Unconventional phase transitions in strongly anisotropic 2D (pseudo)spin systems

Vitaly Konev,Yury Panov,Alexander Moskvin,Vasily Ulitko,Evgeny Vasinovich

doi:10.1051/epjconf/201818508006

Abstract

We have applied a generalized mean-field approach and quantum Monte-Carlo technique for the model 2D S = 1 (pseudo)spin system to find the ground state phase with its evolution under application of the (pseudo)magnetic field. The comparison of the two methods allows us to clearly demonstrate the role of quantum effects. Special attention is given to the role played by an effective single-ion anisotropy ("on-site correlation").

Highlights

These days spin algebra and spin Hamiltonians are used in the traditional fields of spin magnetism and magnetic resonance but in so-called pseudospin lattice systems with the on-site occupation constraint
Standard pseudospin formalism represents a variant of the equivalent operators technique widely known in different physical problems from classical and quantum lattice gases, binary alloys,ferroelectrics,.. to neural networks, usually for simplest s = 1/2 pseudospin value
At variance with quantum s = 1/2 systems the Hamiltonian of S = 1 spin lattices in general is characterized by several additional terms such as a single ion anisotropy, biquadratic isotropic and anisotropic exchange couplings, that results in their rich phase diagrams and novel types of the order such as quantum paramagnet and spin-nematic order

Summary

Introduction

These days spin algebra and spin Hamiltonians are used in the traditional fields of spin magnetism and magnetic resonance but in so-called pseudospin lattice systems with the on-site occupation constraint (see, e.g., Ref. [1]). Studying anisotropic S = 1 spin and pseudospin systems is of a great importance for quantum magnets but for different bosonic-like systems with the on-site Hilbert space truncated to the three lowest occupation states n = 0, 1, 2, in particular, for so-called semi-hard core bosons. S = 1 (pseudo)spin Hamiltonian for bosonic-like systems with the on-site Hilbert space truncated to the three lowest occupation states n = 0, 1, 2, in particular, for so-called semi-hard core bosons, or for model high-Tc cuprate with the on-site Hilbert space reduced to the only triplet of the three effective valence centers [CuO4]7−,6−,5− (nominally Cu1+;2+;3+). In the strong on-site attraction limit of the model (large easy-axis pseudospin on-site anisotropy) we arrive at the Hamiltonian of the hard-core, or local, bosons which was earlier considered to be a starting point for explanation of the cuprate high-Tc superconductivity [8]. The simplified model can be directly applied to a description of bosonic systems with suppressed one-particle hopping

Mean-field approximation

Quantum Monte-Carlo calculations

Conclusions