  Second sound is known as the thermal transport regime where heat is carried
by temperature waves. Its experimental 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. This effect is demonstrated in bulk
Ge between 7 kelvin and room temperature, studying the phase lag of the thermal
response under a harmonic high frequency external thermal excitation,
addressing the relaxation time and the propagation velocity of the heat waves.
These results provide a new route to investigate the potential of wave-like
heat transport in almost any material, opening opportunities to control heat
through its oscillatory nature.
