dimensional quantum magnets. Experimental evidence, however, for such anomalous
transport has been lacking ever since. Here we provide experimental evidence for ballistic
heat transport in a S= 1/2 Heisenberg chain. In particular, we investigate high purity samples
of the chain cuprate SrCuO 2 and observe a huge magnetic heat conductivity κ mag. An
extremely large spinon mean-free path of more than a micrometer demonstrates that κ mag …