32 research outputs found
Origin of Intrinsic Josephson Coupling in the Cuprates and Its Relation to Order Parameter Symmetry: An Incoherent Hopping Model
Experiments on the cuprate superconductors demonstrate that these materials
may be viewed as a stack of Josephson junctions along the c-direction. In this
paper, we present a model which describes this intrinsic Josephson coupling in
terms of incoherent quasiparticle hopping along the c-axis arising from
wave-function overlap, impurity-assisted hopping, and boson-assisted hopping.
We use this model to compute the magnitude and temperature T dependence of the
resulting Josephson critical current j_c (T) for s- and d-wave superconductors.
Contrary to other approaches, d-wave pairing in this model is compatible with
an intrinsic Josephson effect at all hole concentrations and leads to j_c (T)
\propto T at low T. By parameterizing our theory with c-axis resistivity data
from YBCO, we estimate j_c (T) for optimally doped and underdoped members of
this family. Our estimates suggest that further experiments on this compound
would be of great help in elucidating the validity of our model in general and
the pairing symmetry in particular. We also discuss the implications of our
model for LSCO and BSCCO.Comment: 28 pages, REVTEX, 5 compressed PostScript figures. Substantially
expanded and revised from the earlier version. To appear in Physica
Dynamics of 2D pancake vortices in layered superconductors
The dynamics of 2D pancake vortices in Josephson-coupled
superconducting/normal - metal multilayers is considered within the
time-dependent Ginzburg-Landau theory. For temperatures close to a
viscous drag force acting on a moving 2D vortex is shown to depend strongly on
the conductivity of normal metal layers. For a tilted vortex line consisting of
2D vortices the equation of viscous motion in the presence of a transport
current parallel to the layers is obtained. The specific structure of the
vortex line core leads to a new dynamic behavior and to substantial deviations
from the Bardeen-Stephen theory. The viscosity coefficient is found to depend
essentially on the angle between the magnetic field and the
axis normal to the layers. For field orientations close to the layers
the nonlinear effects in the vortex motion appear even for slowly moving vortex
lines (when the in-plane transport current is much smaller than the
Ginzburg-Landau critical current). In this nonlinear regime the viscosity
coefficient depends logarithmically on the vortex velocity .Comment: 15 pages, revtex, no figure
Effects of columnar disorder on flux-lattice melting in high-temperature superconductors
The effect of columnar pins on the flux-lines melting transition in
high-temperature superconductors is studied using Path Integral Monte Carlo
simulations. We highlight the similarities and differences in the effects of
columnar disorder on the melting transition in YBaCuO
(YBCO) and the highly anisotropic BiSrCaCuO (BSCCO) at
magnetic fields such that the mean separation between flux-lines is smaller
than the penetration length. For pure systems, a first order transition from a
flux-line solid to a liquid phase is seen as the temperature is increased. When
adding columnar defects to the system, the transition temperature is not
affected in both materials as long as the strength of an individual columnar
defect (expressed as a flux-line defect interaction) is less than a certain
threshold for a given density of randomly distributed columnar pins. This
threshold strength is lower for YBCO than for BSCCO. For higher strengths the
transition line is shifted for both materials towards higher temperatures, and
the sharp jump in energy, characteristic of a first order transition, gives way
to a smoother and gradual rise of the energy, characteristic of a second order
transition. Also, when columnar defects are present, the vortex solid phase is
replaced by a pinned Bose glass phase and this is manifested by a marked
decrease in translational order and orientational order as measured by the
appropriate structure factors. For BSCCO, we report an unusual rise of the
translational order and the hexatic order just before the melting transition.
