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 La 0.7 Sr 0.3 MnO 3 /SrTiO 3 superlattice (SL) into the substrate on timescales from 100 ps to 4 μ s. Transient thermal lattice expansion was determined with an accuracy of 10 −7 , corresponding to a sensitivity to temperature changes down to 0.01 K. We follow the heat flow within the SL and into the substrate after the impulsive laser heating leads to a small temperature rise of Δ T = 6 K. 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 SrRuO 3 for more than 200 ps. The disagreement could be resolved by assuming that the heat conductivity changes in the first hundred picoseconds.