Volborthite [Cu3V2O7(OH)2⋅2H2O] has two-dimensional kagomé structure of S = 1/2 spins. In this system, geometrical frustration prevents the formation of long-range orderings even at low temperature as is the case in triangular lattice. By the thermal relaxation technique, we measured low-temperature heat capacity of volborthite to investigate sample dependences in the low-temperature anomaly around 1 K. The temperature range in this experiment is 0.13 – 3.5 K. We found a rather broad peak-like structure around 1 K and Schottky anomalies owing to Cu nuclei below 0.6 K. The 1 K anomaly seems to have a character of phase transition rather than glass freezing in the new sample.
Fig. 1. Magnetic heat capacity of volborthite at low temperature around 1 K in a CmagT −1 vs T plot. The extrapolations of the date at H = 0 above and below 1 K down to T = 0 give the coefficients of the T-liner term in the magnetic heat capacity. The T-liner term disappears under magnetic fields above 5 T.
Fig. 2. Magnetic heat capacity of volborthite at low temperature around 1 K in aCmagT −1 vs T plot. Rather broad peak-like structure which is sensitive to magnetic field was observed. Schottky anomalies owing to Cu nuclei were observed below 0.6 K.
Fig. 3. Magnetic heat capacity of volborthite below 0.6 K in a CmagT 2 vs T plot. Dot-lines show high temperature limit approximation of nuclear Schottky anomalies.