Numerical study of magnetic field induced ordering in BaCuSi$_2$O$_6$ and related systems

TL;DR

This study investigates the magnetic field-induced ordering in BaCuSi$_2$O$_6$ using quantum Monte Carlo and mean field theory, proposing a novel boson model that explains the phase diagram asymmetry and the Bose-Einstein condensation mechanism.

Contribution

It introduces a new boson model allowing up to two occupancy per site, explaining field-induced ordering and phase diagram asymmetry in related spin systems.

Findings

Identification of superfluid density as the square of in-plane staggered magnetization

Comparison of QMC results with mean field and simple boson models

Explanation of phase diagram asymmetry in related materials

Abstract

Thermodynamics of spin dimer system BaCuSi_2O_6 is studied using a quantum Monte Carlo calculation (QMC) and a bond-operator mean field theory. We propose that a new type of boson, which, rather than being hard-core, allows up to two occupancy at each site, is responsible for the Bose Einstein condensation of field induced ordering. Its superfluid density is identified as the square of the in-plane staggered magnetization m_{xy} in the ordered phase. We also compare our QMC result of the spin Heisenberg model to those predicted by mean field theory as well as by the simple hard core boson model for both large and small intra-dimer coupling J. The asymmetry of the phase diagram of m_{xy}(h) of small coupling J in related systems such as NiCl_2-4SC(NH_2)_2 is explained with our new boson operator.