Spectrum and correlation functions of anisotropic Heisenberg antiferromagnet III spin-flop phase in the generalized Hartree–Fock approximation

The matrix Green function is studied which is constructed on Pauli operators and describes transversal components of dynamical susceptibility tensor of two-sublattice anisotropic Heisenberg antiferromagnet of spin 1/2 in longitudinal and transversal magnetic fields. In the generalised Hartree–Fock approximation, the renormalised magnon spectrum and one-particle correlation functions in noncollinear (spin-flop) phase are found and phase boundaries on the plane “magnetic field – temperature” are calculated. The estimate for the “bicritical” point is obtained for the “easy axis” model. It is shown that in the low temperature region, contribution of the mass operator in higher approximations must be taken into account for satisfying the symmetry conditions and the Bogoliubov–Goldstone theorem in the case of “easy axis” and “easy plane” models in longitudinal and transversal magnetic field, respectively.