The Ising Limit of the XXZ Heisenberg Magnet and Certain Thermal Correlation Functions

The spin-1/2 XXZ Heisenberg magnet is considered for the case of the anisotropy parameter tending to infinity (so-called, Ising limit). A thermal correlation function of the ferromagnetic string is calculated over the ground state. The approach to the calculation of the correlation functions in the limit of infinite anisotropy is based on the observation that the wave function is expressed in terms of the symmetric Schur functions. It is demonstrated that at low temperatures the amplitude of the asymptotical expression of this correlation function is proportional to the squared numbers of strict boxed plane partitions.