We present results of the thermal conductivity of $\rm La_2CuO_4$ and $\rm La_{ 1.8}Eu_{ 0.2}CuO_4$ single-crystals which represent model systems for the two-dimensional spin-1/2 Heisenberg antiferromagnet on a square lattice. We find large anisotropies of the thermal conductivity, which are explained in terms of two-dimensional heat conduction by magnons within the CuO$_2$ planes. Non-magnetic Zn substituted for Cu gradually suppresses this magnon thermal conductivity $\kappa_{ \mathrm{ mag}}$. A semiclassical analysis of $\kappa_{ \mathrm{ mag}}$ is shown to yield a magnon mean free path which scales linearly with the reciprocal concentration of Zn-ions.
Categories: cond-mat.str-el