1,986 research outputs found
Geometric dependence of Nb-BiTe-Nb topological Josephson junction transport parameters
Superconductor-topological insulator-superconductor Josephson junctions have
been fabricated in order to study the width dependence of the critical current,
normal state resistance and flux periodicity of the critical current modulation
in an external field. Previous literature reports suggest anomalous scaling in
topological junctions due to the presence of Majorana bound states. However,
for most realised devices, one would expect that trivial -periodic
Andreev levels dominate transport. We also observe anomalous scaling behaviour
of junction parameters, but the scaling can be well explained by mere geometric
effects, such as the parallel bulk conductivity shunt and flux focusing
Experimental realization of SQUIDs with topological insulator junctions
We demonstrate topological insulator (BiTe) dc SQUIDs, based on
superconducting Nb leads coupled to nano-fabricated Nb-BiTe-Nb
Josephson junctions. The high reproducibility and controllability of the
fabrication process allows the creation of dc SQUIDs with parameters that are
in agreement with design values. Clear critical current modulation of both the
junctions and the SQUID with applied magnetic fields have been observed. We
show that the SQUIDs have a periodicity in the voltage-flux characteristic of
, of relevance to the ongoing pursuit of realizing interferometers for
the detection of Majorana fermions in superconductor- topological insulator
structures
Assessing positive matrix factorization model fit: a new method to estimate uncertainty and bias in factor contributions at the daily time scale
International audienceA Positive Matrix Factorization receptor model for aerosol pollution source apportionment was fit to a synthetic dataset simulating one year of daily measurements of ambient PM2.5 concentrations, comprised of 39 chemical species from nine pollutant sources. A novel method was developed to estimate model fit uncertainty and bias at the daily time scale, as related to factor contributions. A balanced bootstrap is used to create replicate datasets, with the same model then fit to the data. Neural networks are trained to classify factors based upon chemical profiles, as opposed to correlating contribution time series, and this classification is used to align factor orderings across results associated with the replicate datasets. Factor contribution uncertainty is assessed from the distribution of results associated with each factor. Comparing modeled factors with input factors used to create the synthetic data assesses bias. The results indicate that variability in factor contribution estimates does not necessarily encompass model error: contribution estimates can have small associated variability yet also be very biased. These results are likely dependent on characteristics of the data
Rotational and translational self-diffusion in concentrated suspensions of permeable particles
In our recent work on concentrated suspensions of uniformly porous colloidal
spheres with excluded volume interactions, a variety of short-time dynamic
properties were calculated, except for the rotational self-diffusion
coefficient. This missing quantity is included in the present paper. Using a
precise hydrodynamic force multipole simulation method, the rotational
self-diffusion coefficient is evaluated for concentrated suspensions of
permeable particles. Results are presented for particle volume fractions up to
45%, and for a wide range of permeability values. From the simulation results
and earlier results for the first-order virial coefficient, we find that the
rotational self-diffusion coefficient of permeable spheres can be scaled to the
corresponding coefficient of impermeable particles of the same size. We also
show that a similar scaling applies to the translational self-diffusion
coefficient considered earlier. From the scaling relations, accurate analytic
approximations for the rotational and translational self-diffusion coefficients
in concentrated systems are obtained, useful to the experimental analysis of
permeable-particle diffusion. The simulation results for rotational diffusion
of permeable particles are used to show that a generalized
Stokes-Einstein-Debye relation between rotational self-diffusion coefficient
and high-frequency viscosity is not satisfied.Comment: 4 figure
Assessing positive matrix factorization model fit: a new method to estimate uncertainty and bias in factor contributions at the measurement time scale
A Positive Matrix Factorization receptor model for aerosol pollution source apportionment was fit to a synthetic dataset simulating one year of daily measurements of ambient PM<sub>2.5</sub> concentrations, comprised of 39 chemical species from nine pollutant sources. A novel method was developed to estimate model fit uncertainty and bias at the daily time scale, as related to factor contributions. A circular block bootstrap is used to create replicate datasets, with the same receptor model then fit to the data. Neural networks are trained to classify factors based upon chemical profiles, as opposed to correlating contribution time series, and this classification is used to align factor orderings across the model results associated with the replicate datasets. Factor contribution uncertainty is assessed from the distribution of results associated with each factor. Comparing modeled factors with input factors used to create the synthetic data assesses bias. The results indicate that variability in factor contribution estimates does not necessarily encompass model error: contribution estimates can have small associated variability across results yet also be very biased. These findings are likely dependent on characteristics of the data
Very large magnetoresistance in lateral ferromagnetic (Ga,Mn)As wires with nanoconstrictions
We have fabricated (Ga,Mn)As nanostructures in which domain walls can be
pinned by sub-10 nm constrictions. Controlled by shape anisotropy, we can
switch the regions on either side of the constriction to either parallel or
antiparallel magnetization. All samples exhibit a positive magnetoresistance,
consistent with domain-wall trapping. For metallic samples we find a
magnetoresistance up to 8%, which can be understood from spin accumulation. In
samples where, due to depletion at the constriction, a tunnel barrier is
formed, we observe a magnetoresistance of up to 2000 %.Comment: 4 pages, 3 figures, submited to Phys. Rev. Let
Correlated enhancement of Hc2 and Jc in carbon nanotube-doped MgB2
The use of MgB2 in superconducting applications still awaits for the
development of a MgB2-based material where both current-carrying performance
and critical magnetic field are optimized simultaneously. We achieved this by
doping MgB2 with double-wall carbon nanotubes (DWCNT) as a source of carbon in
polycrystalline samples. The optimum nominal DWCNT content for increasing the
critical current density, Jc is in the range 2.5-10%at depending on field and
temperature. Record values of the upper critical field, Hc2(4K) = 41.9 T (with
extrapolated Hc2(0) ~ 44.4 T) are reached in a bulk sample with 10%at DWCNT
content. The measured Hc2 vs T in all samples are successfully described using
a theoretical model for a two-gap superconductor in the dirty limit first
proposed by Gurevich et al.Comment: 12 pages, 3 figure
Active Microrheology of Networks Composed of Semiflexible Polymers. II. Theory and comparison with simulations
Building on the results of our computer simulation (ArXiv cond-mat/0503573)we
develop a theoretical description of the motion of a bead, embedded in a
network of semiflexible polymers, and responding to an applied force. The
theory reveals the existence of an osmotic restoring force, generated by the
piling up of filaments in front of the moving bead and first deduced through
computer simulations. The theory predicts that the bead displacement scales
like x ~ t^alfa with time, with alfa=0.5 in an intermediate- and alfa=1 in a
long-time regime. It also predicts that the compliance varies with
concentration like c^(-4/3) in agreement with experiment.Comment: 18 pages and 2 figure
Bogoliubov - de Gennes versus Quasiclassical Description of Josephson Structures
The applicability of the quasiclassical theory of superconductivity in
Josephson multi-layer structures is analyzed. The quasiclassical approach is
compared with the exact theory based on the Bogoliubov - de Gennes equation.
The angle and energy resolved (coarse-grain) currents are calculated using both
techniques. It is shown that the two approaches agree in geometries
after the coarse-grain averaging. A quantitative discrepancy, which exceeds the
quasiclassical accuracy, is observed when three or more interfaces are present.
The invalidity of the quasiclassical theory is attributed to the presence of
closed trajectories formed by sequential reflections on the interfaces.Comment: revtex4,12 pages, 12 figure
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