Pressure-Induced Large Volume Collapse, Plane-to-Chain, Insulator to Metal Transition in CaMn$_2$Bi$_2$

TL;DR

This study reveals that CaMn$_2$Bi$_2$ undergoes a pressure-induced structural transition from a plane to chain configuration, leading to a large volume collapse and a transition from semiconducting to metallic behavior, with associated electronic and magnetic changes.

Contribution

It reports the discovery of a unique pressure-induced plane-to-chain structural transition and associated electronic and magnetic property changes in CaMn$_2$Bi$_2$, a novel quantum material.

Findings

Large volume collapse at 2-3 GPa

Transition from semiconducting to metallic behavior

Emergence of quasi-1D electronic states

Abstract

In-situ high pressure single crystal X-ray diffraction study reveals that the quantum material CaMn$_2$Bi$_2$ undergoes a unique plane to chain structural transition between 2 and 3 GPa, accompanied by a large volume collapse. CaMn2Bi2 displays a new structure type above 2.3 GPa, with the puckered Mn honeycomb lattice of the trigonal ambient-pressure structure converting to one-dimensional (1D) zigzag chains in the high-pressure monoclinic structure. Single crystal measurements reveal that the pressure-induced structural transformation is accompanied by a dramatic two order of magnitude drop of resistivity; although the ambient pressure phase displays semiconducting behavior at low temperatures, metallic temperature dependent resistivity is observed for the high pressure phase, as, surprisingly, are two resistivity anomalies with opposite pressure dependences. Based on the electronic structure calculations, we hypothesized that the newly emerged electronic state under high pressure is associated with a Fermi surface instability of the quasi-1D Mn chains, while we infer that the other is a magnetic transition. Assessment of the total energies for hypothetical magnetic structures for high pressure CaMn$_2$Bi$_2$ indicates that ferrimagnetism is thermodynamically favored.