Thermodynamics of integrable chains with alternating spins

We consider a two-parameter (c¯,c̃) family of quantum integrable isotropic Hamiltonians for a chain of alternating spins of spin s=1/2 and s=1. We determine the thermodynamics for low-temperature T and small external magnetic field H, with T≪H. In the antiferromagnetic (c¯>0,c̃>0) case, the model has two gapless excitations. In particular, for c¯=c̃, the model is conformally invariant and has central charge ${\mathit{c}}_{\mathrm{vir}}$=2. When one of these parameters is zero, the Bethe ansatz equations admit an infinite number of solutions with lowest energy.