Magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$

TL;DR

This study uses high-field ESR to investigate magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$, revealing a persistent energy gap above the ordering temperature and suggesting dominant short-range correlations.

Contribution

It provides the first detailed ESR frequency-field diagrams for Cs$_2$CuBr$_4$ and compares experimental results with spin-wave theory, highlighting the robustness of the energy gap.

Findings

The zero-field energy gap is approximately 9.5 K.

The energy gap persists well above the magnetic ordering temperature.

High-energy spin dynamics are governed by short-range correlations.

Abstract

We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuBr$_4$. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, $\Delta\approx9.5$ K, observed in the low-temperature excitation spectrum of Cs$_2$CuBr$_4$ [Zvyagin $et~al.$, Phys. Rev. Lett. 112, 077206 (2014)], is present well above $T_N$. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below $T_N$ the high-energy spin dynamics in Cs$_2$CuBr$_4$ is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangle-lattice antiferromagnet.