Low-energy spin dynamics of the quantum spin liquid candidate NaYbSe2

The family of rare-earth chalcogenides $A{\mathrm{RECh}}_{2}$ ($A$ = alkali or monovalent ions, RE = rare earth, and Ch = O, S, Se, and Te) appears as an inspiring playground for studying quantum spin liquids (QSLs). The crucial low-energy spin dynamics remain to be uncovered. By employing muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL in ${\mathrm{NaYbSe}}_{2}$, a representative member with an effective spin-1/2, and explore its unusual spin dynamics. The ZF $\ensuremath{\mu}\mathrm{SR}$ experiments unambiguously rule out spin ordering or freezing in ${\mathrm{NaYbSe}}_{2}$ down to 50 mK, which is two orders of magnitude smaller than the exchange coupling energies. The spin relaxation rate $\ensuremath{\lambda}$ approaches a constant below 0.3 K, indicating that finite spin excitations are featured by a gapless QSL ground state. This is consistently supported by our AC susceptibility measurements. The careful analysis of the longitudinal field (LF) $\ensuremath{\mu}\mathrm{SR}$ spectra reveals a strong spatial correlation and a temporal correlation in the spin-disordered ground state, highlighting the unique feature of spin entanglement in the QSL state. The observations allow us to establish an experimental H-T phase diagram. The study offers insight into the rich and exotic magnetism of the rare-earth family.