Subtle metastability of the layered magnetic topological insulator MnBi2Te4 from weak interactions

Jinliang Ning,Yu Wang,Jianwei Sun,Yanglin Zhu,Zhiqiang Mao,Yingdong Guan,Jamin Kidd

doi:10.1038/s41524-020-00427-y

Jinliang Ning, Yu Wang + Show 5 more

Open Access

https://doi.org/10.1038/s41524-020-00427-y

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Abstract

Layered quantum materials can host interesting properties, including magnetic and topological, for which enormous computational predictions have been done. Their thermodynamic stability is much less visited computationally, which however determines the existence of materials and can be used to guide experimental synthesis. MnBi2Te4 is one of such layered quantum materials that was predicted to be an intrinsic antiferromagnetic topological insulator, and later experimentally realized but in a thermodynamically metastable state. Here, using a combined first-principles-based approach that considers lattice, charge, and spin degrees of freedom, we investigate the metastability of MnBi2Te4 by calculating the Helmholtz free energy for the reaction Bi2Te3 + MnTe → MnBi2Te4. We identify a temperature range (~500–873 K) in which the compound is stable with respect to the competing binary phases, consistent with experimental observation. We validate the predictions by comparing the calculated specific heats contributed from different degrees of freedom with experimental results. Our findings indicate that the degrees of freedom responsible for the van der Waals interaction, lattice vibration, magnetic coupling, and nontrivial band topology in MnBi2Te4 not only enable emergent phenomena but also play a crucial role in determining its thermodynamic stability. This conclusion lays the foundation for the future computational material synthesis of novel layered systems.

Highlights

Layered quantum materials, such as graphene and twodimensional (2D) semiconductors of transition metal dichalcogenides, have revolutionized many fields in condensed matter physics and materials science because of their exotic quantum properties[1]
It is well known that layered materials are bound by vdW interactions between layers, while the popular Perdew–Burke–Ernzerhof (PBE) density functional[22] misses most vdW interactions and barely binds layered materials[23,24]
Since 11 meV/atom is comparable to the finite temperature contributions to the reaction free energy from the electronic, magnetic, and vibrational degrees of freedom, PBE will falsely predict that MnBi2Te4 cannot be synthesized below its melting temperature, even in a metastable phase

Summary

INTRODUCTION

Layered quantum materials, such as graphene and twodimensional (2D) semiconductors of transition metal dichalcogenides, have revolutionized many fields in condensed matter physics and materials science because of their exotic quantum properties[1]. These materials have different kinds of chemical bonds ranging from strong intralayer covalent bonds to weak interlayer vdW interactions, with interaction strengths across almost 3 orders of magnitude (from ~1 eV for strong bonds to ~1 meV for vdW interactions). MnBi2Te4 with respect to the competing binaries on average by ~11, ~2.5, ~5, and ~5 meV/atom, respectively, while the electronic thermal excitation is negligible This suggests that SOC, vdW, magnetic coupling, and lattice vibrations play important roles in determining this compound’s metastability accurately with temperature, given that MnBi2Te4 is only more stable than

RESULTS AND DISCUSSION

Experimental methods

Computational methods