9,830 research outputs found
Interplay Between Yu-Shiba-Rusinov States and Multiple Andreev Reflections
Motivated by recent scanning tunneling microscopy experiments on single
magnetic impurities on superconducting surfaces, we present here a
comprehensive theoretical study of the interplay between Yu-Shiba-Rusinov bound
states and (multiple) Andreev reflections. Our theory is based on a combination
of an Anderson model with broken spin degeneracy and nonequilibrium Green's
function techniques that allows us to describe the electronic transport through
a magnetic impurity coupled to superconducting leads for arbitrary junction
transparency. Using this combination we are able to elucidate the different
tunneling processes that give a significant contribution to the subgap
transport. In particular, we predict the occurrence of a large variety of
Andreev reflections mediated by Yu-Shiba-Rusinov bound states that clearly
differ from the standard Andreev processes in non-magnetic systems. Moreover,
we provide concrete guidelines on how to experimentally identify the subgap
features originating from these tunneling events. Overall, our work provides
new insight into the role of the spin degree of freedom in Andreev transport
physics.Comment: 15 pages, 10 figure
Mapping all classical spin models to a lattice gauge theory
In our recent work [Phys. Rev. Lett. 102, 230502 (2009)] we showed that the
partition function of all classical spin models, including all discrete
standard statistical models and all Abelian discrete lattice gauge theories
(LGTs), can be expressed as a special instance of the partition function of a
4-dimensional pure LGT with gauge group Z_2 (4D Z_2 LGT). This provides a
unification of models with apparently very different features into a single
complete model. The result uses an equality between the Hamilton function of
any classical spin model and the Hamilton function of a model with all possible
k-body Ising-type interactions, for all k, which we also prove. Here, we
elaborate on the proof of the result, and we illustrate it by computing
quantities of a specific model as a function of the partition function of the
4D Z_2 LGT. The result also allows one to establish a new method to compute the
mean-field theory of Z_2 LGTs with d > 3, and to show that computing the
partition function of the 4D Z_2 LGT is computationally hard (#P hard). The
proof uses techniques from quantum information.Comment: 21 pages, 21 figures; published versio
Innovative evaluation of dexterity in pediatrics.
STUDY DESIGN: Review paper.
INTRODUCTION: Hand dexterity is multifaceted and essential to the performance of daily tasks. Timed performance and precision demands are the most common features of quantitative dexterity testing. Measurement concepts such as rate of completion, in-hand manipulation and dynamic force control of instabilities are being integrated into assessment tools for the pediatric population.
PURPOSE: To review measurement concepts inherent in pediatric dexterity testing and introduce concepts that are infrequently measured or novel as exemplified with two assessment tools.
METHODS: Measurement concepts included in common assessment tools are introduced first. We then describe seldom measured and novel concepts embedded in two instruments; the Functional Dexterity Test (FDT) and the Strength-Dexterity (SD) Test.
DISCUSSION: The inclusion of novel yet informative tools and measurement concepts in our assessments could aid our understanding of atypical dexterity, and potentially contribute to the design of targeted therapy programs
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
Tilt-angle landscapes and temperature dependence of the conductance in biphenyl-dithiol single-molecule junctions
Using a density-functional-based transport method we study the conduction
properties of several biphenyl-derived dithiol (BPDDT) molecules wired to gold
electrodes. The BPDDT molecules differ in their side groups, which control the
degree of conjugation of the pi-electron system. We have analyzed the
dependence of the low-bias zero-temperature conductance on the tilt angle phi
between the two phenyl ring units, and find that it follows closely a
cos^2(phi) law, as expected from an effective pi-orbital coupling model. We
show that the tilting of the phenyl rings results in a decrease of the
zero-temperature conductance by roughly two orders of magnitude, when going
from a planar conformation to a configuration in which the rings are
perpendicular. In addition we demonstrate that the side groups, apart from
determining phi, have no influence on the conductance. All this is in agreement
with the recent experiment by Venkataraman et al. [Nature 442, 904 (2006)].
Finally, we study the temperature dependence of both the conductance and its
fluctuations and find qualitative differences between the examined molecules.
In this analysis we consider two contributions to the temperature behavior, one
coming from the Fermi functions and the other one from a thermal average over
different contact configurations. We illustrate that the fluctuations of the
conductance due to temperature-induced changes in the geometric structure of
the molecule can be reduced by an appropriate design.Comment: 9 pages, 6 figures; submitted to Phys. Rev.
