Antiferromagnetic Order and Possible Quantum Spin Liquid in Kagome Antiferromagnet LuCu₃(OH)₆Br₂Br_x(OH)_1-x

Recent studies on the kagome lattice YCu_3(OH)_6+xX_3-x (X = Cl, Br) have provided promising evidence for the existence of Dirac quantum spin liquids. In this study, we synthesized a new compound, LuCu_3(OH)_6Br_2Br_x(OH)_1-x, which has two types of structures with two different space groups, P\bar3m1 and R\bar3. The type-I sample with a space group of P\bar3m1 had undistorted Cu^2+ kagome planes and orders magnetically below approximately 1.5 K. The type-II sample had a larger unit cell with a space group of R\bar3 and distorted kagome planes. No magnetic ordering was observed, and the low-temperature specific heat behaved like a Dirac quantum spin liquid, including a quadratic temperature dependence at zero field and an additional linear T term under magnetic fields. By comparing the structures with those of YCu_3(OH)_6+xX_3-x (X = Cl, Br), we show that the quantum spin liquid ground state may have a deep connection with the J_⎔-J-J' Heisenberg model in the kagome lattice.