Field-dependent thermodynamic properties of spin-ladder and bilayer systems

Field-dependent thermodynamic properties of two gapped spin systems, two-leg spin-ladder and square-lattice bilayer, are studied using finite-temperature perturbation theory in the strong-coupling limit. The treatment gives reasonable description of the gapped disordered phase of the systems. The free energy F(H,T), susceptibility chi(H,T) and specific heat C(H,T) are calculated. The susceptibility displays a maximum depending strongly on the external field, while it is insensitive to H at higher temperature. With the external field close to the lower critical value, the specific heat exhibits two-round-peak structure. These results agree quite well with the recent experimental measurements on the strongly coupled ladder material Cu-2(C5H12N2)(2)Cl-4 by Hammar et al. [S0163-1829(99)02229-8].