Nanoscale heat transport studied by high-resolution time-resolved x-ray diffraction

We report on synchrotron-based high-repetition rate ultrafast x-ray diffraction (UXRD) experiments monitoring the transport of heat from an epitaxial La0.7Sr0.3MnO3/SrTiO3 superlattice (SL) into the substrate on timescales from 100ps to 4µs. Transient thermal lattice expansion was determined with an accuracy of 10 7 , corresponding to a sensitivity to temperature changes down to 0.01K. We follow the heat flow within the SL and into the substrate after the impulsive laser heating leads to a small temperature rise of 1T = 6K. The transient lattice temperature can be simulated very well using the bulk heat conductivities. This contradicts the interpretation of previous UXRD measurements, which predicted a long-lasting expansion of SrRuO3 for more than 200ps. The disagreement could be resolved by assuming that the heat conductivity changes in the first hundred picoseconds.