Quantum distortion caused by magneto-elastic coupling for antiferromagnetic tetrahedral cluster of spin-1/2

TL;DR

This paper investigates how magneto-elastic coupling influences the ground state degeneracy in a spin-1/2 antiferromagnetic tetrahedral cluster, revealing quantum effects that prevent degeneracy lifting and proposing a new model for structural phase transitions in spinels.

Contribution

It introduces a quantum mechanical model showing that degeneracy is preserved under quantum fluctuations, contrasting with classical predictions, and proposes a novel explanation for spinel phase transitions.

Findings

Degeneracy is lifted by classical displacement but not by quantum displacement.

Quantum fluctuations induce tetrahedral distortion without lifting degeneracy.

A new model explains tetragonal distortion in spinels via quantum fluctuation.

Abstract

We study the effects of magneto-elastic coupling on the degenerate ground spin-state of the antiferromagnetic Heisenberg model on a regular tetrahedron of spin-1/2. When displacement of spin is considered as a classical variable, the degeneracy is lifted through the distance dependence of exchange coupling, i.e., a kind of Jahn-Teller effect. On the other hand, when displacement is considered as a quantum variable, the degeneracy of the ground spin-state is not lifted although the tetrahedron is distorted by quantum fluctuation caused by magneto-elastic coupling for a component of the normal coordinates of the degenerate mode. We propose a new model for the structural phase transition of vanadium and nickel spinels: the tetragonal distortion is caused by quantum fluctuation and there exists a kind of hidden order with respect to the non-magnetic ground spin-states.