1,450 research outputs found
Metallic properties of magnesium point contacts
We present an experimental and theoretical study of the conductance and
stability of Mg atomic-sized contacts. Using Mechanically Controllable Break
Junctions (MCBJ), we have observed that the room temperature conductance
histograms exhibit a series of peaks, which suggests the existence of a shell
effect. Its periodicity, however, cannot be simply explained in terms of either
an atomic or electronic shell effect. We have also found that at room
temperature, contacts of the diameter of a single atom are absent. A possible
interpretation could be the occurrence of a metal-to-insulator transition as
the contact radius is reduced, in analogy with what it is known in the context
of Mg clusters. However, our first principle calculations show that while an
infinite linear chain can be insulating, Mg wires with larger atomic
coordinations, as in realistic atomic contacts, are alwaysmetallic. Finally, at
liquid helium temperature our measurements show that the conductance histogram
is dominated by a pronounced peak at the quantum of conductance. This is in
good agreement with our calculations based on a tight-binding model that
indicate that the conductance of a Mg one-atom contact is dominated by a single
fully open conduction channel.Comment: 14 pages, 5 figure
Mechanical properties of Pt monatomic chains
The mechanical properties of platinum monatomic chains were investigated by
simultaneous measurement of an effective stiffness and the conductance using
our newly developed mechanically controllable break junction (MCBJ) technique
with a tuning fork as a force sensor. When stretching a monatomic contact
(two-atom chain), the stiffness and conductance increases at the early stage of
stretching and then decreases just before breaking, which is attributed to a
transition of the chain configuration and bond weakening. A statistical
analysis was made to investigate the mechanical properties of monatomic chains.
The average stiffness shows minima at the peak positions of the
length-histogram. From this result we conclude that the peaks in the
length-histogram are a measure of the number of atoms in the chains, and that
the chains break from a strained state. Additionally, we find that the smaller
the initial stiffness of the chain is, the longer the chain becomes. This shows
that softer chains can be stretched longer.Comment: 6 pages, 5 figure
Quantum Hall Resistance Overshoot in 2-Dimensional Electron Gases - Theory and Experiment
We present a systematical experimental investigation of an unusual transport
phenomenon observed in two dimensional electron gases in Si/SiGe
heterostructures under integer quantum Hall effect (IQHE) conditions. This
phenomenon emerges under specific experimental conditions and in different
material systems. It is commonly referred to as Hall resistance overshoot,
however, lacks a consistent explanation so far. Based on our experimental
findings we are able to develop a model that accounts for all of our
observations in the framework of a screening theory for the IQHE. Within this
model the origin of the overshoot is attributed to a transport regime where
current is confined to co-existing evanescent incompressible strips of
different filling factors.Comment: 26 pages, 10 figure
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
Electronic and atomic shell structure in aluminum nanowires
We report experiments on aluminum nanowires in ultra-high vacuum at room
temperature that reveal a periodic spectrum of exceptionally stable structures.
