76 research outputs found
Quantum interference structures in the conductance plateaus of gold nanojunctions
The conductance of breaking metallic nanojunctions shows plateaus alternated
with sudden jumps, corresponding to the stretching of stable atomic
configurations and atomic rearrangements, respectively. We investigate the
structure of the conductance plateaus both by measuring the voltage dependence
of the plateaus' slope on individual junctions and by a detailed statistical
analysis on a large amount of contacts. Though the atomic discreteness of the
junction plays a fundamental role in the evolution of the conductance, we find
that the fine structure of the conductance plateaus is determined by quantum
interference phenomenon to a great extent.Comment: 4 pages, 4 figure
Conduction channels of superconducting quantum point contacts
Atomic quantum point contacts accommodate a small number of conduction
channels. Their number N and transmission coefficients {T_n} can be determined
by analyzing the subgap structure due to multiple Andreev reflections in the
current-voltage (IV) characteristics in the superconducting state. With the
help of mechanically controllable break-junctions we have produced Al contacts
consisting of a small number of atoms. In the smallest stable contacts, usually
three channels contribute to the transport. We show here that the channel
ensemble {T_n} of few atom contacts remains unchanged up to temperatures and
magnetic fields approaching the critical temperature and the critical field,
respectively, giving experimental evidence for the prediction that the
conduction channels are the same in the normal and in the superconducting
state.Comment: 8 pages, 5 .eps figures. To be published in Physica B 22
Electronic shell effects and the stability of alkali nanowires
Experimental conductance histograms for Na nanowires are analyzed in detail
and compared to recent theoretical results on the stability of cylindrical and
elliptical nanowires, using the free-electron model. We find a one-to-one
correspondence between the peaks in the histograms and the most stable nanowire
geometries, indicating that several of the commonly observed nanowires have
elliptical cross sections
Conductance fluctuations as a tool vor investigating the quantum modes in atomic size metallic contacts
Quantum Matter and Optic
Observation of Supershell Structure in Alkali Metal Nanowires
Nanowires are formed by indenting and subsequently retracting two pieces of
sodium metal. Their cross-section gradually reduces upon retraction and the
diameters can be obtained from the conductance. In previous work we have
demonstrated that when one constructs a histogram of diameters from large
numbers of indentation-retraction cycles, such histograms show a periodic
pattern of stable nanowire diameters due to shell structure in the conductance
modes. Here, we report the observation of a modulation of this periodic
pattern, in agreement with predictions of a supershell structure.Comment: Phys. Rev. Lett., in prin
Density of states and magnetoconductance of disordered Au point contacts
We report the first low temperature magnetotransport measurements on
electrochemically fabricated atomic scale gold nanojunctions. As , the
junctions exhibit nonperturbatively large zero bias anomalies (ZBAs) in their
differential conductance. We consider several explanations and find that the
ZBAs are consistent with a reduced local density of states (LDOS) in the
disordered metal. We suggest that this is a result of Coulomb interactions in a
granular metal with moderate intergrain coupling. Magnetoconductance of atomic
scale junctions also differs significantly from that of less geometrically
constrained devices, and supports this explanation.Comment: 5 pages, 5 figures. Accepted to PRB as Brief Repor
SINR Analysis of Opportunistic MIMO-SDMA Downlink Systems with Linear Combining
Opportunistic scheduling (OS) schemes have been proposed previously by the
authors for multiuser MIMO-SDMA downlink systems with linear combining. In
particular, it has been demonstrated that significant performance improvement
can be achieved by incorporating low-complexity linear combining techniques
into the design of OS schemes for MIMO-SDMA. However, this previous analysis
was performed based on the effective signal-to-interference ratio (SIR),
assuming an interference-limited scenario, which is typically a valid
assumption in SDMA-based systems. It was shown that the limiting distribution
of the effective SIR is of the Frechet type. Surprisingly, the corresponding
scaling laws were found to follow with , rather
than the conventional form.
Inspired by this difference between the scaling law forms, in this paper a
systematic approach is developed to derive asymptotic throughput and scaling
laws based on signal-to-interference-noise ratio (SINR) by utilizing extreme
value theory. The convergence of the limiting distribution of the effective
SINR to the Gumbel type is established. The resulting scaling law is found to
be governed by the conventional form. These novel results are
validated by simulation results. The comparison of SIR and SINR-based analysis
suggests that the SIR-based analysis is more computationally efficient for
SDMA-based systems and it captures the asymptotic system performance with
higher fidelity.Comment: Proceedings of the 2008 IEEE International Conference on
Communications, Beijing, May 19-23, 200
Heat dissipation in atomic-scale junctions
Atomic and single-molecule junctions represent the ultimate limit to the
miniaturization of electrical circuits. They are also ideal platforms to test
quantum transport theories that are required to describe charge and energy
transfer in novel functional nanodevices. Recent work has successfully probed
electric and thermoelectric phenomena in atomic-scale junctions. However, heat
dissipation and transport in atomic-scale devices remain poorly characterized
due to experimental challenges. Here, using custom-fabricated scanning probes
with integrated nanoscale thermocouples, we show that heat dissipation in the
electrodes of molecular junctions, whose transmission characteristics are
strongly dependent on energy, is asymmetric, i.e. unequal and dependent on both
the bias polarity and the identity of majority charge carriers (electrons vs.
holes). In contrast, atomic junctions whose transmission characteristics show
weak energy dependence do not exhibit appreciable asymmetry. Our results
unambiguously relate the electronic transmission characteristics of
atomic-scale junctions to their heat dissipation properties establishing a
framework for understanding heat dissipation in a range of mesoscopic systems
where transport is elastic. We anticipate that the techniques established here
will enable the study of Peltier effects at the atomic scale, a field that has
been barely explored experimentally despite interesting theoretical
predictions. Furthermore, the experimental advances described here are also
expected to enable the study of heat transport in atomic and molecular
junctions, which is an important and challenging scientific and technological
goal that has remained elusive.Comment: supporting information available in the journal web site or upon
reques
Effect of quantum interference in the nonlinear conductance of microconstrictions
The influence of the interference of electron waves, which were scattered by
single impurities, on nonlinear quantum conductance of metallic
microconstrictions (as was recently investigated experimentally) is studied
theoretically. The dependence of the interference pattern in the conductance
on the contact diameter and the spatial distribution of impurities is
analyzed. It is shown that the amplitude of conductance oscillation is strongly
depended on the position of impurities inside the constriction.Comment: 6 pages, 4 figures, To appear in PR
Shot noise suppression at room temperature in atomic-scale Au junctions
Shot noise encodes additional information not directly inferable from simple
electronic transport measurements. Previous measurements in atomic-scale metal
junctions at cryogenic temperatures have shown suppression of the shot noise at
particular conductance values. This suppression demonstrates that transport in
these structures proceeds via discrete quantum channels. Using a high frequency
technique, we simultaneously acquire noise data and conductance histograms in
Au junctions at room temperature and ambient conditions. We observe noise
suppression at up to three conductance quanta, with possible indications of
current-induced local heating and noise in the contact region at high
biases. These measurements demonstrate the quantum character of transport at
room temperature at the atomic scale. This technique provides an additional
tool for studying dissipation and correlations in nanodevices.Comment: 15 pages, 4 figures + supporting information (6 pages, 6 figures
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