Spinon heat transport and spin-phonon interaction in the spin-1/2 Heisenberg chain cuprates Sr2CuO3 and SrCuO2

We have investigated the thermal conductivity k(mag) of high-purity single crystals of the spin chain compound Sr2CuO3, which is considered an excellent realization of the one-dimensional spin-1/2 antiferromagnetic Heisenberg model. We find that the spinon heat conductivity k(mag) is strongly enhanced as compared to previous results obtained on samples with lower chemical purity. The analysis of k(mag) allows the computation of the spinon mean free path l(mag) as a function of temperature. At low temperature we find l(mag) similar to 0.5 mu m, corresponding to more than 1200 chain unit cells. Upon increasing the temperature, the mean free path decreases strongly and approaches an exponential decay similar to(1/T) exp (T-u*/T), which is characteristic for Umklapp processes with the energy scale k(B)T(u)*. Based on Matthiessen's rule we decompose lmag into a temperature-independent spinon-defect scattering length l(0) and a temperature-dependent spinon-phonon scattering length l(sp)(T). By comparing l(mag)(T) of Sr2CuO3 with that of SrCuO2, we show that the spin-phonon interaction, as expressed by l(sp), is practically the same in both systems. The comparison of the empirically derived l(sp) with model calculations for the spin-phonon interaction of the one-dimensional spin-1/2 XY model yields reasonable agreement with the experimental data.