Anisotropic field-induced ordering in the triangular-lattice quantum spin liquid NaYbSe₂

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⊥c direction, a field-induced magnetic phase transition is observed above 2 T from the C_P(T ) data, agreeing with a clear M_s/3 plateau of M(H), which is associated with an up-up-down spin arrangement. For the H ǀǀ c direction, a field-induced transition could be evidenced at a much higher field range (9–21 T). The ²³Na 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_⊥ ≃ 4.7 K and J_z ≃ 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 4 f 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.