Magnetic field-induced phase transition of quantum spin system Cu2Cl4•H8C4SO2

The magnetic properties of Cu2Cl4.H8C4S02 are described by an S = 1/2 double spin chain with strong exchange interactions along the leg direction. This system has a singlet ground state with an excitation gap of Delta/k(B) = 5.13 K. We have measured the magnetization and the specific heat of Cu2Cl4.H8C4SO2. The magnetic phase transition due to interchain interactions was observed for H > 4 T. Magnetic phase diagrams were obtained for H vertical bar vertical bar b- and c-axes. The phase boundaries for these two different field directions can be represented by the power law (g/2) [H-N (T)-H-g] proportional to T-phi with (g/2)H-g = 3.83 T and phi = 1.98. The experimental exponent phi = 1.98 is somewhat larger than the value phi(BEC) = 3/2 predicted by the magnon Bose-Einstein condensation theory.