516 research outputs found
Local modulations of the spin-fluctuation mediated pairing interaction by impurities in d-wave superconductors
We present a self-consistent real space formulation of spin-fluctuation
mediated d-wave pairing. By calculating all relevant inhomogeneous spin and
charge susceptibilities in real space within the random phase approximation
(RPA), we obtain the effective pairing interaction and study its spatial
dependence near both local potential and hopping impurities. A remarkably large
enhancement of the pairing interaction may be obtained near the impurity site.
We discuss the relevance of our result to inhomogeneities observed by scanning
tunneling spectroscopy on the surface of cuprate superconductors.Comment: 8 pages, 7 figure
Andreev bound states at a cuprate grain boundary junction: A lower bound for the upper critical field
We investigate in-plane quasiparticle tunneling across thin film grain
boundary junctions (GBJs) of the electron-doped cuprate
LaCeCuO in magnetic fields up to T, perpendicular to
the CuO layers. The differential conductance in the superconducting state
shows a zero bias conductance peak (ZBCP) due to zero energy surface Andreev
bound states. With increasing temperature , the ZBCP vanishes at the
critical temperature K if B=0, and at K for B=16 T. As
the ZBCP is related to the macroscopic phase coherence of the superconducting
state, we argue that the disappearance of the ZBCP at a field
must occur below the upper critical field of the superconductor. We
find T which is at least a factor of 2.5 higher than
previous estimates of .Comment: 4 pages, 4 figure
Structure of BSCCO supermodulation from ab initio calculations
We present results of density functional theory (DFT) calculation of the
structural supermodulation in BSCCO-2212 structure, and show that the
supermodulation is indeed a spontaneous symmetry breaking of the nominal
crystal symmetry, rather than a phenomenon driven by interstitial O dopants.
The structure obtained is in excellent quantitative agreement with recent x-ray
studies, and reproduces several qualitative aspects of scanning tunnelling
microscopy (STM) experiments as well. The primary structural modulation
affecting the CuO_2 plane is found to be a buckling wave of tilted CuO_5
half-octahedra, with maximum tilt angle near the phase of the supermodulation
where recent STM experiments have discovered an enhancement of the
superconducting gap. We argue that the tilting of the half-octahedra and
concommitant planar buckling are directly modulating the superconducting pair
interaction.Comment: 4 pages, 3 figure
Time scales of epidemic spread and risk perception on adaptive networks
Incorporating dynamic contact networks and delayed awareness into a contagion
model with memory, we study the spreading patterns of infectious diseases in
connected populations. It is found that the spread of an infectious disease is
not only related to the past exposures of an individual to the infected but
also to the time scales of risk perception reflected in the social network
adaptation. The epidemic threshold is found to decrease with the rise
of the time scale parameter s and the memory length T, they satisfy the
equation .
Both the lifetime of the epidemic and the topological property of the evolved
network are considered. The standard deviation of the degree
distribution increases with the rise of the absorbing time , a power-law
relation is found
Comparison of CT colonography, colonoscopy, sigmoidoscopy and faecal occult blood tests for the detection of advanced adenoma in an average risk population.
