajib Rahman, and Zubin Jacob

Phys. Rev. B 108, 224307 – Published 14 December 2023

Abstract

Nonreciprocal gyrotropic materials have attracted significant interest recently in material physics, nanophotonics, and topological physics. Most of the well-known nonreciprocal materials, however, only show nonreciprocity under a strong external magnetic field and within a small segment of the electromagnetic spectrum. Here, through first-principles density-functional theory calculations, we show that due to strong spin-orbit coupling manganese-bismuth (MnBi) exhibits nonreciprocity without any external magnetic field and a large gyrotropy in a broadband long-wavelength infrared regime. Further, we design a multilayer structure based on MnBi to obtain a maximum degree of spin-polarized thermal emission at 7 $µ m$ . The connection established here between large gyrotropy and the spin-polarized thermal emission points to the potential use of MnBi to develop spin-controlled thermal photonics platforms.

Received 15 May 2023
Accepted 20 November 2023

DOI:https://doi.org/10.1103/PhysRevB.108.224307

Physics Subject Headings (PhySH)

First-principles calculations Magneto-optical Kerr effect Optical conductivity Permittivity Spin-orbit coupling Thermionic emission

Ferromagnets

Density functional theory

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Md Roknuzzaman ^1,2, Sathwik Bharadwaj ³, Yifan Wang³, Chinmay Khandekar³, Dan Jiao³, Rajib Rahman¹, and Zubin Jacob ^3,*

¹School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia
²School of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia
³Birck Nanotechnology Center, Elmore Family School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA

^*zjacob@purdue.edu

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Issue

Vol. 108, Iss. 22 — 1 December 2023

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Images

Figure 1
(a) The schematic depicts the emission of spin-polarized thermal radiation originating from the intrinsic fluctuating dipole moments in a MnBi layer. Spin polarization of the thermal radiation can be quantified by the nonvanishing Stokes parameter $S_{3}$ . Crystal structures of the conventional unit cell of the low-temperature phases of MnBi: (b) Stable hexagonal phase and (c) Metastable cubic zinc-blende phase.
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Figure 2
Electronic band structures of hexagonal and zinc-blende phases of MnBi. Spin-polarized band structure for (a) hexagonal and (b) zinc-blende MnBi. Effect of SOC on the band structure for (c) hexagonal and (d) zinc-blende MnBi. We observe that the inclusion of spin-orbit coupling leads to significant changes in the band structures for both the hexagonal and zinc-blende phases. The horizontal dashed lines here represent the band structure obtained with the inclusion of SOC.
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Figure 3
The calculated total and projected density of states for (a) hexagonal and (b) zinc-blende MnBi. It is found from the projected density of states that the $3 d$ orbital of Mn is the major contributor to the total DOS at the Fermi level. Also, the projected densities of states for Mn $4 s$ , Bi 6s, and Bi $6 p$ at around the Fermi level are nearly zero. Hence, the strong SOC found in MnBi is due to half-filled $3 d$ orbital of the Mn atom.
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Figure 4
LWIR dielectric function for hexagonal and zinc-blende phases of MnBi. (a) Independent components of the imaginary part of the dielectric tensor due to off-diagonal and diagonal elements. (b) Independent components of the real part of the dielectric tensor due to off-diagonal and diagonal elements.
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Figure 5
LWIR optical conductivity of hexagonal and zinc-blende phases of MnBi. (a) Independent components of the real part of the conductivity tensor due to off-diagonal and diagonal parts. (b) Independent components of the imaginary part of the dielectric tensor due to off-diagonal and diagonal parts.
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Figure 6
The gyrotropy ratio calculated from the DFT calculations, and the simulated circularly polarized (CP) thermal emission for hexagonal and zinc-blende MnBi in the LWIR regime. Gyrotropy of (a) hexagonal and (b) zinc-blende MnBi. Emissivity for right circularly polarized (RCP) denoted as $η_{(+)}$ and left circularly polarized (LCP) denoted as $η_{(-)}$ as well as Stokes parameter $(S_{3})$ for (c), (e) hexagonal and (d), (f) zinc-blende MnBi for two different device geometries. Observed asymmetry in $η_{(+)}$ and $η_{(-)}$ translates into a spin-polarized thermal emission in MnBi with maximum spin polarization at 7 $µ m$ .
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