149 research outputs found
BCS superconductivity of Dirac electrons in graphene layers
Possible superconductivity of electrons with the Dirac spectrum is analyzed
using the BCS model. We calculate the critical temperature, the superconducting
energy gap, and supercurrent as functions of the doping level and of the
pairing interaction strength. Zero doping is characterized by existence of the
quantum critical point such that the critical temperature vanishes below some
finite value of the interaction strength. However, the critical temperature
remains finite for any nonzero electron or hole doping level when the Fermi
energy is shifted away from the Dirac point of the normal-state electron
spectrum. We analyze the behavior of the characteristic length scales, i.e.,
the London penetration depth and the coherence length, which determine the
critical magnetic fields
rf-studies of vortex dynamics in isotropic type-II superconductors
We have measured the surface impedance of thick superconductors in the mixed
state over a broad 2 kHz - 20 MHz frequency range. The depinning cross-over is
observed; but it is much broader than expected from classical theories of
pinning. A striking result is the existence of size effects which invalidate
the common interpretation of the low-frequency surface inductance in terms of a
single penetration depth. Instead, a two-mode description of vortex dynamics,
assuming free vortex flow in the bulk and surface pinning, accounts
quantitatively for the spectrum of the complex apparent penetration depth.Comment: 20 pages, 6 figures, 28 reference
Depinning transition in type-II superconductors
The surface impedance Z(f) of conventional isotropic materials has been
carefully measured for frequencies f ranging from 1 kHz to 3 MHz, allowing a
detailed investigation of the depinning transition. Our results exhibit the
irrelevance of classical ideas to the dynamics of vortex pinning. We propose a
new picture, where the linear ac response is entirely governed by disordered
boundary conditions of a rough surface, whereas in the bulk vortices respond
freely. The universal law for Z(f) thus predicted is in remarkable agreement
with experiment, and tentatively applies to microwave data in YBaCuO films.Comment: 4 pages, 4 figures, 14 reference
Longitudinal Force on a Moving Potential
We show a formal result of the longitudinal force acting on a moving
potential. The potential can be velocity-dependent, which appears in various
interesting physical systems, such as electrons in the presence of a magnetic
flux-line, or phonons scattering off a moving vortex. By using the phase-shift
analysis, we are able to show the equivalence between the adiabatic
perturbation theory and the kinetic theory for the longitudinal force in the
dilute gas limit.Comment: RevTeX, 4 pages, revised tex
Vortex vs spinning string: Iordanskii force and gravitational Aharonov-Bohm effect
We discuss the transverse force acting on the spinning cosmic string, moving
in the background matter. It comes from the gravitational Aharonov-Bohm effect
and corresponds to the Iordanskii force acting on the vortex in superfluids,
when the vortex moves with respect to the normal component of the liquid.Comment: Latex file, 9 pages, no figures, references are added, version
submitted to JETP Let
Penetration of Josephson vortices and measurement of the c-axis penetration depth in : Interplay of Josephson coupling, surface barrier and defects
The first penetration field H_{J}(T) of Josephson vortices is measured
through the onset of microwave absorption in the locked state, in slightly
overdoped single crystals (T_{c} ~ 84
K). The magnitude of H_{J}(T) is too large to be accounted for by the first
thermodynamic critical field H_{c1}(T). We discuss the possibility of a
Bean-Livingston barrier, also supported by irreversible behavior upon flux
exit, and the role of defects, which relates H_{J}(T) to the c-axis penetration
depth . The temperature dependence of the latter, determined by
a cavity perturbation technique and a theoretical estimate of the
defect-limited penetration field are used to deduce from H_{J}(T) the absolute
value of .Comment: 9 pages, 6 figure
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
Vortex Dynamics in Selfdual Maxwell-Higgs Systems with Uniform Background Electric Charge Density
We introduce selfdual Maxwell-Higgs systems with uniform background electric
charge density and show that the selfdual equations satisfied by topological
vortices can be reduced to the original Bogomol'nyi equations without any
background. These vortices are shown to carry no spin but to feel the Magnus
force due to the shielding charge carried by the Higgs field. We also study the
dynamics of slowly moving vortices and show that the spin-statistics theorem
holds to our vortices.Comment: 24 pages + 2 figures ( not included), Cu-TP-611, IASSNS-HEP-93/33,
NSF-ITP-93-13
Guiding superconducting vortices by magnetic domain walls
We demonstrate a unique prospect for inducing anisotropic vortex pinning and
manipulating the directional motion of vortices using the stripe domain
patterns of a uniaxial magnetic film in a the superconducting/ferromagnetic
hybrid. Our observations can be described by a model, which considers
interactions between magnetic charges of vortices and surface magnetic charges
of domains resulting in the enhanced pinning of vortices on domain walls.Comment: 12 pages, 6 figure
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