Heat conductivity of the spin-Peierls compounds TiOCl and TiOBr

We report experimental results on the heat conductivity kappa of the S=1/2 spin chain compounds TiOBr and TiOCl for temperatures 5 K <T <300 K and magnetic fields up to 14 T. 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 T(c1) and T(c2). Moreover, we find an overall but anisotropic suppression of kappa in the intermediate phase which extends even to temperatures higher than T(c2). An external magnetic field causes a slight downshift of the transition at T(c1) and enhances the suppression of kappa up to T(c2). We interpret our findings in terms of strong spin-phonon coupling and phonon scattering arising from spin-driven lattice distortions.