Spontaneous symmetry breaking and the local freeze-out in ${\mathrm{Tl}}^{2+}$ -doped K${\mathrm{H}}_2$As${\mathrm{O}}_4$

The slowing down of the local order-parameter fluctuations and the spontaneous dynamic symmetry breaking at a defect site in an order-disorder hydrogen-bonded system—describable by an Ising model in a transverse-field Hamiltonian—has been evaluated and shown to depend on the range of the forces with which the defect couples to the host lattice. The spontaneous symmetry breaking observed far above $T_c$ in the EPR spectra of defect centers in K${\mathrm{H}}_2$As${\mathrm{O}}_4$-type crystals is shown to be compatible with such a local slowing down and "pseudo-freeze-out" around defects which couple to the surrounding pseudospins via short-range forces which are significantly stronger than the interaction forces in the undisturbed lattice. No such freeze-out occurs for the ${\mathrm{Tl}}^{2+}$ ions which are coupled to the surrounding pseudospins via long-range forces.