214 research outputs found
Observation of electronic and atomic shell effects in gold nanowires
The formation of gold nanowires in vacuum at room temperature reveals a
periodic spectrum of exceptionally stable diameters. This is identified as
shell structure similar to that which was recently discovered for alkali metals
at low temperatures. The gold nanowires present two competing `magic' series of
stable diameters, one governed by electronic structure and the other by the
atomic packing.Comment: 4 pages, 4 figure
Landau diamagnetism and magnetization of interacting diffusive conductors
We show how the orbital magnetization of an interacting disordered diffusive
electron gas can be simply related to the magnetization of the non-interacting
system having the same geometry. This result is applied to the persistent
current of a mesoscopic ring and to the relation between Landau diamagnetism
and the interaction correction to the magnetization of diffusive systems. The
field dependence of this interaction contribution can be deduced directly from
the de Haas-van Alphen oscillations of the free electron gas. Known results for
the free orbital magnetism of finite systems can be used to derive the
interaction contribution in the diffusive regime in various geometries.Comment: 4 pages, 2 figure
Electronic and optical properties of electromigrated molecular junctions
Electromigrated nanoscale junctions have proven very useful for studying
electronic transport at the single-molecule scale. However, confirming that
conduction is through precisely the molecule of interest and not some
contaminant or metal nanoparticle has remained a persistent challenge,
typically requiring a statistical analysis of many devices. We review how
transport mechanisms in both purely electronic and optical measurements can be
used to infer information about the nanoscale junction configuration. The
electronic response to optical excitation is particularly revealing. We briefly
discuss surface-enhanced Raman spectroscopy on such junctions, and present new
results showing that currents due to optical rectification can provide a means
of estimating the local electric field at the junction due to illumination.Comment: 19 pages, 8 figures, invited paper for forthcoming special issue of
Journal of Physics: Condensed Matter. For other related papers, see
http://www.ruf.rice.edu/~natelson/publications.htm
Method to determine defect positions below a metal surface by STM
The oscillatory voltage dependence of the conductance of a quantum point
contact in the presence of a single point-like defect has been analyzed
theoretically. Such signals are detectable and may be exploited to obtain
information on defect positions below a metal surface. Both tunnel junctions
and ballistic contacts of adiabatic shape have been considered. The effect of
quantum interference has been taking into account between the principal wave
that is directly transmitted through the contact and the partial wave that is
scattered by the contact and the defect. This effect leads to oscillations of
the conductance as a function of applied voltage. We obtain the dependence of
the period and amplitude of the conductance oscillations on the position of the
defect inside the metal.Comment: 16 pages, 7 figure
Large tunable image-charge effects in single-molecule junctions
The characteristics of molecular electronic devices are critically determined
by metal-organic interfaces, which influence the arrangement of the orbital
levels that participate in charge transport. Studies on self-assembled
monolayers (SAMs) show (molecule-dependent) level shifts as well as
transport-gap renormalization, suggesting that polarization effects in the
metal substrate play a key role in the level alignment with respect to the
metal's Fermi energy. Here, we provide direct evidence for an electrode-induced
gap renormalization in single-molecule junctions. We study charge transport in
single porphyrin-type molecules using electrically gateable break junctions. In
this set-up, the position of the occupied and unoccupied levels can be followed
in situ and with simultaneous mechanical control. When increasing the electrode
separation, we observe a substantial increase in the transport gap with level
shifts as high as several hundreds of meV for displacements of a few \aa
ngstroms. Analysis of this large and tunable gap renormalization with
image-charge calculations based on atomic charges obtained from density
functional theory confirms and clarifies the dominant role of image-charge
effects in single-molecule junctions
Theory of oscillations in the STM conductance resulting from subsurface defects (Review Article)
In this review we present recent theoretical results concerning
investigations of single subsurface defects by means of a scanning tunneling
microscope (STM). These investigations are based on the effect of quantum
interference between the electron partial waves that are directly transmitted
through the contact and the partial waves scattered by the defect. In
particular, we have shown the possibility imaging the defect position below a
metal surface by means of STM. Different types of subsurface defects have been
discussed: point-like magnetic and non-magnetic defects, magnetic clusters in a
nonmagnetic host metal, and non-magnetic defects in a s-wave superconductor.
The effect of Fermi surface anisotropy has been analyzed. Also, results of
investigations of the effect of a strong magnetic field to the STM conductance
of a tunnel point contact in the presence of a single defect has been
presented.Comment: 31 pages, 10 figuers Submitted to Low. Temp. Phy
Orbital Magnetism and Current Distribution of Two-Dimensional Electrons under Confining Potential
The spatial distribution of electric current under magnetic field and the
resultant orbital magnetism have been studied for two-dimensional electrons
under a harmonic confining potential V(\vecvar{r})=m \omega_0^2 r^2/2 in
various regimes of temperature and magnetic field, and the microscopic
conditions for the validity of Landau diamagnetism are clarified. Under a weak
magnetic field (\omega_c\lsim\omega_0, \omega_c being a cyclotron frequency)
and at low temperature (T\lsim\hbar\omega_0), where the orbital magnetic
moment fluctuates as a function of the field, the currents are irregularly
distributed paramagnetically or diamagnetically inside the bulk region. As the
temperature is raised under such a weak field, however, the currents in the
bulk region are immediately reduced and finally there only remains the
diamagnetic current flowing along the edge. At the same time, the usual Landau
diamagnetism results for the total magnetic moment. The origin of this dramatic
temperature dependence is seen to be in the multiple reflection of electron
waves by the boundary confining potential, which becomes important once the
coherence length of electrons gets longer than the system length. Under a
stronger field (\omega_c\gsim\omega_0), on the other hand, the currents in
the bulk region cause de Haas-van Alphen effect at low temperature as
T\lsim\hbar\omega_c. As the temperature gets higher (T\gsim\hbar\omega_c)
under such a strong field, the bulk currents are reduced and the Landau
diamagnetism by the edge current is recovered.Comment: 15 pages, 11 figure
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