225 research outputs found
Vortex deformation and breaking in superconductors: A microscopic description
Vortex breaking has been traditionally studied for nonuniform critical
current densities, although it may also appear due to nonuniform pinning force
distributions. In this article we study the case of a
high-pinning/low-pinning/high-pinning layered structure. We have developed an
elastic model for describing the deformation of a vortex in these systems in
the presence of a uniform transport current density for any arbitrary
orientation of the transport current and the magnetic field. If is above a
certain critical value, , the vortex breaks and a finite effective
resistance appears. Our model can be applied to some experimental
configurations where vortex breaking naturally exists. This is the case for
YBaCuO (YBCO) low angle grain boundaries and films on vicinal
substrates, where the breaking is experienced by Abrikosov-Josephson vortices
(AJV) and Josephson string vortices (SV), respectively. With our model, we have
experimentally extracted some intrinsic parameters of the AJV and SV, such as
the line tension and compared it to existing predictions based on
the vortex structure.Comment: 11 figures in 13 files; minor changes after printing proof
Surface-Barrier Effects in the Microwave Second-Harmonic Response of Superconductors in the Mixed State
We report on transient effects in the microwave second-order response of
different type of superconductors in the mixed state. The samples have
contemporarily been exposed to a dc magnetic field, varying with a constant
rate of 60 Oe/s, and a pulsed microwave magnetic field. The time evolution of
the signal radiated at the second-harmonic frequency of the driving field has
been measured for about 500 s from the instant in which the dc-field sweep has
been stopped, with sampling time of about 0.3 s. We show that the
second-harmonic signal exhibits two relaxation regimes; an initial exponential
decay, which endures roughly 10 s, and a logarithmic decay in the time scale of
minutes. Evidence is given that the decay in the time scale of minutes is ruled
by magnetic relaxation over the surface barrier.Comment: 6 pages, 6 embedded figure
Reducing vortex density in superconductors using the ratchet effect
A serious obstacle that impedes the application of low and high temperature
superconductor (SC) devices is the presence of trapped flux. Flux lines or
vortices are induced by fields as small as the Earth's magnetic field. Once
present, vortices dissipate energy and generate internal noise, limiting the
operation of numerous superconducting devices. Methods used to overcome this
difficulty include the pinning of vortices by the incorporation of impurities
and defects, the construction of flux dams, slots and holes and magnetic
shields which block the penetration of new flux lines in the bulk of the SC or
reduce the magnetic field in the immediate vicinity of the superconducting
device. Naturally, the most desirable would be to remove the vortices from the
bulk of the SC. There is no known phenomenon, however, that could form the
basis for such a process. Here we show that the application of an ac current to
a SC that is patterned with an asymmetric pinning potential can induce vortex
motion whose direction is determined only by the asymmetry of the pattern. The
mechanism responsible for this phenomenon is the so called ratchet effect, and
its working principle applies to both low and high temperature SCs. As a first
step here we demonstrate that with an appropriate choice of the pinning
potential the ratchet effect can be used to remove vortices from low
temperature SCs in the parameter range required for various applications.Comment: 7 pages, 4 figures, Nature (in press
Acoustic black holes for relativistic fluids
We derive a new acoustic black hole metric from the Abelian Higgs model. In
the non-relativistic limit, while the Abelian Higgs model becomes the
Ginzburg-Landau model, the metric reduces to an ordinary Unruh type. We
investigate the possibility of using (type I and II) superconductors as the
acoustic black holes. We propose to realize experimental acoustic black holes
by using spiral vortices solutions from the Navier-stokes equation in the
non-relativistic classical fluids.Comment: 16 pages. typos corrected, contents expande
Charge Induced Vortex Lattice Instability
It has been predicted that superconducting vortices should be electrically
charged and that this effect is particularly enhanced for, high temperature
superconductors.\cite{kho95,bla96} Hall effect\cite{hag91} and nuclear magnetic
resonance (NMR) experiments\cite{kum01} suggest the existence of vortex
charging, but the effects are small and the interpretation controversial. Here
we show that the Abrikosov vortex lattice, characteristic of the mixed state of
superconductors, will become unstable at sufficiently high magnetic field if
there is charge trapped on the vortex core. Our NMR measurements of the
magnetic fields generated by vortices in BiSrCaCuO
single crystals\cite{che07} provide evidence for an electrostatically driven
vortex lattice reconstruction with the magnitude of charge on each vortex
pancake of x, depending on doping, in line
with theoretical estimates.\cite{kho95,kna05}Comment: to appear in Nature Physics; 6 pages, 7 figure
Evidence of Andreev bound states as a hallmark of the FFLO phase in -(BEDT-TTF)Cu(NCS)
Superconductivity is a quantum phenomena arising, in its simplest form, from
pairing of fermions with opposite spin into a state with zero net momentum.
