A variational approach to the low energy properties of even-legged d-dimensional quantum spin systems

TL;DR

This paper introduces a variational method to analyze the low energy spectra of frustrated quantum spin systems, demonstrating the existence of a finite energy gap in ladder systems of various dimensions.

Contribution

It develops a variational formalism applicable to frustrated quantum spin systems, extending to higher dimensions and arbitrary coupling ranges.

Findings

Finite energy gap in the variational ground state.

Applicability to ladders of any size and higher-dimensional systems.

Potential for level degeneracies at specific coupling strengths.

Abstract

We develop a variational formalism in order to study the structure of low energy spectra of frustrated quantum spin systems. It is first applied to trial wavefunctions of ladders with one spin-1/2 on each site. We determine energy minima of these states. The variational ground state shows a finite energy gap with respect to the energies of states which span the Hilbert space and are orthogonal to it. This is the case for any size of the system. Under some justifiable approximations the argument can be extended to even-legged ladder systems in 2d and higher dimensional spaces. The Hamiltonian can contain spin-spin coupling interactions of any range. For specific values of the coupling strengths level degeneracies can occur.