INSIGHTS | PERSPECTIVES
1126 17 MARCH 2017 • VOL 355 ISSUE 6330 sciencemag.org SCIENCE
fundamental issues in quantum thermal
transport were carefully scrutinized (see the
figure, left panel).
Cui et al. found that Au contacts displayed the quantum of thermal conductance in the atomic limit, whereas Pt
junctions did not resolve this quantization.
First-principle calculations confirmed that
Au supports a single “fully open” channel
for charge conduction, but the electronic
structure of Pt results in multiple, imperfect transmission channels, each operating
below the quantum bound. The Wiedemann-Franz law, however, is firmly verified for both Au and Pt atomic contacts, in
agreement with the assumed phase-coher-ent conduction mechanism.
Probing the quantum of thermal transport in atomic junctions requires exceptional thermal sensitivity, stability, and
reproducibility, and the work of Cui et al.
constitutes a major technical feat. In the experiment, the atomic junction was formed
between a hot substrate (TS = 315 K) and
a cold scanning probe (TP = 295 K), whose
change in temperature DTP was monitored
by measuring changes in its electrical resistance. The heat balance equation organizes
the thermal conductance of the atomic
junction as GTh = (DTP/RP)/( TS – TP – DTP),
where RP is the thermal resistance of the
probe. Thus, to resolve quantitatively the
thermal conductance quantum (times 2 for
spin degeneracy) at room temperature of
~560 p W K–1, an excellent temperature resolution and a high thermal resistance had to
By designing a setup with the character-
istics of DTP 0.6 mK and RP 1.3 × 106
K W–1, Cui et al. reached a truly remark-
able thermal conductance resolution of 25
p W K–1. Thorough simulations reconstructed
the experiment and offered a clear view
into this microscopic world (see the figure,
right panel). Relevant structures were iden-
tified (atomic “dimers”), atomic motion
and charge flow were followed, and the
contribution of phonons to the measured
thermal conductance was estimated and
found to be noninfluential.
The experimental tools described by Cui
et al. will enable fundamental explorations
of thermal transport in molecular electronic
devices with tailored properties (11) and in
interacting systems that deviate from the
well-established Landauer framework (4).
Enhancing thermal conduction in electronic
circuits improves stability and performance,
whereas suppressing it is important for
other functions, such as thermoelectric efficiency. In either case, the work of Cui et al.
surely promises exciting advances to both
basic science and technology. j
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Withdrawing a calorimetric scanning tip creates an
atomic point contact.
conductance of the
atomic junction is
measured by the tip.
The Landauer quantum transport theory combines
contributions from multiple conduction channels.
In this model,
phases in and out
of the channels.
For gold chains, only a single channel
contributes, and the transmission probability
t(E) reaches the maximum value of 1.
say, no Sir
of an aggregated protein
controls yeast cell aging
By Aaron D. Gitler1 and Daniel F. Jarosz2,3
Is aging ineluctable, or are there genetic programs of aging that could be ma- nipulated to extend life span? Research over the past two decades has provided powerful evidence that aging is indeed regulated by genes that control highly
conserved pathways (1). For example, mutations in single genes in model organisms
like flies and worms not only allow these
animals to live longer, but rejuvenate them
as well (2). Even the single-celled budding
yeast Saccharomyces cerevisiae ages, and
specific genes likewise control this process
(1). Indeed, one of the most powerful genetic manipulations to extend life span was
discovered in yeast. The histone deacetylase silent information regulator 2 (Sir2)
is required for repressing the transcription
of certain mating-type loci, telomeres, and
ribosomal DNA (3–5). The latter had been
linked to aging in yeast, inspiring studies
that revealed Sir2’s importance for this
process (6). Subsequent work in yeast and
animal models established that changes in
Sir2 activity are responsible for much of
the life span–extending effects of caloric
restriction (7). On page 1184 of this issue,
Schlissel et al. (8) report that a particular
facet of aging, which had long been attributed to age-dependent changes in Sir2
function, is caused by a new mechanism.
To study aging in budding yeast, researchers count the number of times a
daughter cell buds off from a mother
cell. A mother has only a fixed number of
times to divide to produce a daughter. As
she gets older (produces more daughters),
she undergoes characteristic physiological
changes, including sterility. That is, haploid mother cells lose the ability to mate.
Haploid cells exist as either one of two
mating types—a or a. Mating-type information is encoded by the MAT locus. Extra
1Department of Genetics, Stanford University School of
Medicine, Stanford, CA 94305, USA. 2Department of Chemical
and Systems Biology, Stanford University School of Medicine,
Stanford, CA 94305, USA. 3Department of Developmental
Biology, Stanford University School of Medicine, Stanford, CA
94305, USA. Email: firstname.lastname@example.org; email@example.com
Quantum transport in single-atom contacts
Cui et al. measured the thermal conductance of the atomic contact with a calorimetric scanning
probe (left). A theoretical model (right) formulates electron transport coefficients. For gold atomic contacts,
a single channel perfectly contributes, corresponding to the observation of the conductance quantum.