No such rise is observed in YBCO.Comment: 32 pages, 13 figures, revte
NMR and NQR Fluctuation Effects in Layered Superconductors
We study the effect of thermal fluctuations of the s-wave order parameter of
a quasi two dimensional superconductor on the nuclear spin relaxation rate near
the transition temperature Tc. We consider both the effects of the amplitude
fluctuations and the Berezinskii-Kosterlitz-Thouless (BKT) phase fluctuations
in weakly coupled layered superconductors. In the treatment of the amplitude
fluctuations we employ the Gaussian approximation and evaluate the longitudinal
relaxation rate 1/T1 for a clean s-wave superconductor, with and without pair
breaking effects, using the static pair fluctuation propagator D. The increase
in 1/T1 due to pair breaking in D is overcompensated by the decrease arising
from the single particle Green's functions. The result is a strong effect on
1/T1 for even a small amount of pair breaking. The phase fluctuations are
described in terms of dynamical BKT excitations in the form of pancake
vortex-antivortex (VA) pairs. We calculate the effect of the magnetic field
fluctuations caused by the translational motion of VA excitations on 1/T1 and
on the transverse relaxation rate 1/T2 on both sides of the BKT transitation
temperature T(BKT)<Tc. The results for the NQR relaxation rates depend strongly
on the diffusion constant that governs the motion of free and bound vortices as
well as the annihilation of VA pairs. We discuss the relaxation rates for real
multilayer systems where the diffusion constant can be small and thus increase
the lifetime of a VA pair, leading to an enhancement of the rates. We also
discuss in some detail the experimental feasibility of observing the effects of
amplitude fluctuations in layered s-wave superconductors such as the
dichalcogenides and the effects of phase fluctuations in s- or d-wave
superconductors such as the layered cuprates.Comment: 38 pages, 12 figure
Energy and Flux Measurements of Ultra-High Energy Cosmic Rays Observed During the First ANITA Flight
The first flight of the Antarctic Impulsive Transient Antenna (ANITA)
experiment recorded 16 radio signals that were emitted by cosmic-ray induced
air showers. For 14 of these events, this radiation was reflected from the ice.
The dominant contribution to the radiation from the deflection of positrons and
electrons in the geomagnetic field, which is beamed in the direction of motion
of the air shower. This radiation is reflected from the ice and subsequently
detected by the ANITA experiment at a flight altitude of 36km. In this paper,
we estimate the energy of the 14 individual events and find that the mean
energy of the cosmic-ray sample is 2.9 EeV. By simulating the ANITA flight, we
calculate its exposure for ultra-high energy cosmic rays. We estimate for the
first time the cosmic-ray flux derived only from radio observations. In
addition, we find that the Monte Carlo simulation of the ANITA data set is in
agreement with the total number of observed events and with the properties of
those events.Comment: Added more explanation of the experimental setup and textual
improvement
Acceleration of Relativistic Protons during the 20 January 2005 Flare and CME
The origin of relativistic solar protons during large flare/CME events has
not been uniquely identified so far.We perform a detailed comparative analysis
of the time profiles of relativistic protons detected by the worldwide network
of neutron monitors at Earth with electromagnetic signatures of particle
acceleration in the solar corona during the large particle event of 20 January
2005. The intensity-time profile of the relativistic protons derived from the
neutron monitor data indicates two successive peaks. We show that microwave,
hard X-ray and gamma-ray emissions display several episodes of particle
acceleration within the impulsive flare phase. The first relativistic protons
detected at Earth are accelerated together with relativistic electrons and with
protons that produce pion decay gamma-rays during the second episode. The
second peak in the relativistic proton profile at Earth is accompanied by new
signatures of particle acceleration in the corona within approximatively 1
solar radius above the photosphere, revealed by hard X-ray and microwave
emissions of low intensity, and by the renewed radio emission of electron beams
and of a coronal shock wave. We discuss the observations in terms of different
scenarios of particle acceleration in the corona.Comment: 22 pages, 5 figure
The Antarctic Impulsive Transient Antenna Ultra-high Energy Neutrino Detector Design, Performance, and Sensitivity for 2006-2007 Balloon Flight
We present a detailed report on the experimental details of the Antarctic
Impulsive Transient Antenna (ANITA) long duration balloon payload, including
the design philosophy and realization, physics simulations, performance of the
instrument during its first Antarctic flight completed in January of 2007, and
expectations for the limiting neutrino detection sensitivity. Neutrino physics
results will be reported separately.Comment: 50 pages, 49 figures, in preparation for PR
The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres
Laboratory astrophysics and astrochemistr
Equilibrium intermediate-state patterns in a type-I superconducting slab in an arbitrarily oriented applied magnetic field
The equilibrium topology of superconducting and normal domains in flat type-I superconductors is investigated. Important improvements with respect to previous work are that (1) the energy of the external magnetic field, as deformed by the presence of superconducting domains, is calculated in the same way for three different topologies and (2) calculations are made for arbitrary orientation of the applied field. A phase diagram is presented for the minimum-energy topology as a function of applied field magnitude and angle. For small (large) applied fields, normal (superconducting) tubes are found, while for intermediate fields, parallel domains have a lower energy. The range of field magnitudes for which the superconducting-tubes structure is favored shrinks when the field is more in-plane oriented. © 2013 American Physical Society