Expansion for quantum perturbations in random spin systems
Energy eigenstates in the random transverse field Edwards-Anderson (EA) model and the random bond quantum Heisenberg XYZ model in a $d$-dimensional finite cubic lattice are obtained for sufficiently weak interactions. The Datta-Kennedy-Kirkwood-Thomas convergent perturbative expansion using the contraction mapping theorem is developed for quantum spin systems with site- and bond-dependent interactions. This expansion enables us to obtain energy eigenstates in the random transverse field free spin model perturbed by sufficiently weak longitudinal exchange interactions. This expansion is useful also for the EA model perturbed by sufficiently weak transverse fields and bond-dependent XY exchange interactions. In these models, their perturbations split the two fold degenerate energy eigenvalues because of the ${\mathbb Z}_2$ symmetry in the unperturbed EA model. It is shown that the energy gap between split energy eigenvalues is exponentially small in the system size. We provide a sufficient condition on the perturbation for absence of level crossing between arbitrary energy eigenstates.