Magnon heat conductivity and mean free paths in two-leg spin ladders: A model-independent determination

Abstract

The magnon thermal conductivity κmag of the spin ladders in Sr14 Cu24-x Znx O41 has been investigated at low doping levels x=0, 0.125, 0.25, 0.5, and 0.75. The Zn-impurities generate nonmagnetic defects which define an upper limit for lmag and, therefore, allow a clear-cut relation between lmag and κmag to be established independently of any model. Over a large temperature range we observe a progressive suppression of κmag with increasing Zn-content and find in particular that with respect to pure Sr14 Cu24 O41 κmag is strongly suppressed even in the case of tiny impurity densities where lmag 374. This shows unambiguously that large lmag 3000 which have been reported for Sr14 Cu24 O41 and La5 Ca9 Cu24 O41 on basis of a kinetic model are in the correct order of magnitude. © 2006 The American Physical Society.