Ballistic heat transport of quantum spin excitations

We provide experimental evidence for ballistic heat transport in the spin chain material SrCuO2. In particular, we investigate high purity samples of this double chain cuprate and observe a huge magnetic heat conductivity κmag. An extremely large spinon mean free path of more than a micrometer demonstrates that κmag is only limited by extrinsic scattering processes which is a clear signature of ballistic transport in the underlying spin model. Furthermore we study the influence of magnetic Ni and non-magnetic Mg impurities on κmag of SrCuO2. While Ni-doping has a large impact on the magnetic thermal conductivity, Mg-doping shows no influence. In order to clarify this surprising behavior we compare κmag to measurements of the single chain compound Sr2CuO3.