Transient Hydrodynamic Lattice Cooling by Picosecond Laser Irradiation\n of Graphite

Abstract

Recent theories and experiments have suggested hydrodynamic phonon transport\nfeatures in graphite at unusually high temperatures. Here, we report a\npico-second pump-probe thermal reflectance measurement of heat pulse\npropagation in graphite. The measurement results reveal transient lattice\ncooling near the adiabatic center of a 15 $\\mu$m diameter ring-shape pump beam\nat temperatures between 80 and 120 K. While such lattice cooling has not been\nreported in recent diffraction measurements of second sound in graphite, the\nobservation here is consistent with both hydrodynamic phonon transport theory\nand prior heat pulse measurements of second sound in bulk sodium fluoride.\n