Off-Diagonal Induction of Short-Range Order: Spin Pair Correlation of the Quantum XXZ-Model

It is widely accepted among statistical physicists that quantum effects usually prevent a growth of long-range order of diagonal variables, which is originally present in classical systems. For example, iIi the two-dimensional (2D) Ising model the long-range spin-pair correlation below the transition temperature is broken when some amounts of exchange interactions of off-diagonal spin components are added to the original Hamiltonian of the Ising model. This breakdown of correlation results in the absence of phase transitions in 2D quantum isotropic Heisenb6:rg model. l }