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

The family of rare-earth chalcogenides ARECh2 (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 (μSR) and zero-field (ZF) AC susceptibility down to 50 mK, we are able to identify the gapless QSL in NaYbSe2, a representative member with an effective spin-1/2, and explore its unusual spin dynamics. The ZF μSR experiments unambiguously rule out spin ordering or freezing in NaYbSe2 down to 50 mK, which is two orders of magnitude smaller than the exchange coupling energies. The spin relaxation rate λ 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) μSR 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.