Magnon-phonon coupling in the spin-ladder compound Ca 9La5Cu24O41 measured by time-domain thermoreflectance

Below room temperature, the apparent thermal conductivity of the two-leg spin ladder, Ca9La5Cu24O41, as measured by time-domain thermoreflectance (TDTR), depends on the frequency of the applied surface heating. This occurs because the thermal penetration depth in the TDTR experiment is comparable to the length scale for the equilibration of the magnons that are the dominant channel for heat conduction and the phonons that dominate the heat capacity. We apply a two-temperature model to analyze the TDTR data and extract an effective volumetric magnon-phonon coupling parameter g for Ca9La5Cu24O41 at temperatures from 75 K to 300 K; g varies by approximately two orders of magnitude over this range of temperature and has the value g=1015Wm-3K-1 near the peak of the thermal conductivity at T≈180 K.