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Abstract

High-quality single crystals of NaYbSe2, which resembles a perfect triangular-lattice antiferromagnet without intrinsic disorder, are investigated by magnetization and specific heat, as well as the local probe techniques nuclear magnetic resonance (NMR) and electron spin resonance. The low-field measurements confirm the absence of any spin freezing or long-range magnetic order down to 50 mK, which suggests a quantum spin liquid ground (QSL) state with gapless excitations. Instability of the QSL state is observed upon applying magnetic fields. For the H perpendicular to c direction, a field-induced magnetic phase transition is observed above 2 T from the C-p(T ) data, agreeing with a clear M-x/3 plateau of M(H), which is associated with an up-up-down spin arrangement. For the H parallel to c direction, a field-induced transition could be evidenced at a much higher field range (9-21 T). The Na-23 NMR measurements provide microscopic evidence of field-induced ordering for both directions. A reentrant behavior of T-N, originating from the thermal and quantum spin fluctuations, is observed for both directions. The anisotropic exchange interactions J _ similar or equal to 4.7 K and J(z) similar or equal to 2.33 K are extracted from the modified bond-dependent XXZ model for the spin-1/2 triangular-lattice antiferromagnet. The absence of magnetic long-range order at zero fields is assigned to the effect of strong bond frustration, arising from the complex spin-orbit entangled 4f ground state. Finally, we derive the highly anisotropic magnetic phase diagram, which is discussed in comparison with the existing theoretical models for spin-1/2 triangular-lattice antiferromagnets.