According to Fourier’s law,
a temperature difference across
a material results in a linear temperature profile and a thermal conductance
that decreases inversely proportional to the system length. These
are the hallmarks of diffusive heat flow. Here, we report heat flow
in ultrathin (25 nm) GaP nanowires in the absence of a temperature
gradient within the wire and find that the heat conductance is independent
of wire length. These observations deviate from Fourier’s law
and are direct proof of ballistic heat flow, persisting for wire lengths
up to at least 15 μm at room temperature. When doubling the
wire diameter, a remarkably sudden transition to diffusive heat flow
is observed. The ballistic heat flow in the ultrathin wires can be
modeled within Landauer’s formalism by ballistic phonons with
an extraordinarily long mean free path.
In this work we present a method to quantitatively measure the optical absorption of single nanowires that can be applied over a wide range of temperatures and with a high enough sensitivity to enable the measurement of below-band-gap absorption (as well as the absorption of single molecules). The method is based on accurately measuring the heat flow coming from a nanowire when it is illuminated by a laser beam. We experimentally verify this method by measuring the absorption of both a zincblende and a wurtzite GaAs, a wurtzite GaP, and a superlattice Zn 3 P 2 nanowire. Furthermore, we find that the Zn 3 P 2 nanowires have the largest absorption of all these materials. We analyze the advantages and disadvantages of the method and study its range of applicability.
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