Thermodynamics of frustrated ferromagnetic spin-1/2 Heisenberg chains: Role of interchain coupling

摘要

The thermodynamics of coupled frustrated ferromagnetic chains is studied within a spin-rotation-invariant Green's-function approach. We consider an isotropic Heisenberg spin-half system with a ferromagnetic in-chain coupling J_1 < 0 between nearest neighbors and a frustrating antiferromagnetic next-nearest-neighbor in-chain coupling J_2 > 0. We focus on the moderate strength of frustration J_2 < |J_1|/4 such that the in-chain spin-spin correlations are predominantly ferromagnetic. We consider two interchain couplings (ICs) J_(⊥,y) and J_(⊥,z), corresponding to the two axes perpendicular to the chain, where ferromagnetic as well as antiferromagnetic ICs are taken into account. We discuss the influence of frustration on the ground-state properties for antiferromagnetic ICs, where the ground state is of a quantum nature. The major part of our study is devoted to the finite-temperature properties. We calculate the critical temperature T_c as a function of the competing exchange couplings J_2,J_(⊥,y),J_(⊥,z). We find that for fixed ICs, T_c decreases monotonically with increasing frustration J_2, where as J_2 →|J_1|/4 the T_c(J_2) curve drops down rapidly. To characterize the magnetic ordering below and above T_c, we calculate the spin-spin correlation functions (S_OS_R), the magnetic order parameter M, the uniform static susceptibility x_0, as well as the correlation length ξ. Moreover, we discuss the specific heat C_V and the temperature dependence of the excitation spectrum ω_q. As J_2 → |J_1|/4, some unusual frustration-induced features were found, such as an increase of the in-chain spin stiffness (in the case of ferromagnetic ICs) or of the in-chain spin-wave velocity (in the case of antiferromagnetic ICs) with growing temperature.