Magnon heat transport in (Sr,Ca,La)₁₄Cu₂₄O₄₁

We have measured the thermal heat conductivity κ of the compounds Sr₁₄Cu₂₄O₄₁ and Ca₉La₅Cu₂₄O₄₁ containing doped and undoped spin ladders, respectively. We find a huge anisotropy of both, the size and the temperature dependence of κ that we interpret in terms of a very large heat conductivity due to the magnetic excitations of the one-dimensional spin ladders. This magnon heat conductivity decreases with increasing hole doping of the ladders. Magnon heat transport is analyzed theoretically using a simple kinetic model. From this analysis we determine the spin gap and the temperature-dependent mean free path of the magnons, which measures about several thousand angstrom at low temperature. The relevance of several scattering channels for the magnon transport is discussed.