Whether superconductivity can occur in fermionic systems with unequal number of
two species distinguished by spin, atomic hyperfine states, flavor, presents an
important open question in condensed matter, cold atoms, and quantum
chromodynamics, physics. In the former case the imbalance between spin-up and
spin-down electrons forming the Cooper pairs is indyced by the magnetic field.
Nearly fifty years ago Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed
that such imbalanced system can lead to exotic superconductivity in which pairs
acquire finite momentum. The finite pair momentum leads to spatially
inhomogeneous state consisting of of a periodic alternation of "normal" and
"superconducting" regions. Here, we report nuclear magnetic resonance (NMR)
measurements providing microscopic evidence for the existence of this new
superconducting state through the observation of spin-polarized quasiparticles
forming so-called Andreev bound states.Comment: 6 pages, 5 fig
Theory of flux cutting and flux transport at the critical current of a type-II superconducting cylindrical wire
I introduce a critical-state theory incorporating both flux cutting and flux
transport to calculate the magnetic-field and current-density distributions
inside a type-II superconducting cylinder at its critical current in a
longitudinal applied magnetic field. The theory is an extension of the elliptic
critical-state model introduced by Romero-Salazar and Perez-Rodriguez. The
vortex dynamics depend in detail upon two nonlinear effective resistivities for
flux cutting (\rho_\parallel) and flux flow (\rho_\perp), and their ratio r =
\rho_\parallel/\rho_\perp. When r < 1, the low relative efficiency of flux
cutting in reducing the magnitude of the internal magnetic-flux density leads
to a paramagnetic longitudinal magnetic moment. As a model for understanding
the experimentally observed interrelationship between the critical currents for
flux cutting and depinning, I calculate the forces on a helical vortex arc
stretched between two pinning centers when the vortex is subjected to a current
density of arbitrary angle \phi. Simultaneous initiation of flux cutting and
flux transport occurs at the critical current density J_c(\phi) that makes the
vortex arc unstable.Comment: 14 pages, 13 figure
Hard Superconductivity of a Soft Metal in the Quantum Regime
Superconductivity is inevitably suppressed in reduced dimensionality.
Questions of how thin superconducting wires or films can be before they lose
their superconducting properties have important technological ramifications and
go to the heart of understanding coherence and robustness of the
superconducting state in quantum-confined geometries. Here, we exploit quantum
confinement of itinerant electrons in a soft metal to stabilize superconductors
with lateral dimensions of the order of a few millimeters and vertical
dimensions of only a few atomic layers. These extremely thin superconductors
show no indication of defect- or fluctuation-driven suppression of
superconductivity and sustain supercurrents of up to 10% of the depairing
current density. The extreme hardness of the critical state is attributed to
quantum trapping of vortices. This study paints a conceptually appealing,
elegant picture of a model nanoscale superconductor with calculable critical
state properties. It indicates the intriguing possibility of exploiting robust
superconductivity at the nanoscale.Comment: 15 pages, 4 figures, submitted to Nature Physic
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