17 MARCH 2017 • VOL 355 ISSUE 6330 1125 SCIENCE sciencemag.org
By Dvira Segal
Theexistenceofuniversalupperbounds (limits) for the rates of transport of electricity and thermal energy is a strikingmanifestationofquantumme- chanics. These fundamental bounds can be revealed in low-dimensional
constrictions defining a single transport
channel. The discrete unit (quantum) of electrical conductance G0 has been observed in
many experiments dating back to 1988 (1, 2),
but the quantum of thermal conductance G0, Th
has been much more challenging to probe.
Unlike tiny electrical currents, it is much
harder to measure minute heat currents in
a reproducible manner. On page 1192 of this
issue, Cui et al. (3) conquer this challenge
by developing an experimental platform for
studying quantum thermal transport at the
atomic limit. Their work reveals that thermal
conductance can be quantized even at room
temperature, as well as the fundamental relation between the thermal and electrical
conductances. This study paves the way for
investigations of thermal phenomena in individual molecular devices, with technological
ramifications for controlling energy transport in nanoscale electronic circuitry.
The Landauer theory (4) describes transport of wavelike electrons. It predicts that
the electrical conductance (the inverse of
resistance) of perfect channels is given by
G0 = 2e2/h, where e is the electron charge and h
is Planck’s constant (the factor of 2 accounts
for spin-up and spin-down electrons). This
value, an upper bound for the electrical conductance of a single channel, only depends
on fundamental constants. Thermal conductance is quantized in the basic unit of
G0,Th = p2kB2 T/(3h), where kB is Boltzmann’s
constant and T is temperature. This value
represents the maximum rate at which
thermal energy can be transported through
a single channel and is truly universal. It is
indifferent to the material properties and
the type (statistics) of energy carriers, and
has been verified experimentally for transport by phonons (5), photons (6–8), and
electrons (9, 10).
It is evident from the above that the thermal conductance quantum is related to the
electrical conductance quantum by fundamental constants and temperature, but this
relation is valid more generally and in radically distinct scenarios. First, for phase-co-herent quantum transport (possibly through
multiple and imperfect channels), Landauer
theory asserts that the electronic contribution to the thermal conductance GTh,e
relates to the electrical conductance Ge as
G Th,e = TL0Ge, where L0 = (p2/3)(kB/e)2 is the
Lorenz number. This relation holds so long as
the electronic transmission function across
the constriction only varies weakly with
energy in the relevant range (around the
Fermi energy), on the scale of the thermal
energy kBT.
Nevertheless, it is intriguing to recall that
macroscopic, classical (ohmic) metal wires
follow the same relation, originally put forward in 1853, in an empirical form. This
Wiedemann-Franz law can thus be observed
in different situations, whether transport is
governed by quantum or classical principles.
Its assessment, and possibly invalidation,
can reveal microscopic details about transport, such as which coherent regime dominates charge transport in a given system, or
if many-body interactions are influential or
the noninteracting-electron model holds.
To address basic questions in electronic
thermal transport at the nanoscale, measurements in the ultimate microscopic
limit are required. Cui et al. report on an
experimental setup with precise and decisive measurements of the thermal conductance of single-atom junctions, with the
quantum of conductance observed at room
temperature. By studying the electrical and
thermal conductances of both gold (Au)
and platinum (Pt) junctions concurrently,
NANOSCALE TRANSPORT
Probing the limits of heat flow
Studies of atomic point contacts reveal the fundamental
quantum of heat transport
Department of Chemistry and Centre for Quantum
Information and Quantum Control, University of Toronto, 80
Saint George Street, Toronto, Ontario M5S 3H6, Canada.
Email: dsegal@chem.utoronto.ca
“Probing the quantum
of thermal transport in
atomic junctions requires
exceptional thermal
sensitivity, stability, and
reproducibility…”
diphosphate (UDP) mediates changes in
appetite by activating purinergic P2Y6
receptors on hypothalamic neurons that
release Agouti-related protein, a potent
appetite stimulator. These neurons belong
to a brain circuitry that controls systemic
metabolism (6). This effect of UDP likely
offers a glimpse into the complexity of how
changing uridine plasma concentrations
may differentially affect metabolic homeostasis in an organ-specific manner.
Metabolic studies on uridine, its regulation, and the effects on energy metabolism
are particularly intriguing in the light of
how mammals respond to food shortage.
A uridine-induced reduction in body temperature would save energy, and promotion of glucose uptake by cells may help
to overcome cellular energy deficits. Uridine may act in a way similar to that of a
small-molecule hunger signal. Although
uridine supplements appear to have no effects because they are excreted in bile, this
clearance system appears to be not fully
developed in human infants, where uridine
monophosphates enriched in mother’s milk
can enter into and potentially control the
infant’s metabolism (7). In this situation,
uridine may act as a nutrient-derived hormone (“nutrikine”).
This discovery of uridine’s physiological
roles in maintaining systemic metabolic
balance is undoubtedly of fundamental and
broad importance to biology and medicine.
Plasma uridine controls dietary responses
and modulates leptin signaling, thereby
governing glucose and insulin sensitivity.
Specifically, targeting UDP signaling may
offer intriguing potential for treating the
metabolic disease states characterized by
adiposity, impaired glucose tolerance, and
hypercholesterinemia. Tissue-specific approaches could include the hypothalamic
inhibition of the P2Y6 receptor to mitigate
food intake and to improve systemic insulin sensitivity (8).
If uridine can help to reverse the world-
wide obesity and diabetes pandemic, this
fifth base will surely take center stage in
metabolism research. j
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2. P. P. Bera et al., J. Chem. Phys. 144, 144308 (2016).
3. Z. Martins etal.,Earth Planet.Sci.Lett. 270, 130 (2008).
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6. S. M. Steculorum etal.,Cell 162, 1404 (2015).
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8. S. M. Steculorum etal.,Cell Rep. 18, 1587 (2017).
