Magnetic field-temperature phase diagram of spin-1/2 triangular lattice antiferromagnet KYbSe_2

A quantum spin liquid (QSL) is a state of matter characterized by fractionalized quasiparticle excitations, quantum entanglement, and a lack of long-range magnetic order. However, QSLs have evaded definitive experimental observation. Several Yb^3+-based triangular lattice antiferromagnets with effective S = 1/2 have been suggested to stabilize the QSL state as the ground state. Here, we build a comprehensive magnetic temperature phase diagram of a high-quality single crystalline KYbSe_2 via heat capacity and magnetocaloric effect down to 30 mK with magnetic field applied along the a-axis. At zero magnetic field, we observe the magnetic long-range order at T_N = 0.29 K entering 120 degrees ordered state in heat capacity, consistent with neutron scattering studies. Analysis of the low-temperature (T) specific heat (C) at zero magnetic field indicates linear T-dependence of C/T and a broad hump of C/T in the proximate QSL region above T_N. By applying magnetic field, we observe the up-up-down phase with 1/3 magnetization plateau and oblique phases, in addition to two new phases. These observations strongly indicate that while KYbSe_2 closely exhibits characteristics resembling an ideal triangular lattice, deviations may exist, such as the effect of the next-nearest-neighbor exchange interaction, calling for careful consideration for spin Hamiltonian modeling. Further investigations into tuning parameters, such as chemical pressure, could potentially induce an intriguing QSL phase in the material.