Cep164, a novel centriole appendage protein required for primary cilium formation
Primary cilia (PC) function as microtubule-based sensory antennae projecting from the surface of many eukaryotic cells. They play important roles in mechano- and chemosensory perception and their dysfunction is implicated in developmental disorders and severe diseases. The basal body that functions in PC assembly is derived from the mature centriole, a component of the centrosome. Through a small interfering RNA screen we found several centrosomal proteins (Ceps) to be involved in PC formation. One newly identified protein, Cep164, was indispensable for PC formation and hence characterized in detail. By immunogold electron microscopy, Cep164 could be localized to the distal appendages of mature centrioles. In contrast to ninein and Cep170, two components of subdistal appendages, Cep164 persisted at centrioles throughout mitosis. Moreover, the localizations of Cep164 and ninein/Cep170 were mutually independent during interphase. These data implicate distal appendages in PC formation and identify Cep164 as an excellent marker for these structures
Influence of Fermi surface topology on the quasiparticle spectrum in the vortex state
We study the influence of Fermi surface topology on the quasiparticle density
of states in the vortex state of type II superconductors. We observe that the
field dependence and the shape of the momentum and spatially averaged density
of states is affected significantly by the topology of the Fermi surface. We
show that this behavior can be understood in terms of characteristic Fermi
surface functions and that an important role is played by the number of points
on the Fermi surface at which the Fermi velocity is directed parallel to the
magnetic field. A critical comparison is made with a broadened BCS type density
of states, that has been used frequently in analysis of tunneling data. We
suggest a new formula as a replacement for the broadened BCS model for the
special case of a cylindrical Fermi surface. We apply our results to the two
gap superconductor MgB and show that in this particular case the field
dependence of the partial densities of states of the two gaps behaves very
differently due to the different topologies of the corresponding Fermi
surfaces, in qualitative agreement with recent tunneling experiments.Comment: 12 pages 12 figure
Sensitivity of the superconducting state and magnetic susceptibility to key aspects of electronic structure in ferropnictides
Experiments on the iron-pnictide superconductors appear to show some
materials where the ground state is fully gapped, and others where low-energy
excitations dominate, possibly indicative of gap nodes. Within the framework of
a 5-orbital spin fluctuation theory for these systems, we discuss how changes
in the doping, the electronic structure or interaction parameters can tune the
system from a fully gapped to nodal sign-changing gap with s-wave ()
symmetry (). In particular we focus on the role of the hole pocket at
the point of the unfolded Brillouin zone identified as crucial to
the pairing by Kuroki {\it et al.}, and show that its presence leads to
additional nesting of hole and electron pockets which stabilizes the isotropic
state. The pocket's contribution to the pairing can be tuned by doping,
surface effects, and by changes in interaction parameters, which we examine.
Analytic expressions for orbital pairing vertices calculated within the RPA
fluctuation exchange approximation allow us to draw connections between aspects
of electronic structure, interaction parameters, and the form of the
superconducting gap
Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy
To test the dust torus model for active galactic nuclei directly, we study
the extent and morphology of the nuclear dust distribution in the Circinus
galaxy using high resolution interferometric observations in the mid-infrared
with the MIDI instrument at the Very Large Telescope Interferometer. We find
that the dust distribution in the nucleus of Circinus can be explained by two
components, a dense and warm disk-like component of 0.4 pc size and a slightly
cooler, geometrically thick torus component with a size of 2.0 pc. The disk
component is oriented perpendicular to the ionisation cone and outflow and
seems to show the silicate feature at 10 micron in emission. It coincides with
a nuclear maser disk in orientation and size. From the energy needed to heat
the dust, we infer a luminosity of the accretion disk corresponding to 20% of
the Eddington luminosity of the nuclear black hole. We find that the
interferometric data are inconsistent with a simple, smooth and axisymmetric
dust emission. The irregular behaviour of the visibilities and the shallow
decrease of the dust temperature with radius provide strong evidence for a
clumpy or filamentary dust structure. We see no evidence for dust reprocessing,
as the silicate absorption profile is consistent with that of standard galactic
dust. We argue that the collimation of the ionising radiation must originate in
the geometrically thick torus component. Our findings confirm the presence of a
geometrically thick, torus-like dust distribution in the nucleus of Circinus,
as required in unified schemes of Seyfert galaxies. Several aspects of our data
require that this torus is irregular, or "clumpy".Comment: 20 pages, 16 figures, accepted for publication by A&
Magnetic order in orbital models of the iron pnictides
We examine the appearance of the experimentally-observed stripe
spin-density-wave magnetic order in five different orbital models of the iron
pnictide parent compounds. A restricted mean-field ansatz is used to determine
the magnetic phase diagram of each model. Using the random phase approximation,
we then check this phase diagram by evaluating the static spin susceptibility
in the paramagnetic state close to the mean-field phase boundaries. The momenta
for which the susceptibility is peaked indicate in an unbiased way the actual
ordering vector of the nearby mean-field state. The dominant orbitally resolved
contributions to the spin susceptibility are also examined to determine the
origin of the magnetic instability. We find that the observed stripe magnetic
order is possible in four of the models, but it is extremely sensitive to the
degree of the nesting between the electron and hole Fermi pockets. In the more
realistic five-orbital models, this order competes with a strong-coupling
incommensurate state which appears to be controlled by details of the
electronic structure below the Fermi energy. We conclude by discussing the
implications of our work for the origin of the magnetic order in the pnictides.Comment: 19 pages, 19 figures; published version, typos corrected, references
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