Spin-1/2 Antiferromagnetic Ladder

From heat capacity (Cp) measurements on a single crystal sample of the spin (S) 1/2 two-leg spin-ladder material Cu(DEP)Br2 above a critical magnetic field (Hc0) where the energy gap between the singlet ground state and the excited magnetic state closes, we have observed a peak anomaly, which is indicative of a magnetic long-range ordering. We have examined how the position of the anomaly in Cp changes with temperature (T) and magnetic field (H) to obtain the experimental H–T phase diagram of this S=1/2 two-leg spin ladder material. From the low temperature specific heat below Hc0, we have evaluated the field-dependent energy gap that vanishes at about 7.6 T.