Novel approach to description of quantum magnets with large singe-ion easy-plane anisotropy

摘要

We introduce a new representation of an integer spin $S$ via bosonicoperators which is useful in describing the paramagnetic phase and transitionsto magnetically ordered phases in magnetic systems with large single-ioneasy-plane anisotropy $D$. Considering the exchange interaction between spinsas a perturbation and using the diagram technique we derive the elementaryexcitation spectrum and the ground state energy in the third order of theperturbation theory. In the special case of S=1 we obtain these expressionsalso using simpler spin representations some of which were introduced before.Comparison with results of previous numerical studies of 2D systems with S=1demonstrates that our approach works better than other analytical methodsapplied before for such systems. We apply our results for the elementaryexcitation spectrum analysis obtained experimentally in $\rm NiCl_2$-$\rm4SC(NH_2)_2$ (DTN). It is demonstrated that a set of model parameters (exchangeconstants and $D$) which has been used for DTN so far describes badly theexperimentally obtained spectrum. A new set of parameters is proposed usingwhich we fit the spectrum and values of two critical fields of DTN.