Renormalization-group approach to weakly interacting spin chains

The thermodynamic properties of the weakly interacting spin-1/2 and spin-1 chains with the intrachain coupling J and interchain coupling $\tilde{J}$ are studied by using the linear renormalization-group method. The magnetic susceptibility of the weakly interacting $s=1/2\mathrm{}$ quantum anisotropic Heisenberg chains in the longitudinal field (h) is investigated. The scaling shifts of the parallel susceptibility maxima in the external magnetic field are considered. The same method is used to study the specific heat and susceptibility of the weakly interacting $S=1\mathrm{}$ Ising models. In the special case (Blume-Capel model) the phase diagram with critical and first-order transition lines is found. The location of the tricritical point for $\tilde{J}/J=0.2\mathrm{}$ is estimated.