We present a comprehensive experimental investigation of the temperature evolution of magnetic states in triangular-lattice delafossite ${\mathrm{YbCuSe}}_2$. Magnetization measurements on high-quality single crystals reveal easy-plane anisotropy. Specific heat, magnetization, and muon spin relaxation ($\mu \mathrm{SR}$) establish the absence of magnetic order or spin freezing down to 0.03 K ($\le J_{\mathrm{avg}}/250$), demonstrating a dynamically fluctuating quantum spin liquid (QSL) ground state. Thermodynamic measurements uncover multiple characteristic energy scales at $T_H\approx 4.5\mathrm{K}, T_L\approx 1.8\mathrm{K}$, and $T^{*}\approx 0.7\mathrm{K}$. Below $T^{*}, \mu \mathrm{SR}$ 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 $\mu \mathrm{SR}$ 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 ${\mathrm{YbCuSe}}_2$ as a unique QSL platform, providing valuable insights for further experimental and theoretical exploration.

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• Frustrated magnetism
• Quantum spin liquid
• Muon spin relaxation & rotation
• Specific heat measurements