2,204 research outputs found
Gate-controlled superconductivity in diffusive multiwalled carbon nanotube
We have investigated electrical transport in a diffusive multiwalled carbon
nanotube contacted using superconducting leads made of Al/Ti sandwich
structure. We find proximity-induced superconductivity with measured critical
currents up to I_cm = 1.3 nA, tunable by gate voltage down to 10 pA. The
supercurrent branch displays a finite zero bias resistance which varies as R_0
proportional to I_cm^-alpha with alpha=0.74. Using IV-characteristics of
junctions with phase diffusion, a good agreement is obtained with Josephson
coupling energy in the long, diffusive junction model of A.D Zaikin and G.F.
Zharkov (Sov. J. Low Temp. Phys. 7, 184 (1981)).Comment: 5 pages, 4 figure
Assessing T cell clonal size distribution: a non-parametric approach
Clonal structure of the human peripheral T-cell repertoire is shaped by a
number of homeostatic mechanisms, including antigen presentation, cytokine and
cell regulation. Its accurate tuning leads to a remarkable ability to combat
pathogens in all their variety, while systemic failures may lead to severe
consequences like autoimmune diseases. Here we develop and make use of a
non-parametric statistical approach to assess T cell clonal size distributions
from recent next generation sequencing data. For 41 healthy individuals and a
patient with ankylosing spondylitis, who undergone treatment, we invariably
find power law scaling over several decades and for the first time calculate
quantitatively meaningful values of decay exponent. It has proved to be much
the same among healthy donors, significantly different for an autoimmune
patient before the therapy, and converging towards a typical value afterwards.
We discuss implications of the findings for theoretical understanding and
mathematical modeling of adaptive immunity.Comment: 13 pages, 3 figures, 2 table
Supercurrent fluctuations in short filaments
We evaluate the average and the standard deviation of the supercurrent in
superconducting nanobridges, as functions of the temperature and the phase
difference, in an equilibrium situation. We also evaluate the autocorrelation
of the supercurrent as a function of the elapsed time. The behavior of
supercurrent fluctuations is qualitatively different from from that of the
normal current: they depend on the phase difference, have a different
temperature dependence, and for appropriate range their standard deviation is
independent of the probing time. We considered two radically different
filaments and obtained very similar results for both. Fluctuations of the
supercurrent can in principle be measured
Phase diffusion and charging effects in Josephson junctions
The supercurrent of a Josephson junction is reduced by phase diffusion. For
ultrasmall capacitance junctions the current may be further decreased by
Coulomb blockade effects. We calculate the Cooper pair current by means of
time-dependent perturbation theory to all orders in the Josephson coupling
energy and obtain the current-voltage characteristic in closed form in a range
of parameters of experimental interest. The results comprehend phase diffusion
of the coherent Josephson current in the classical regime as well as the
supercurrent peak due to incoherent Cooper pair tunneling in the strong Coulomb
blockade regime.Comment: 4 pages, 3 figures, RevTe
Operation of a superconducting nanowire quantum interference device with mesoscopic leads
A theory describing the operation of a superconducting nanowire quantum
interference device (NQUID) is presented. The device consists of a pair of
thin-film superconducting leads connected by a pair of topologically parallel
ultra-narrow superconducting wires. It exhibits intrinsic electrical
resistance, due to thermally-activated dissipative fluctuations of the
superconducting order parameter. Attention is given to the dependence of this
resistance on the strength of an externally applied magnetic field aligned
perpendicular to the leads, for lead dimensions such that there is essentially
complete and uniform penetration of the leads by the magnetic field. This
regime, in which at least one of the lead dimensions lies between the
superconducting coherence and penetration lengths, is referred to as the
mesoscopic regime. The magnetic field causes a pronounced oscillation of the
device resistance, with a period not dominated by the Aharonov-Bohm effect
through the area enclosed by the wires and the film edges but, rather, in terms
of the geometry of the leads, in contrast to the well-known Little-Parks
resistance of thin-walled superconducting cylinders. A theory, encompassing
this phenomenology, is developed through extensions, to the setting of parallel
superconducting wires, of the Ivanchenko-Zil'berman-Ambegaokar-Halperin theory
for the case of short wires and the Langer-Ambegaokar-McCumber-Halperin theory
for the case of longer wires. It is demonstrated that the NQUID acts as a probe
of spatial variations in the superconducting order parameter.Comment: 20 pages, 18 figure
Fractional Fokker-Planck dynamics: Numerical algorithm and simulations
Anomalous transport in a tilted periodic potential is investigated
numerically within the framework of the fractional Fokker-Planck dynamics via
the underlying CTRW. An efficient numerical algorithm is developed which is
applicable for an arbitrary potential. This algorithm is then applied to
investigate the fractional current and the corresponding nonlinear mobility in
different washboard potentials. Normal and fractional diffusion are compared
through their time evolution of the probability density in state space.
