Bond disorder and spinon heat transport in the S=1/2 Heisenberg spin chain compound Sr2CuO3: From clean to dirty limits

We investigate the effect of disorder on the heat transport properties of the S = 1/2 Heisenberg spin chain compound Sr2CuO3 upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu-O-Cu spin chains, bond disorder, i.e., a spatial variation of the exchange interaction J, is expected to be realized in these chains. We observe that the magnetic heat conductivity (kappa(mag)) due to spinons propagating in the chains is gradually but strongly suppressed with increasing amounts of Ca, where the doping dependence can be understood in terms of increased scattering of spinons due to Ca-induced disorder. This is also reflected in the spinon mean-free path which can be separated in a doping-independent but temperature-dependent scattering length due to spinon-phonon scattering, and a temperature-independent but doping-dependent spinon-defect scattering length. The latter spans from very large (>1300 lattice spacings) to very short (similar to 12 lattice spacings) and scales with the average distance between two neighboring Ca atoms. Thus, the Ca-induced disorder acts as an effective defect within the spin chain, and the doping scheme allows us to cover the whole doping regime between the clean and dirty limits. Interestingly, at maximum impurity levels, we observe in Ca-doped Sr2CuO3 an almost linear increase of kappa(mag) at temperatures above 100 K which reflects the intrinsic low-temperature behavior of heat transport in a Heisenberg spin chain. These findings are quite different from that observed for the Ca-doped double spin chain compound, SrCuO2, where the effect of Ca seems to saturate already at intermediate doping levels.