242 research outputs found
Mesoscopic superconductors in the London limit: equilibrium properties and metastability
We present a study of the behaviour of metastable vortex states in mesoscopic
superconductors. Our analysis relies on the London limit within which it is
possible to derive closed analytical expressions for the magnetic field and the
Gibbs free energy. We consider in particular the situation where the vortices
are symmetrically distributed along a closed ring. There, we obtain expressions
for the confining Bean-Livingston barrier and for the magnetization which turns
out to be paramagnetic away from thermodynamic equilibrium. At low temperature,
the barrier is high enough for this regime to be observable. We propose also a
local description of both thermodynamic and metastable states based on
elementary topological considerations; we find structural phase transitions of
vortex patterns between these metastable states and we calculate the
corresponding critical fields.Comment: 24 pages, 20 figure
Superconducting properties of mesoscopic cylinders with enhanced surface superconductivity
The superconducting state of an infinitely long superconducting cylinder
surrounded by a medium which enhances its superconductivity near the boundary
is studied within the nonlinear Ginzburg-Landau theory. This enhancement can be
due to the proximity of another superconductor or due to surface treatment.
Quantities like the free energy, the magnetization and the Cooper-pair density
are calculated. Phase diagrams are obtained to investigate how the critical
field and the critical temperature depend on this surface enhancement for
different values of the Ginzburg-Landau parameter \kappa. Increasing the
superconductivity near the surface leads to higher critical fields and critical
temperatures. For small cylinder diameters only giant vortex states nucleate,
while for larger cylinders multivortices can nucleate. The stability of these
multivortex states also depends on the surface enhancement. For type-I
superconductors we found the remarkable result that for a range of values of
the surface extrapolation length the superconductor can transit from the
Meissner state into superconducting states with vorticity L > 1. Such a
behaviour is not found for the case of large \kappa, i.e. type-II
superconductivity.Comment: submitted to Phys. Rev.
Weak localisation in bilayer graphene
We have performed the first experimental investigation of quantum
interference corrections to the conductivity of a bilayer graphene structure. A
negative magnetoresistance - a signature of weak localisation - is observed at
different carrier densities, including the electro-neutrality region. It is
very different, however, from the weak localisation in conventional
two-dimensional systems. We show that it is controlled not only by the
dephasing time, but also by different elastic processes that break the
effective time-reversal symmetry and provide invervalley scattering.Comment: 4 pages, 4 figures (to be published in PRL
Paraxial propagation of a quantum charge in a random magnetic field
The paraxial (parabolic) theory of a near forward scattering of a quantum
charged particle by a static magnetic field is presented. From the paraxial
solution to the Aharonov-Bohm scattering problem the transverse transfered
momentum (the Lorentz force) is found. Multiple magnetic scattering is
considered for two models: (i) Gaussian -correlated random magnetic
field; (ii) a random array of the Aharonov-Bohm magnetic flux line. The
paraxial gauge-invariant two-particle Green function averaged with respect to
the random field is found by an exact evaluation of the Feynman integral. It is
shown that in spite of the anomalous character of the forward scattering, the
transport properties can be described by the Boltzmann equation. The Landau
quantization in the field of the Aharonov-Bohm lines is discussed.Comment: Figures and references added. Many typos corrected. RevTex, 25 pages,
9 figure
A dual point description of mesoscopic superconductors
We present an analysis of the magnetic response of a mesoscopic
superconductor, i.e. a system of sizes comparable to the coherence length and
to the London penetration depth. Our approach is based on special properties of
the two dimensional Ginzburg-Landau equations, satisfied at the dual point
Closed expressions for the free energy and the
magnetization of the superconductor are derived. A perturbative analysis in the
vicinity of the dual point allows us to take into account vortex interactions,
using a new scaling result for the free energy. In order to characterize the
vortex/current interactions, we study vortex configurations that are out of
thermodynamical equilibrium. Our predictions agree with the results of recent
experiments performed on mesoscopic aluminium disks.Comment: revtex, 20 pages, 9 figure
Quantum Hall activation gaps in bilayer graphene
We have measured the quantum Hall activation gaps in bilayer graphene at
filling factors and in high magnetic fields up to 30 T.
We find that energy levels can be described by a 4-band relativistic hyperbolic
dispersion. The Landau level width is found to contain a field independent
background due to an intrinsic level broadening and a component which increases
linearly with magnetic field.Comment: 4 pages, accepted version (just removed a few typos), will appear as
Fast Track Communication in Solid State Commu
Graphene-hexagonal boron nitride resonant tunneling diodes as high-frequency oscillators
We assess the potential of two-terminal graphene-hexagonal boron nitride-graphene resonant tunneling diodes as high-frequency oscillators, using self-consistent quantum transport and electrostatic simulations to determine the time-dependent response of the diodes in a resonant circuit. We quantify how the frequency and power of the current oscillations depend on the diode and circuit parameters including the doping of the graphene electrodes, device geometry, alignment of the graphene lattices, and the circuit impedances. Our results indicate that current oscillations with frequencies of up to several hundred GHz should be achievable
Demonstration of a quantum nondemolition sum gate
The sum gate is the canonical two-mode gate for universal quantum computation
based on continuous quantum variables. It represents the natural analogue to a
qubit C-NOT gate. In addition, the continuous-variable gate describes a quantum
nondemolition (QND) interaction between the quadrature components of two light
fields. We experimentally demonstrate a QND sum gate, employing the scheme by
R. Filip, P. Marek, and U.L. Andersen [\pra {\bf 71}, 042308 (2005)], solely
based on offline squeezed states, homodyne measurements, and feedforward. The
results are verified by simultaneously satisfying the criteria for QND
measurements in both conjugate quadratures.Comment: 4 pages, 4 figure
Surface Instabilities on Liquid Oxygen in an Inhomogeneous Magnetic Field
Liquid oxygen exhibits surface instabilities when subjected to a sufficiently
strong magnetic field. A vertically oriented magnetic field gradient both
increases the magnetic field value at which the pattern forms and shrinks the
length scale of the surface patterning. We show that these effects of the field
gradient may be described in terms of an ``effective gravity'', which in our
experiments may be varied from 1g to 360g.Comment: 4 pages, 5 embedded figures in eps forma
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