Bond impurity induced bound states in disordered spin-1/2 spin ladders

MARCELO ARLEGO; WOLFRAM BRENIG; DANIEL CABRA; FABIAN HEIDRICH-MEISNER; ANDREAS HONECKER; GERARDO ROSSINI

PHYSICAL REVIEW B - CONDENSED MATTER AND MATERIALS PHYSICS

The American Physical Society

Año: 2004 vol. 70 p. 14436 - 14444

0163-1829

We discuss the effect of weak  bond-disorder in two-leg spin ladders on the dispersion relation of the elementary triplet excitations with a particular focus on the appearance of bound states in the spin gap. Both the case of modified exchange couplings on  the rungs  and the legs of the ladder are analyzed.Based on a projection on the single-triplet subspace, the single impurity and small cluster problems are treated analytically in the  strong-coupling limit. Numerically, we study the problem of a single impurity in a spin ladder by exact diagonalization  to obtain the low lying excitations.At finite concentrations and to leading order in the inter-rung coupling, we compare the spectra obtainedfrom numerical diagonalization of large systems within the single-triplet subspace with the results of diagrammatic techniques, namely low-concentration and coherent-potential approximation. The contribution of  small impurity clusters to the density of states is also discussed.