Thermodynamic functions of the S=(1/2 one-dimensional ferromagnet via the renormalization-group approach and Green's function technique.

Abstract

The model applicable to (${\mathrm{C}}_{6}$${\mathrm{H}}_{11}$${\mathrm{NH}}_{3}$)${\mathrm{CuBr}}_{3}$ is studied within the framework of the real-space renormalization-group approach as well as the Green's function technique. The zero-field specific heat is evaluated in the first-order generalized-cumulant expansion and qualitative agreement with the reliable numerical data and experimental results is found. Some crossover effects observed at low temperatures are also discussed. The in-plane magnetization is calculated by having recourse to the first-order Green's function approach and an instability of the spin-wave contribution under the thermal renormalization of magnons is demonstrated.