Bound states in weakly disordered spin ladders

TL;DR

This paper investigates how weak bond disorder in spin-1/2 ladders leads to bound states within the spin gap, using analytical and numerical methods to understand impurity effects and density of states.

Contribution

It introduces a projection method for bound state analysis in disordered spin ladders and compares analytical results with numerical simulations for impurity clusters.

Findings

Bound states appear in the spin gap due to weak disorder.

Analytical predictions match numerical results for impurity clusters.

Density of states structure is influenced by impurity cluster configurations.

Abstract

We study the appearance of bound states in the spin gap of spin-1/2 ladders induced by weak bond disorder. Starting from the strong-coupling limit, i.e., the limit of weakly coupled dimers, we perform a projection on the single-triplet subspace and derive the position of bound states for the single impurity problem of one modified coupling as well as for small impurity clusters. The case of a finite concentration of impurities is treated with the coherent-potential approximation in the strong-coupling limit and compared with numerical results. Furthermore, we analyze the details in the structure of the density of states and relate their origin to the influence of impurity clusters.