Two "magic" series of stable structures are observed: At low conductance, the
formation of stable nanowires is governed by electronic shell effects whereas
for larger contacts atomic packing dominates. The crossover between the two
regimes is found to be smooth. A detailed comparison of the experimental
results to a theoretical stability analysis indicates that while the main
features of the observed electron-shell structure are similar to those of
alkali and noble metals, a sequence of extremely stable wires plays a unique
role in Aluminum. This series appears isolated in conductance histograms and
can be attributed to "superdeformed" non-axisymmetric nanowires.Comment: 15 pages, 9 figure
Giant negative magnetoresistance in semiconductors doped by multiply charged deep impurities
A giant negative magnetoresistance has been observed in bulk germanium doped
with multiply charged deep impurities. Applying a magnetic field the resistance
may decrease exponentially at any orientation of the field. A drop of the
resistance as much as about 10000% has been measured at 6 T. The effect is
attributed to the spin splitting of impurity ground state with a very large
g-factor in the order of several tens depending on impurity.Comment: 4 pages, 4 figure
The visibility of IQHE at sharp edges: Experimental proposals based on interactions and edge electrostatics
The influence of the incompressible strips on the integer quantized Hall
effect (IQHE) is investigated, considering a cleaved-edge overgrown (CEO)
sample as an experimentally realizable sharp edge system. We propose a set of
experiments to clarify the distinction between the large-sample limit when bulk
disorder defines the IQHE plateau width and the small-sample limit smaller than
the disorder correlation length, when self-consistent edge electrostatics
define the IQHE plateau width. The large-sample or bulk QH regime is described
by the usual localization picture, whereas the small-sample or edge regime is
discussed within the compressible/incompressible strips picture, known as the
screening theory of QH edges. Utilizing the unusually sharp edge profiles of
the CEO samples, a Hall bar design is proposed to manipulate the edge potential
profile from smooth to extremely sharp. By making use of a side-gate
perpendicular to the two dimensional electron system, it is shown that the
plateau widths can be changed or even eliminated altogether. Hence, the
visibility of IQHE is strongly influenced when adjusting the edge potential
profile and/or changing the dc current direction under high currents in the
non-linear transport regime. As a second investigation, we consider two
different types of ohmic contacts, namely highly transmitting (ideal) and
highly reflecting (non-ideal) contacts. We show that if the injection contacts
are non-ideal, however still ohmic, it is possible to measure directly the
non-quantized transport taking place at the bulk of the CEO samples. The
results of the experiments we propose will clarify the influence of the edge
potential profile and the quality of the contacts, under quantized Hall
conditions.Comment: Substantially revised version of manuscript arXiv:0906.3796v1,
including new figures et
On the Surface Structure of Strange Superheavy Nuclei
Bound, strange, neutral superheavy nuclei, stable against strong decay, may
exist. A model effective field theory calculation of the surface energy and
density of such systems is carried out assuming vector meson couplings to
conserved currents and scalar couplings fit to data where it exists. The
non-linear relativistic mean field equations are solved assuming local baryon
sources. The approach is calibrated through a successful calculation of the
known nuclear surface tension.Comment: 12 pages, 9 figure
Density dependent hadron field theory for hypernuclei
The Density Dependent Relativistic Hadron Field (DDRH) theory, previously
introduced and applied to isospin nuclei, is extended to hypernuclei by
including the octet hyperons. Infinite matter Dirac-Brueckner theory for octet
baryons and the derivation of in-medium DDRH baryon-meson vertices is
discussed. From the properties of Dirac-Brueckner interactions it is found that
hyperon and nucleon self-energies and vertices are related by the ratios of
free space coupling constants. This leads to simple scaling laws for the
in-medium hyperon and nucleon vertices. The model is applied in relativistic
DDRH mean-field calculations to singl$\Lambda nuclei. Free space N-Lambda
T-matrix results are used for the scalar vertex. As the only free parameter the
hyperon vector vertex scaling factor is adjusted to a selected set of
hypernuclear data. Spectroscopic data of single Lambda hypernuclei over the
full mass range are well described. The reduced Lambda spin-orbit splitting is
reproduced and found to be related closely the medium dependence of scalar and
vector interactions.Comment: 38 pages, 9 figure
Statistics of Resonances and Delay Times in Random Media: Beyond Random Matrix Theory
We review recent developments on quantum scattering from mesoscopic systems.
Various spatial geometries whose closed analogs shows diffusive, localized or
critical behavior are considered. These are features that cannot be described
by the universal Random Matrix Theory results. Instead one has to go beyond
this approximation and incorporate them in a non-perturbative way. Here, we pay
particular emphasis to the traces of these non-universal characteristics, in
the distribution of the Wigner delay times and resonance widths. The former
quantity captures time dependent aspects of quantum scattering while the latter
is associated with the poles of the scattering matrix.Comment: 30 pages, 15 figures (submitted to Journal of Phys. A: Math. and
General, special issue on "Aspects of Quantum Chaotic Scattering"
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