Thermodynamic Properties of the Kagome Lattice in Volborthite

Thermodynamic investigations on volborthite [Cu3V2O7(OH)(2)center dot 2H(2)O], which possesses a two-dimensional kagome structure of S-1/2 spins, are presented. The low-temperature heat capacities of volborthite and its Zn analogue compound were measured by the relaxation calorimetry technique between 0.8 and 45 K. The magnetic heat capacity of volborthite is characterized by two contributions of T-linear and T-2 dependences, the former of which is large above 1K, giving evidence of dense gapless excitations. We find a distinct kink in CpT-1 at T* = 1 K, which demonstrates a thermodynamic phase transition of short-range nature to a novel ground state as reported in previous V-51-NMR experiments. The T-linear term becomes smaller but remains finite even in the low-temperature state below T*, while it is gradually reduced with increasing magnetic field and vanishes at 5 T, which is close to the critical field for the field-induced transition observed in the first magnetization step.