Thermal conductivity of doped La2CuO4 as an example for heat transport by optical phonons in complex materials

We investigate the phonon thermal conductivity kappa(ph) of doped La2CuO4 based on out-of-plane thermal conductivity measurements. When room temperature is approached the temperature dependence of kappa(ph) strongly deviates from the T-1-decrease which is usually expected for heat transport by acoustic phonons. Instead, kappa(ph) decreases much weaker or even increases with rising temperature. Simple arguments suggest that such unusual temperature dependencies of kappa(ph) are caused by heat transport via dispersive optical phonons.