Anomalous thermal conductivity of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_4$: Possibility of a polaronic glass

We present measurements of the thermal conductivity of ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$ and ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$Cu${\mathrm{O}}_4$. The results on the pure sample show that lattice conduction is predominant with a $T^{1.8}$ dependence at low temperatures and a strong $T$-independent scattering above 30 K. We observe a similar behavior for the superconductor but with a $T^{1.4}$ law at low temperature and a small electronic contribution in the normal state. These results together with the behavior of the specific heat and sound attenuation are consistent with the existence of a two-level tunneling system for an amorphous state.