Toy models of crossed Andreev reflection
We propose toy models of crossed Andreev reflection in multiterminal hybrid
structures containing out-of-equilibrium conductors. We apply the description
to two possible experiments: (i) to a device containing a large quantum dot
inserted in a crossed Andreev reflection circuit. (ii) To a device containing
an Aharonov-Bohm loop inserted in a crossed Andreev reflection circuit.Comment: 5 pages, 9 figures, minor modification
Mean Free Path and Energy Fluctuations in Quantum Chaotic Billiards
The elastic mean free path of carriers in a recently introduced model of
quantum chaotic billiards in two and three dimensions is calculated. The model
incorporates surface roughness at a microscopic scale by randomly choosing the
atomic levels at the surface sites between -W/2 and W/2. Surface roughness
yields a mean free path l that decreases as L/W^2 as W increases, L being the
linear size of the system. But this diminution ceases when the surface layer
begins to decouple from the bulk for large enough values of W, leaving more or
less unperturbed states on the bulk. Consequently, the mean free path shows a
minimum of about L/2 for W of the order of the band width. Energy fluctuations
reflect the behavior of the mean free path. At small energy scales, strong
level correlations manifest themselves by small values of the number of levels
variance Sigma^2(E) that are close to Random Matrix Theory (RMT) in all cases.
At larger energy scales, fluctuations are below the logarithmic behavior of RMT
for l > L, and above RMT value when l < L.Comment: 8 twocolumn pages, seven figures, revtex and epsf macros. To be
published in Physical Review B
Full Counting Statistics of Multiple Andreev Reflections in incoherent diffusive superconducting junctions
We present a theory for the full distribution of current fluctuations in
incoherent diffusive superconducting junctions, subjected to a voltage bias.
This theory of full counting statistics of incoherent multiple Andreev
reflections is valid for arbitrary applied voltage. We present a detailed
discussion of the properties of the first four cumulants as well as the low and
high voltage regimes of the full counting statistics. The work is an extension
of the results of Pilgram and the author, Phys. Rev. Lett. 94, 086806 (2005).Comment: Included in special issue Spin Physics of Superconducting
heterostructures of Applied Physics A: Materials Science & Processin
Nonlinear Excitations, Stability Inversions and Dissipative Dynamics in Quasi-one-dimensional Polariton Condensates
We consider the existence, stability and dynamics of the ground state and
nonlinear excitations, in the form of dark solitons, for a
quasi-one-dimensional polariton condensate in the presence of pumping and
nonlinear damping. We find a series of remarkable features that can be directly
contrasted to the case of the typically energy-conserving ultracold alkali-atom
Bose-Einstein condensates. For some sizeable parameter ranges, the nodeless
("ground") state becomes {\it unstable} towards the formation of {\em stable}
nonlinear single or {\em multi} dark-soliton excitations. It is also observed
that for suitable parametric choices, the instability of single dark solitons
can nucleate multi-dark-soliton states. Also, for other parametric regions,
{\em stable asymmetric} sawtooth-like solutions exist. Finally, we consider the
dragging of a defect through the condensate and the interference of two
initially separated condensates, both of which are capable of nucleating dark
multi-soliton dynamical states.Comment: 9 pages, 10 figure
On the analogy between streamlined magnetic and solid obstacles
Analogies are elaborated in the qualitative description of two systems: the
magnetohydrodynamic (MHD) flow moving through a region where an external local
magnetic field (magnetic obstacle) is applied, and the ordinary hydrodynamic
flow around a solid obstacle. The former problem is of interest both
practically and theoretically, and the latter one is a classical problem being
well understood in ordinary hydrodynamics. The first analogy is the formation
in the MHD flow of an impenetrable region -- core of the magnetic obstacle --
as the interaction parameter , i.e. strength of the applied magnetic field,
increases significantly. The core of the magnetic obstacle is streamlined both
by the upstream flow and by the induced cross stream electric currents, like a
foreign insulated insertion placed inside the ordinary hydrodynamic flow. In
the core, closed streamlines of the mass flow resemble contour lines of
electric potential, while closed streamlines of the electric current resemble
contour lines of pressure. The second analogy is the breaking away of attached
vortices from the recirculation pattern produced by the magnetic obstacle when
the Reynolds number , i.e. velocity of the upstream flow, is larger than a
critical value. This breaking away of vortices from the magnetic obstacle is
similar to that occurring past a real solid obstacle. Depending on the inlet
and/or initial conditions, the observed vortex shedding can be either symmetric
or asymmetric.Comment: minor changes, accepted for PoF, 26 pages, 7 figure
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