Dr. Sebastian Reparaz 
Materials Department
Material Science Institute of Barcelona (ICMAB-CSIC)
08193 Bellaterra (Spain)
departments.icmab.es/nanopto/

Aspects of Current Interest in Nanoscale Thermal Transport: Second Sound and Thermal Anisotropy
Second sound is known as the thermal transport regime where heat is carried by temperature waves. Its experi-
mental observation was previously restricted to a small number of materials, usually in rather narrow temperature
windows. We show that it is possible to overcome these limitations by driving the system with a rapidly varying
temperature field. High-frequency second sound is demonstrated in bulk natural Ge between 7 K and room tem-
perature by studying the phase lag of the thermal response under a harmonic high-frequency external thermal
excitation and addressing the relaxation time and the propagation velocity of the heat waves. These results pro-
vide a route to investigate the potential of wave-like heat transport in almost any material, opening opportunities
to control heat through its oscillatory nature.