Semiclassical theory for liquid-like behaviour of the frustrated magnet $\mathrm{Ca}_{10}\mathrm{Cr}_{7}\mathrm{O}_{28}$

TL;DR

This paper develops a semiclassical theory for the low-temperature liquid-like behavior of the frustrated magnet Ca10Cr7O28, revealing how spin fluctuations and effective Hamiltonians explain experimental observations and possible phase transitions.

Contribution

It introduces an effective Hamiltonian with S=3/2 moments and uses semiclassical spinwave theory along with simulations to explain the material's low-temperature magnetic properties and structure factors.

Findings

Singular spinwave fluctuations destabilize spiral order.

Simulations reproduce liquid-like structure factors.

Predicted transition to nematic order at lower temperatures.

Abstract

We identify the low energy effective Hamiltonian that is expected to describe the low temperature properties of the frustrated magnet $\mathrm{Ca}_{10}\mathrm{Cr}_{7}\mathrm{O}_{28}$. Motivated by the fact that this effective Hamiltonian has $S=3/2$ effective moments as its degrees of freedom, we use semiclassical spinwave theory to study the $T=0$ physics of this effective model and argue that singular spinwave fluctuations destabilize the spiral order favoured by the exchange couplings of this effective Hamiltonian. We also use a combination of classical Monte-Carlo simulations and molecular dynamics, as well as analytical approximations, to study the physics at low, nonzero temperatures. The results of these nonzero temperature calculations capture the liquid-like structure factors observed in the temperature range accessed by recent experiments. Additionally, at still lower temperatures, they predict that a transition to nematic order in the bond energies reflects itself in the spin channel in the form of a crossover to a regime with large but finite correlation length for spiral spin correlations and a corresponding slowing down of spin dynamics.