Schwinger boson mean field theory of the Heisenberg Ferrimagnetic Spin Chain

TL;DR

This paper applies Schwinger boson mean field theory to the quantum ferrimagnetic Heisenberg chain, analyzing ground state order, excitations, and thermodynamic properties, with results aligning well with numerical data.

Contribution

It introduces a mean field approach to analyze ferrimagnetic chains, providing analytical insights into their ground state, excitations, and finite-temperature behavior.

Findings

Ground state exhibits ferrimagnetic long-range order.

Two branches of low-lying excitations are identified.

Thermodynamic quantities match numerical results at low temperatures.

Abstract

The Schwinger boson mean field theory is applied to the quantum ferrimagnetic Heisenberg chain. There is a ferrimagnetic long range order in the ground state. We observe two branches of the low lying excitation and calculate the spin reduction, the gap of the antiferromagnetic branch, and the spin fluctuation at $T=0K$. These results agree with the established numerical results quite well. At finite temperatures, the long range order is destroyed because of the disappearance of the Bose condensation. The thermodynamic observables, such as the free energy, magnetic susceptibility, specific heat, and the spin correlation at $T>0K$, are calculated. The $T\chi_{uni}$ has a minimum at intermediate temperatures and the spin correlation length behaves as $T^{-1}$ at low temperatures. These qualitatively agree with the numerical results and the difference is small at low temperatures.