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

Bhattacharya, K.*; Tokiwa, Yoshifumi  ; Majumder, M.*

We present a comprehensive experimental investigation of the temperature evolution of magnetic states in the triangular-lattice delafossite YbCuSe. Magnetization measurements on high-quality single crystals reveal easy-plane anisotropy. Specific heat, magnetization, and muon spin relaxation (SR) measurements establish the absence of magnetic order or spin freezing down to 0.03~K (), demonstrating a dynamically fluctuating quantum spin liquid (QSL) ground state. Thermodynamic measurements uncover multiple characteristic energy scales at ~K, ~K, and ~K. Below , SR detects a dynamical phase separation in which the majority of the spins form a QSL state, while the remaining spins constitute a sporadic, disorder-induced state decoupled from the dominant QSL component. Remarkably, the unconventional temperature dependence of the SR relaxation rate indicates the emergence of roton-like excitations between and , a feature not previously observed in any QSL system, preceding the stabilization of the low-temperature QSL below 0.3~K. These findings identify YbCuSe as a unique QSL platform, providing valuable insights for further experimental and theoretical exploration.