2,290 research outputs found
Cooper-pair propagation and superconducting correlations in graphene
We investigate the Cooper-pair propagation and the proximity effect in
graphene under conditions in which the distance L between superconducting
electrodes is much larger than the width W of the contacts. In the case of
undoped graphene, supercurrents may exist with a spatial decay proportional to
W^2/L^3. This changes upon doping into a 1/L^2 behavior, opening the
possibility to observe a supercurrent over length scales above 1 micron at
suitable doping levels. We also show that there is in general a crossover
temperature T ~ v_F/k_B L that marks the onset of the strong decay of the
supercurrent, and that corresponds to the scale below which the Cooper pairs
are not disrupted by thermal effects during their propagation.Comment: 5 pages, 2 figures; corrected discussio
Impurity Effects on the Flux Phase Quantum Critical Point Scenario
Impurity substitution of Zn in La-214 and (Y,Ca)-123 high-T_c superconductors
suppresses T_c but does not affect appreciably the onset of the pseudogap phase
in the underdoped region nor optimal doping or the position of the inferred
quantum critical point. Based on a 1/N expansion of the t-J model we explain
these findings as well as the similar dependence on a magnetic field in terms
of a quantum critical point scenario where a flux phase causes the pseudogap.Comment: europhys.sty, 3 eps figures included, to appear on Europhysics
Letter
Color Magnetic Flux Tubes in Dense QCD
QCD is expected to be in the color-flavor locking phase in high baryon
density, which exhibits color superconductivity. The most fundamental
topological objects in the color superconductor are non-Abelian vortices which
are topologically stable color magnetic flux tubes. We present numerical
solutions of the color magnetic flux tube for diverse choices of the coupling
constants. We also analytically study its asymptotic profiles and find that
they are different from the case of usual superconductors. We propose the width
of color magnetic fluxes and find that it is larger than naive expectation of
the Compton wave length of the massive gluon when the gluon mass is larger than
the scalar mass.Comment: 24 pages, 5 figures; v2: typos corrected, references added, minor
changes; v3: published versio
Ground State and Tkachenko Modes of a Rapidly Rotating Bose-Einstein Condensate in the Lowest Landau Level State
The Letter considers the ground state and the Tkachenko modes for a rapidly
rotating Bose-Einstein condensate (BEC), when its macroscopic wave function is
a coherent superposition of states analogous to the lowest Landau levels of a
charge in a magnetic field. As well as in type II superconductors close to the
critical magnetic field , this corresponds to a periodic vortex
lattice. The exact value of the shear elastic modulus of the vortex lattice,
which was known from the old works on type II superconductors, essentially
exceeds the values calculated recently for BEC. This is important for
comparison with observation of the Tkachenko mode in the rapidly rotating BEC.Comment: 5 pages, 1 figure; discussion edited, references added, numerical
factors and typos correcte
Josephson super-current in graphene-superconductor junction
Within the tunneling Hamiltonian formulation for the eight-component
spinors,the Josephson critical super-current has been calculated in a planar
superconductor-normal graphene-superconductor junction. Coupling between
superconductor regions and graphene is taken into account by a tunneling
Hamiltonian which contains two types of tunneling, intra-valley and
inter-valley tunneling. Within the present tunneling approach, we find that the
contributions of two kinds of tunneling to the critical super-current, are
completely separable. Therefore, it is possible to consider the effect of the
inter-valley tunnelings in the critical super-current. The incorporation of
these type of processes into the tunneling Hamiltonian, exposes a special
feature of the graphene Josephson junctions. The effect of inter-valley
tunneling appears in the length dependence plot of critical current in the form
of oscillations. We also present the results for temperature dependence of
critical super-current and compare with experimental results and other
theoretical calculations
Enhancing in ferromagnetic semiconductors
We theoretically investigate disorder effects on the ferromagnetic transition
('Curie') temperature in dilute IIIMnV magnetic
semiconductors (e.g. GaMnAs) where a small fraction () of the cation atoms (e.g. Ga) are randomly replaced by the magnetic
dopants (e.g. Mn), leading to long-range ferromagnetic ordering for . We
find that is a complicated function of at least eight different
parameters including carrier density, magnetic dopant density, and carrier mean
free path; nominally macroscopically similar samples could have substantially
different Curie temperatures. We provide simple physically appealing
prescriptions for enhancing in diluted magnetic semiconductors, and
discuss the magnetic phase diagram in the system parameter space.Comment: 5 pages with 4 figure
Ginzburg-Landau Vortex Lattice in Superconductor Films of Finite Thickness
The Ginzburg-Landau equations are solved for ideally periodic vortex lattices
in superconducting films of arbitrary thickness in a perpendicular magnetic
field. The order parameter, current density, magnetic moment, and the
3-dimensional magnetic field inside and outside the film are obtained in the
entire ranges of the applied magnetic field, Ginzburg Landau parameter kappa,
and film thickness. The superconducting order parameter varies very little near
the surface (by about 0.01) and the energy of the film surface is small. The
shear modulus c66 of the triangular vortex lattice in thin films coincides with
the bulk c66 taken at large kappa. In thin type-I superconductor films with
kappa < 0.707, c66 can be positive at low fields and negative at high fields.Comment: 12 pages including 14 Figures, corrected, Fig.14 added, appears in
Phys. Rev. B 71, issue 1 (2005
Pipelike current-carrying vortices in two-component condensates
We study straight vortices with global longitudinal currents in the
Bogomol'ny limit of the Abelian Higgs model with two charged scalar fields. The
model possesses global SU(2) and local electromagnetic U(1) symmetries
spontaneously broken to global U(1) group, and corresponds to a semilocal limit
of the standard electroweak model. We show that the contribution of the global
SU(2) current to the vortex energy is proportional to the total current
squared. Locally, these vortices carry also longitudinal electromagnetic
currents, while the total electromagnetic current flowing through a transverse
section of the vortex is always zero. The vortices with high winding numbers
have, in general, a nested pipelike structure. The magnetic field of the vortex
is concentrated at a certain distance from the geometric center of the vortex,
thus resembling a "pipe." This magnetic pipe is layered between two
electrically charged pipes that carry longitudinal electric currents in
opposite directions.Comment: 11 pages, 14 figures, RevTeX 4.1; v2: references added, minor
changes, Figure 8 (a visualization of the nested structure of the pipelike
vortex) is replaced, published versio
Flux quantization for a vortex in two-gap superconductor
Contrary to recent theoretical prediction, we show that the magnetic flux of
a vortex in SU(2) model of two-gap superconductor is quantized in units of
2\pi/g, not 4 \pi/g. For the U(1) version of this model, the flux is quantized
in units of 2 \pi\alpha/g where 0 < \alpha < 1. The parameter \alpha depends on
the masses and concentrations of the Cooper pairs in the two condensates.Comment: 7 page
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