Moreover, we discuss the stationary probability density of the fractional
current values.Comment: 10 pages, 9 figure
Evidence of Josephson-coupled superconducting regions at the interfaces of Highly Oriented Pyrolytic Graphite
Transport properties of a few hundreds of nanometers thick (in the graphene
plane direction) lamellae of highly oriented pyrolytic graphite (HOPG) have
been investigated. Current-Voltage characteristics as well as the temperature
dependence of the voltage at different fixed input currents provide evidence
for Josephson-coupled superconducting regions embedded in the internal
two-dimensional interfaces, reaching zero resistance at low enough
temperatures. The overall behavior indicates the existence of superconducting
regions with critical temperatures above 100 K at the internal interfaces of
oriented pyrolytic graphite.Comment: 6 Figures, 5 page
Towards the observation of phase locked Bloch oscillations in arrays of small Josephson junctions
We have designed an experiment and performed extensive simulations and
preliminary measurements to identify a set of realistic circuit parameters that
should allow the observation of constant-current steps at I=2ef in short arrays
of small Josephson junctions under external AC drive of frequency f.
Observation of these steps demonstrating phase lock of the Bloch oscillations
with the external drive requires a high-impedance environment for the array,
which is provided by on-chip resistors close to the junctions. We show that the
width and shape of the steps crucially depend on the shape of the drive and the
electron temperature in the resistors
Exact analytical solution of the problem of current-carrying states of the Josephson junction in external magnetic fields
The classical problem of the Josephson junction of arbitrary length W in the
presence of externally applied magnetic fields (H) and transport currents (J)
is reconsidered from the point of view of stability theory. In particular, we
derive the complete infinite set of exact analytical solutions for the phase
difference that describe the current-carrying states of the junction with
arbitrary W and an arbitrary mode of the injection of J. These solutions are
parameterized by two natural parameters: the constants of integration. The
boundaries of their stability regions in the parametric plane are determined by
a corresponding infinite set of exact functional equations. Being mapped to the
physical plane (H,J), these boundaries yield the dependence of the critical
transport current Jc on H. Contrary to a wide-spread belief, the exact
analytical dependence Jc=Jc(H) proves to be multivalued even for arbitrarily
small W. What is more, the exact solution reveals the existence of unquantized
Josephson vortices carrying fractional flux and located near one of the
junction edges, provided that J is sufficiently close to Jc for certain finite
values of H. This conclusion (as well as other exact analytical results) is
illustrated by a graphical analysis of typical cases.Comment: 21 pages, 9 figures, to be published in Phys. Rev.
Current and universal scaling in anomalous transport
Anomalous transport in tilted periodic potentials is investigated within the
framework of the fractional Fokker-Planck dynamics and the underlying
continuous time random walk. The analytical solution for the stationary,
anomalous current is obtained in closed form. We derive a universal scaling law
for anomalous diffusion occurring in tilted periodic potentials. This scaling
relation is corroborated with precise numerical studies covering wide parameter
regimes and different shapes for the periodic potential, being either symmetric
or ratchet-like ones
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