We investigate the effect of disorder on the heat transport properties of the $S=tfrac{1}{2}$ Heisenberg chain compound Sr$_2$CuO$_3$ upon chemically substituting Sr by increasing concentrations of Ca. As Ca occupies sites outside but near the Cu-O-C
u 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 ($kappa_{mathrm{mag}}$) due to spinons propagating in the chains is gradually but strongly suppressed with increasing amount 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 ($sim$ 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 to cover the whole doping regime between the clean and the dirty limits. Interestingly, at maximum impurity level we observe, in Ca-doped Sr$_2$CuO$_3$, an almost linear increase of $kappa_{mathrm{mag}}$ 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, SrCuO$_2$, where the effect of Ca seems to saturate already at intermediate doping levels.
We have investigated the thermal conductivity kappa_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 th
e spinon heat conductivity kappa_mag is strongly enhanced as compared to previous results obtained on samples with lower chemical purity. The analysis of kappa_mag allows to compute the spinon mean free path l_mag as a function of temperature. At low-temperature we find l_magsim0.5mum, corresponding to more than 1200 chain unit cells. Upon increasing the temperature, the mean free path decreases strongly and approaches an exponential decay ~1/T*exp(T*/T) which is characteristic for umklapp processes with the energy scale k_B T*. Based on Matthiesens rule we decompose l_mag into a temperature-independent spinon-defect scattering length l0 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.
We study the impact of a weak bond disorder on the spinon heat transport in the S=1/2 antiferromagnetic (AFM) Heisenberg chain material Sr_{1-x}Ca_xCuO_2. We observe a drastic suppression in the magnetic heat conductivity kappa_mag even at tiny disor
der levels (i.e., Ca-doping levels), in stark contrast to previous findings for kappa_mag of S=1/2 two-dimensional square lattice and two-leg spin-ladder systems, where a similar bond disorder has no effect on kappa_mag. Hence, our results underpin the exceptional role of integrability of the S=1/2 AFM Heisenberg chain model and suggest that the bond disorder effectively destroys the ballistic nature of its heat transport. We further show that the suppression of kappa_mag is captured by an effective spinon-impurity scattering length, which exhibits the same doping dependence as the long-distance exponential decay length of the spin-spin correlation as determined by density-matrix renormalization group calculations.
We report experimental results on the heat conductivity kappa of the S=1/2 spin chain compounds TiOBr and TiOCl for temperatures 5K<T<300K and magnetic fields up to 14. Surprisingly, we find no evidence of a significant magnetic contribution to kappa
, which is in stark contrast to recent results on S=1/2 spin chain cuprates. Despite this unexpected result, the thus predominantly phononic heat conductivity of these spin-Peierls compounds exhibits a very unusual behavior. In particular, we observe strong anomalies at the phase transitions Tc1 and Tc2. Moreover, we find an overall but anisotropic suppression of kappa in the intermediate phase which extends even to temperatures higher than Tc2. An external magnetic field causes a slight downshift of the transition at Tc1 and enhances the suppression of kappa up to Tc2. We interprete our findings in terms of strong spin-phonon coupling and phonon scattering arising from spin-driven lattice distortions.
Fundamental conservation laws predict ballistic, i.e., dissipationless transport behaviour in one-dimensional quantum magnets. Experimental evidence, however, for such anomalous transport has been lacking ever since. Here we provide experimental evid
ence for ballistic heat transport in a S=1/2 Heisenberg chain. In particular, we investigate high purity samples of the chain cuprate SrCuO2 and observe a huge magnetic heat conductivity $kappa_{mag}$. An extremely large spinon mean free path of more than a micrometer demonstrates that $kappa_{mag}$ is only limited by extrinsic scattering processes which is a clear signature of ballistic transport in the underlying spin model.
