Quantum phase transitions of the extended isotropic XY model with long-range interactions

The one-dimensional extended isotropic XY model (s=1/2) in a transverse field with uniform long-range interactions among the z components of the spin is considered. The model is exactly solved by introducing the Gaussian and Jordan–Wigner transformations, which map it in a non-interacting fermion system. The partition function can be determined in closed form at arbitrary temperature and for arbitrary multiplicity of the multiple spin interaction. From this result, all relevant thermodynamic functions are obtained and, due to the long-range interactions, the model can present classical and quantum transitions of first and second orders. The study of its critical behavior is restricted to the quantum transitions, which are induced by the transverse field at T=0. The phase diagram is explicitly obtained for multiplicities p  =2,3,4 and ∞∞, as a function of the interaction parameters, and, in these cases, the critical behavior of the model is studied in detail. Explicit results are also presented for the induced magnetization and isothermal susceptibility χTzz, and a detailed analysis is also carried out for the static longitudinal 〈SjzSlz〉 and transversal 〈SjxSlx〉 correlation functions. The different phases presented by the model can be characterized by the spatial decay of these correlations, and from these results some of these can be classified as quantum spin liquid phases. The static critical exponents and the dynamic one, z, have also been determined, and it is shown that, besides inducing first order phase transition, the long-range interaction also changes the universality class of the model.