310 research outputs found
Observation of shell structure in sodium nanowires
The quantum states of a system of particles in a finite spatial domain in
general consist of a set of discrete energy eigenvalues; these are usually
grouped into bunches of degenerate or close-lying levels, called shells. In
fermionic systems, this gives rise to a local minimum in the total energy when
all the states of a given shell are occupied. In particular, the closed-shell
electronic configuration of the noble gases produces their exceptional
stability. Shell effects have previously been observed for protons and neutrons
in nuclei and for clusters of metal atoms. Here we report the observation of
shell effects in an open system - a sodium metal nanowire connecting two bulk
sodium metal electrodes, which are progressively pulled apart. We measure
oscillations in the statistical distribution of conductance values, for contact
cross-sections containing up to a hundred atoms or more. The period follows the
law expected from shell-closure effects, similar to the abundance peaks at
`magic numbers' of atoms in metal clusters.Comment: The argumentation in favour of shell structure owing to the
fluctuations in the free energy of the nanowires has been strengthened.
Further improvements in the presentation include the plot of the radius of
the wires versus shell number in Fig.
Atomic size oscillations in conductance histograms for gold nanowires and the influence of work hardening
Nanowires of different nature have been shown to self-assemble as a function
of stress at the contact between two macroscopic metallic leads. Here we
demonstrate for gold wires that the balance between various metastable nanowire
configurations is influenced by the microstructure of the starting materials
and we discover a new set of periodic structures, which we interpret as due to
the atomic discreteness of the contact size for the three principal crystal
orientations.Comment: This version corrects an error in attributing the three observed
periods, and includes a comparison with recent model calculation
Crossover from Electronic to Atomic Shell Structure in Alkali Metal Nanowires
After making a cold weld by pressing two clean metal surfaces together, upon
gradually separating the two pieces a metallic nanowire is formed, which
progressively thins down to a single atom before contact is lost. In previous
experiments [1,2] we have observed that the stability of such nanowires is
influenced by electronic shell filling effects, in analogy to shell effects in
metal clusters [3]. For sodium and potassium at larger diameters there is a
crossover to crystalline wires with shell-closings corresponding to the
completion of additional atomic layers. This observation completes the analogy
between shell effects observed for clusters and nanowires.Comment: 4 page
Directional photoelectric current across the bilayer graphene junction
A directional photon-assisted resonant chiral tunneling through a bilayer
graphene barrier is considered. An external electromagnetic field applied to
the barrier switches the transparency in the longitudinal direction from
its steady state value T=0 to the ideal T=1 at no energy costs. The switch
happens because the a.c. field affects the phase correlation between the
electrons and holes inside the graphene barrier changing the whole angular
dependence of the chiral tunneling (directional photoelectric effect). The
suggested phenomena can be implemented in relevant experiments and in various
sub-millimeter and far-infrared optical electronic devices.Comment: 7 pages 5 figure
Effect of disorder on the conductance of a Cu atomic point contact
We present a systematic study of the effect of the disorder in copper point
contacts. We show that peaks in the conductance histogram of copper point
contacts shift upon addition of nickel impurities. The shift increases
initially linerarly with the nickel concentration, thus confirming that it is
due to disorder in the nanowire, in accordance with predictions. In general,
this shift is modelled as a resistance R_s which is placed in series with the
contact resistance R_c. However, we obtain different R_s values for the two
peaks in the histogram, R_s being larger for the peak at higher conductance.Comment: 6 pages, 4 figure
Onset of dissipation in ballistic atomic wires
Electronic transport at finite voltages in free-standing gold atomic chains
of up to 7 atoms in length is studied at low temperatures using a scanning
tunneling microscope (STM). The conductance vs voltage curves show that
transport in these single-mode ballistic atomic wires is non-dissipative up to
a finite voltage threshold of the order of several mV. The onset of dissipation
and resistance within the wire corresponds to the excitation of the atomic
vibrations by the electrons traversing the wire and is very sensitive to
strain.Comment: Revtex4, 4 pages, 3 fig
Andreev experiments on superconductor/ferromagnet point contacts
Andreev reflection is a smart tool to investigate the spin polarisation P of
the current through point contacts between a superconductor and a ferromagnet.
We compare different models to extract P from experimental data and investigate
the dependence of P on different contact parameters.Comment: 14 pages, 5 figures, accepted for publication in Fizika Nizkikh
Temperatu
Radiation Due to Josephson Oscillations in Layered Superconductors
We derive the power of direct radiation into free space induced by Josephson
oscillations in intrinsic Josephson junctions of highly anisotropic layered
superconductors. We consider the super-radiation regime for a crystal cut in
the form of a thin slice parallel to the c-axis. We find that the radiation
correction to the current-voltage characteristic in this regime depends only on
crystal shape. We show that at large enough number of junctions oscillations
are synchronized providing high radiation power and efficiency in the THz
frequency range. We discuss crystal parameters and bias current optimal for
radiation power and crystal cooling.Comment: 4 pages, 1 figure, to be published in Phys. Rev. Let
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