42,445 research outputs found
Antiferromagnetic correlations and impurity broadening of NMR linewidths in cuprate superconductors
We study a model of a d-wave superconductor with strong potential scatterers
in the presence of antiferromagnetic correlations and apply it to experimental
nuclear magnetic resonance (NMR) results on Zn impurities in the
superconducting state of YBCO. We then focus on the contribution of
impurity-induced paramagnetic moments, with Hubbard correlations in the host
system accounted for in Hartree approximation. We show that local magnetism
around individual impurities broadens the line, but quasiparticle interference
between impurity states plays an important role in smearing out impurity
satellite peaks. The model, together with estimates of vortex lattice effects,
provides a semi-quantitative description of the impurity concentration
dependence of the NMR line shape in the superconducting state, and gives a
qualitative description of the temperature dependence of the line asymmetry. We
argue that impurity-induced paramagnetism and resonant local density of states
effects are both necessary to explain existing experiments.Comment: 15 pages, 23 figures, submitted to Phys. Rev.
Extinction of quasiparticle interference in underdoped cuprates with coexisting order
Recent scanning tunnelling spectroscopy measurements [Y. Koksaka et al.,
Nature 454, 1072 (2008)] have shown that dispersing quasiparticle interference
peaks in Fourier transformed conductance maps disappear as the bias voltage
exceeds a certain threshold corresponding to the coincidence of the contour of
constant quasiparticle energy with the antiferromagnetic zone boundary. Here we
argue that this is caused by quasistatic short-range coexisting order present
in the d-wave superconducting phase, and that the most likely origin of this
order is disorder-induced incommensurate antiferromagnetism. We show explicitly
how the peaks are extinguished in the related situation with coexisting
long-range antiferromagnetic order, and discuss the connection with the
realistic disordered case. Since it is the localized quasiparticle interference
peaks rather than the underlying antinodal states themselves which are
destroyed at a critical bias, our proposal resolves a conflict between scanning
tunneling spectroscopy and photoemission regarding the nature of these states.Comment: 10 pages, 9 figure
Disorder-induced freezing of dynamical spin fluctuations in underdoped cuprates
We study the dynamical spin susceptibility of a correlated d-wave
superconductor (dSC) in the presence of disorder, using an unrestricted
Hartree-Fock approach. This model provides a concrete realization of the notion
that disorder slows down spin fluctuations, which eventually "freeze out". The
evolution of disorder-induced spectral weight transfer agrees qualitatively
with experimental observations on underdoped cuprate superconductors. For
sufficiently large disorder concentrations, static spin density wave (SDW)
order is created when droplets of magnetism nucleated by impurities overlap. We
also study the disordered stripe state coexisting with a dSC and compare its
magnetic fluctuation spectrum to that of the disorder-generated SDW phase.Comment: 5 pages, 4 figure
A computer program for anisotropic shallow-shell finite elements using symbolic integration
A FORTRAN computer program for anisotropic shallow-shell finite elements with variable curvature is described. A listing of the program is presented together with printed output for a sample case. Computation times and central memory requirements are given for several different elements. The program is based on a stiffness (displacement) finite-element model in which the fundamental unknowns consist of both the displacement and the rotation components of the reference surface of the shell. Two triangular and four quadrilateral elements are implemented in the program. The triangular elements have 6 or 10 nodes, and the quadrilateral elements have 4 or 8 nodes. Two of the quadrilateral elements have internal degrees of freedom associated with displacement modes which vanish along the edges of the elements (bubble modes). The triangular elements and the remaining two quadrilateral elements do not have bubble modes. The output from the program consists of arrays corresponding to the stiffness, the geometric stiffness, the consistent mass, and the consistent load matrices for individual elements. The integrals required for the generation of these arrays are evaluated by using symbolic (or analytic) integration in conjunction with certain group-theoretic techniques. The analytic expressions for the integrals are exact and were developed using the symbolic and algebraic manipulation language
Is the Redshift Clustering of Long-Duration Gamma-Ray Bursts Significant?
