Quantum phase transitions and thermodynamics of quantum antiferromagnets with competing interactions

摘要

We study the isotropic Hciscnbcrg chain with nearest and ncxt-ncarcst neighbor interactions, I he ground state phase diagram is constructed in dependence on the additional interactions and an external magnetic field. The thermodynamics is studied by use of finite sets of nonlinear integral equations resulting fiom integrability. The equations are solved numerically and analytically in suitable limiting cases We find second and first order transition lines The exponents of the low-temperature asymptotics at the phase transitions are determined.