From one- to two-magnon excitations in the S=3/2 magnet $\beta$-CaCr$_2$O$_4$

TL;DR

This study uses neutron spectroscopy to explore magnetic excitations in the S=3/2 magnet $eta$-CaCr$_2$O$_4$, revealing a transition from classical spin-wave behavior to quantum fluctuation continuum at higher energies.

Contribution

It provides the first detailed analysis of high-energy magnetic excitations in $eta$-CaCr$_2$O$_4$, demonstrating a crossover from classical to quantum magnetic dynamics.

Findings

Low-energy fluctuations resemble large-S linear spin-waves.

High-energy excitations form a broadened continuum indicative of quantum fluctuations.

$eta$-CaCr$_2$O$_4$ is at the boundary between classical and quantum magnetic phases.

Abstract

We apply neutron spectroscopy to measure the magnetic dynamics in the S=3/2 magnet $\beta$-CaCr$_2$O$_4$ (T$_N$=21 K). The low-energy fluctuations, in the ordered state, resemble large-S linear spin-waves from the incommensurate ground state. However, at higher energy transfers, these semi-classical and harmonic dynamics are replaced by an energy and momentum broadened continuum of excitations. Applying kinematic constraints required for energy and momentum conservation, sum rules of neutron scattering, and comparison against exact diagonalization calculations, we show that the dynamics at high-energy transfers resemble low-S one-dimensional quantum fluctuations. $\beta$-CaCr$_2$O$_4$ represents an example of a magnet at the border between classical N\'eel and quantum phases, displaying dual characteristics.