The 26 long-duration gamma-ray bursts (GRBs) with known redshifts form a
distinct cosmological set, selected differently than other cosmological probes
such as quasars and galaxies. Since the progenitors are now believed to be
connected with active star-formation and since burst emission penetrates dust,
one hope is that with a uniformly-selected sample, the large-scale redshift
distribution of GRBs can help constrain the star-formation history of the
Universe. However, we show that strong observational biases in ground-based
redshift discovery hamper a clean determination of the large-scale GRB rate and
hence the connection of GRBs to the star formation history. We then focus on
the properties of the small-scale (clustering) distribution of GRB redshifts.
When corrected for heliocentric motion relative to the local Hubble flow, the
observed redshifts appear to show a propensity for clustering: 8 of 26 GRBs
occurred within a recession velocity difference of 1000 km/s of another GRB.
That is, 4 pairs of GRBs occurred within 30 h_65^-1 Myr in cosmic time, despite
being causally separated on the sky. We investigate the significance of this
clustering. Comparison of the numbers of close redshift pairs expected from the
simulation with that observed shows no significant small-scale clustering
excess in the present sample; however, the four close pairs occur only in about
twenty percent of the simulated datasets (the precise significance of the
clustering is dependent upon the modeled biases). We conclude with some
impetuses and suggestions for future precise GRB redshift measurements.Comment: Published in the Astronomical Journal, June 2003: see
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2003AJ....125.2865
Adapting the interior point method for the solution of LPs on serial, coarse grain parallel and massively parallel computers
In this paper we describe a unified scheme for implementing an interior point algorithm (IPM) over a range of computer architectures. In the inner iteration of the IPM a search direction is computed using Newton's method. Computationally this involves solving a sparse symmetric positive definite (SSPD) system of equations. The choice of direct and indirect methods for the solution of this system, and the design of data structures to take advantage of serial, coarse grain parallel and massively parallel computer architectures, are considered in detail. We put forward arguments as to why integration of the system within a sparse simplex solver is important and outline how the system is designed to achieve this integration
Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors
We investigate the emergent impurity-induced states arising from point-like
scatterers in the spin-density wave phase of iron-based superconductors within
a microscopic five-band model. Independent of the details of the band-structure
and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional
nematogens are formed locally by the impurities. Interestingly, these
nematogens exhibit a dimer structure in the electronic density, are directed
along the antiferromagnetic a-axis, and have typical lengths of order 10
lattice constants in excellent agreement with recent scanning tunnelling
experiments. These electronic dimers provide a natural explanation of the
dopant-induced transport anisotropy found e.g. in the 122 iron pnictides.Comment: 5 pages, 4 figure
Coreshine in L1506C - Evidence for a primitive big-grain component or indication for a turbulent core history?
The recently discovered coreshine effect can aid in exploring the core
properties and in probing the large grain population of the ISM. We discuss the
implications of the coreshine detected from the molecular cloud core L1506C in
the Taurus filament for the history of the core and the existence of a
primitive ISM component of large grains becoming visible in cores. The
coreshine surface brightness of L1506C is determined from IRAC Spitzer images
at 3.6 micron. We perform grain growth calculations to estimate the grain size
distribution in model cores similar in gas density, radius, and turbulent
velocity to L1506C. Scattered light intensities at 3.6 micron are calculated
for a variety of MRN and grain growth distributions to compare with the
observed coreshine. For a core with the overall physical properties of L1506C,
no detectable coreshine is predicted for an MRN size distribution. Extending
the distribution to grain radii of about 0.65 m allows to reproduce the
observed surface brightness level in scattered light. Assuming the properties
of L1506C to be preserved, models for the growth of grains in cores do not
yield sufficient scattered light to account for the coreshine within the
lifetime of the Taurus complex. Only increasing the core density and the
turbulence amplifies the scattered light intensity to a level consistent with
the observed coreshine brightness. The grains could be part of primitive
omni-present large grain population becoming visible in the densest part of the
ISM, could grow under the turbulent dense conditions of former cores, or in
L1506C itself. In the later case, L1506C must have passed through a period of
larger density and stronger turbulence. This would be consistent with the
surprisingly strong depletion usually attributed to high column densities, and
with the large-scale outward motion of the core envelope observed today.Comment: 6 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
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