The magnetic field dependence of specific heat in CuSO4 center dot 5H(2)O S=1/2 quantum spin chain

The experimental magnetic specific heat studies of monocrystalline CuSO4. 5 H(2)O have been performed at the temperature 615 mK in a magnetic field fro m 0 T up to 3 T applied in a direction parallel to the easy axis. The depen dence of the magnetic contribution to the specific heat as a function of th e applied magnetic field at fixed temperature strongly supports the theoret ically predicted double peak structure, which cannot be explained by the ap proximation of spin waves. The acquired results illustrate the importance o f non-linearity for thermodynamics of quantum spin chains even at low tempe ratures.