Antiferromagnetic order and possible quantum spin liquid in kagome antiferromagnet LuCu₃(OH)₆Br₂Br_x(OH)_1-x

Recent studies on the kagome lattice YCu₃(OH)_6+xX_3-x (X = (Cl, Br)) have show promising evidence for the existence of a Dirac quantum spin liquid. We have synthesized a new compound LuCu₃(OH)₆Br₂Br_x(OH)_1-x, which has two types of structures with two different space group, P3m1 and R3. Type-I sample with the space group of P3m1 has undistorted Cu²⁺ kagome planes and orders magnetically below about 1.5 K. Type-II has a larger unit cell with the space group of R3 and distorted kagome planes. No magnetic ordering is observed, and the low-temperature specific heat behaves like the Dirac quantum spin liquid, including the quadratic temperature dependence at zero field and an additional linear-T term under magnetic fields. By comparing the structures with YCu₃(OH)_6+xX_3-x (X = Cl, Br), we show that the QSL ground state may have a deep connection with the J_⎔ - J - J′ Heisenberg model on the kagome lattice.