Universal scaling of specific heat in the S = ? quantum kagome antiferromagnet herbertsmithite

Despite tremendous investigations, a quantum spin liquid (QSL) state realized in a spin-1/2 kagome Heisenberg antiferromagnet remains largely elusive. In herbertsmithite ZnCu3(OH)6Cl2, a quantum spin liquid candidate on the perfect kagome lattice, precisely characterizing the intrinsic physics of the kagome layers is extremely challenging due to the presence of interlayer Cu/Zn antisite disorder within its crystal structure. Here we measured the specific heat and thermal conductivity of single-crystal herbertsmithite in magnetic fields with high resolution. Strikingly, intrinsic magnetic specific heat contribution arising from the kagome layers exhibits excellent scaling collapse as a function of T /H (temperature/magnetic field). In addition, no residual linear term in the thermal conductivity kappa/T(T -> 0) is observed in zero and applied magnetic fields, indicating the absence of itinerant gapless excitations. These results capture a new essential feature of the QSL state of the kagome layers; localized orphan spins are induced by exchange bond randomness, surrounded by a nonitinerant QSL.