Optical probing of anisotropic heat transport in the quantum spin ladder Ca9La5Cu24O41

A transient thermal imaging technique is used to monitor heat diffusion at the surface of the antiferromagnetic spin ladder material Ca9La5Cu24O41. This material shows highly anisotropic thermal conductivity due to a large uni-directional magnetic heat transport along the ladders. The thermal conductivity is measured using optical heating as well as electrical heating, yielding 37 ± 3 W m−1 K−1 for the fast (ladder) direction and 2.5 ± 0.5 W m−1 K−1 for the slow direction, respectively. The fast direction result is in agreement with the thermal conductivity measured using other dynamic methods, but about 60% lower than the thermal conductivity measured using steady state methods.