Berlin 2005 – scientific programme
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TT: Tiefe Temperaturen
TT 16: Posters Correlated Electrons, Measuring Devices, Cryotechnique
TT 16.38: Poster
Saturday, March 5, 2005, 11:00–16:30, Poster TU C
Triplet excitations of bond-disordered spin-1/2 ladders — •Marcelo Arlego1,2, Wolfram Brenig1, Daniel Cabra3, Fabian Heidrich-Meisner1, Andreas Honecker1, and Gerardo Rossini3,2 — 1Technische Universität Braunschweig, Institut für Theoretische Physik, Mendelssohnstraße 3, 38106 Braunschweig, Germany — 2Universidad Nacional de La Plata, Departamento de Fisica, C.C. 67, (1900) La Plata, Argentina — 3Université Louis Pasteur, Laboratoire de Physique Théorique, 3 Rue de l’Université, 67084 Strasbourg Cedex, France
We analyze the effect of weak bond disorder in two-leg spin ladders, focusing on the appearance of bound states in the spin gap. Performing a projection on the single-triplet subspace we analytically derive the position of bound states for single-impurity and small impurity clusters in the strong-coupling limit, i.e., strong dimerization. Both the case of modified exchange couplings on the rungs and on the legs of the ladder are studied. Numerically, we analyze the single-impurity problem in a spin ladder by the Lanczos method to obtain the low-lying excitations. The case of a finite concentration of impurities is treated to leading order in the interung coupling within the single-triplet subspace by numerical impurity averaging of large systems. We compare the spectra obtained numerically with the results of diagrammatic techniques, particularly the coherent-potential approximation (CPA). Since the CPA does not account for the interference of impurities we discuss the contribution of small impurity clusters to the density of states.