Magnetism and Peierls distortion in Dirac semimetal CaMnBi2

Published

November 5, 2025

Author(s)

Aashish Sapkota, Niraj Aryal, Xiao Hu, Masaaki Matsuda, Yan Wu, Guangyong Xu, John Wilde, Andreas Kreyssig, Paul Canfield, Cedomir Petrovic, John Tranquada, Igor Zaliznyak

Abstract

Dirac semimetals of the form AMnX2 (A = alkaline-earth or divalent rare earth; X = Bi, Sb) host conducting square-net Dirac-electron layers of X atoms interleaved with antiferromagnetic MnX layers. In these materials, canted antiferromagnetism can break time-reversal symmetry (TRS) and produce a Weyl semimetallic state. CaMnBi2 was proposed to realize this behavior below T∗ ∼ 50 K, where anomalies in resistivity and optical conductivity were reported. We investigate single-crystal CaMnBi2 using polarized and unpolarized neutron diffraction, x-ray diffraction, and density functional theory (DFT) calculations to elucidate the underlying crystal and magnetic structures. The results show that the observed anomalies do not originate from spin canting or weak ferromagnetism; no measurable uniform Mn spin canting is detected. Instead, CaMnBi2 undergoes a coupled structural and magnetic symmetry-lowering transition at T∗ = 48(2) K, from a tetragonal lattice with C-type antiferromagnetism to an orthorhombic phase with unit-cell doubling along the c axis and minimal impact on magnetism. Analysis of superlattice peak intensities and lattice distortion reveals a continuous second-order transition governed by a single order parameter. The refined atomic displacements correspond to a zigzag bond-order-wave (BOW) modulation of Bi-Bi bonds, consistent with an electronically driven Peierls-type instability in the Dirac-electron Bi layer, long anticipated by Hoffmann and co-workers [W. Tremel and R. Hoffmann, J. Am. Chem. Soc. 109, 124 (1987); G. A. Papoian and R. Hoffmann, Angew. Chem. Int. Ed. 39, 2408 (2000)]

Citation

Physical Review X

Pub Type

Journals

Keywords

Dirac semimetal, neutron scattering, Peierls distortion, topological quantum materials

Citation

Sapkota, A. , Aryal, N. , Hu, X. , Matsuda, M. , Wu, Y. , Xu, G. , Wilde, J. , Kreyssig, A. , Canfield, P. , Petrovic, C. , Tranquada, J. and Zaliznyak, I. (2025), Magnetism and Peierls distortion in Dirac semimetal CaMnBi2, Physical Review X (Accessed March 7, 2026)