Quantum magnets with large single-ion easy-plane anisotropy in transverse magnetic field

Abstract

We discuss low-temperature properties of magnets with integer spin and large single-ion easy-plane anisotropy $D$ in transverse magnetic field $h$. Considering the exchange interaction between spins as a perturbation and using the diagram technique we derive at $h\sim D$ in the first nonvanishing orders of the perturbation theory thermal corrections to the elementary excitation spectrum, magnetization and specific heat. An expression for the boundary $h_{c1}(T)$ is found in the $h$-$T$ plane between the paramagnetic phase and that with the long range magnetic order. The effective interaction is derived between bosons near the quantum critical point $h_{c1}(0)$. The proposed theory describes well experimental data obtained in $\mathrm{NiC}{\mathrm{l}}_2$-$4SC(\mathrm{N}{\mathrm{H}}_2{)}_2$ (DTN).

7 pages, 4 figures, to appear in JETP Letters

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