Quantum spin liquid ground state with the evidence of roton-like excitations at elevated temperatures in the triangular-lattice delafossite YbCuSe$_2$

TL;DR

This study provides experimental evidence of a quantum spin liquid ground state in YbCuSe$_2$, featuring roton-like excitations at elevated temperatures, with detailed insights into its magnetic and thermodynamic properties.

Contribution

First comprehensive investigation revealing roton-like excitations and dynamic phase separation in a triangular-lattice Yb-based QSL, advancing understanding of quantum spin liquids.

Findings

No magnetic order down to 0.03 K, confirming a QSL ground state.

Detection of roton-like excitations between 4.5 K and 1.8 K.

Observation of dynamical phase separation below 0.7 K.

Abstract

We present a comprehensive experimental investigation of the temperature evolution of magnetic states in triangular-lattice delafossite YbCuSe$_2$. Magnetization measurements on high-quality single crystals reveal easy-plane anisotropy. Specific heat, magnetization, and muon spin relaxation ($\mu$SR) establish the absence of magnetic order or spin freezing down to 0.03 K ($\leq J_{\mathrm{avg}}/250$), demonstrating a dynamically fluctuating quantum spin liquid (QSL) ground state. Thermodynamic measurements uncover multiple characteristic energy scales at $T_H \approx 4.5$ K, $T_L \approx 1.8$ K, and $T^* \approx 0.7$ K. Below $T^*$, $\mu$SR detects a dynamical phase separation in which the majority of the spins are forming a QSL state whereas the remaining spins form a sporadic, disorder-induced state decoupled from the dominant QSL component. Remarkably, the unconventional temperature dependence of the $\mu$SR relaxation rate indicates roton-like excitations emerging between $T_H$ and $T_L$, a feature not previously observed in any QSL system, preceding the stabilization of the low-temperature QSL at 0.3 K. These findings identify YbCuSe$_2$ as a unique QSL platform, providing valuable insights for further experimental and theoretical exploration.