16,635 research outputs found
Buckling and d-Wave Pairing in HiTc-Superconductors
We have investigated whether the electron-phonon interaction can support a
d-wave gap-anisotropy. On the basis of models derived from LDA calculations, as
well as LDA linear-response calculations we argue that this is the case, for
materials with buckled or dimpled CuO2 planes, for the so-called buckling
modes, which involve out-of-plane movements of the plane oxygens.Comment: 5pages, Latex2e, 6 Postscript figure
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
The prompt energy release of gamma-ray bursts using a cosmological k-correction
The fluences of gamma-ray bursts (GRBs) are measured with a variety of
instruments in different detector energy ranges. A detailed comparison of the
implied energy releases of the GRB sample requires, then, an accurate
accounting of this diversity in fluence measurements which properly corrects
for the redshifting of GRB spectra. Here, we develop a methodology to
``k-correct'' the implied prompt energy release of a GRB to a fixed co-moving
bandpass. This allows us to homogenize the prompt energy release of 17
cosmological GRBs (using published redshifts, fluences, and spectra) to two
common co-moving bandpasses: 20-2000 keV and 0.1 keV-10 MeV (``bolometric'').
While the overall distribution of GRB energy releases does not change
significantly by using a k-correction, we show that uncorrected energy
estimates systematically undercounts the bolometric energy by ~5% to 600%,
depending on the particular GRB. We find that the median bolometric
isotropic-equivalent prompt energy release is 2.2 x 10^{53} erg with an r.m.s.
scatter of 0.80 dex. The typical estimated uncertainty on a given k-corrected
energy measurement is ~20%.Comment: Accepted to the Astronomical Journal. 21 pages (LaTeX) and 4 figure
W Plus Multiple Jets at the LHC with High Energy Jets
We study the production of a W boson in association with n hard QCD jets (for
n>=2), with a particular emphasis on results relevant for the Large Hadron
Collider (7 TeV and 8 TeV). We present predictions for this process from High
Energy Jets, a framework for all-order resummation of the dominant
contributions from wide-angle QCD emissions. We first compare predictions
against recent ATLAS data and then shift focus to observables and regions of
phase space where effects beyond NLO are expected to be large.Comment: 19 pages, 9 figure
On the Inelastic Collapse of a Ball Bouncing on a Randomly Vibrating Platform
We study analytically the dynamics of a ball bouncing inelastically on a
randomly vibrating platform, as a simple toy model of inelastic collapse. Of
principal interest are the distributions of the number of flights n_f till the
collapse and the total time \tau_c elapsed before the collapse. In the strictly
elastic case, both distributions have power law tails characterised by
exponents which are universal, i.e., independent of the details of the platform
noise distribution. In the inelastic case, both distributions have exponential
tails: P(n_f) ~ exp[-\theta_1 n_f] and P(\tau_c) ~ exp[-\theta_2 \tau_c]. The
decay exponents \theta_1 and \theta_2 depend continuously on the coefficient of
restitution and are nonuniversal; however as one approches the elastic limit,
they vanish in a universal manner that we compute exactly. An explicit
expression for \theta_1 is provided for a particular case of the platform noise
distribution.Comment: 32 page
Out-of-plane instability and electron-phonon contribution to s- and d-wave pairing in high-temperature superconductors; LDA linear-response calculation for doped CaCuO2 and a generic tight-binding model
The equilibrium structure, energy bands, phonon dispersions, and s- and
d-channel electron-phonon interactions (EPIs) are calculated for the
infinite-layer superconductor CaCuO2 doped with 0.24 holes per CuO2. The LDA
and the linear-response full-potential LMTO method were used. In the
equilibrium structure, oxygen is found to buckle slightly out of the plane and,
as a result, the characters of the energy bands near EF are found to be similar
to those of other optimally doped HTSCs. For the EPI we find lambda(s)=0.4, in
accord with previous LDA calculations for YBa2Cu3O7. This supports the common
belief that the EPI mechanism alone is insufficient to explain HTSC.
Lambda(x^2-y^2) is found to be positive and nearly as large as lambda(s). This
is surprising and indicates that the EPI could enhance some other d-wave
pairing mechanism. Like in YBa2Cu3O7, the buckling modes contribute
significantly to the EPI, although these contributions are proportional to the
static buckling and would vanish for flat planes. These numerical results can
be understood from a generic tight-binding model originally derived from the
LDA bands of YBa2Cu3O7. In the future, the role of anharmonicity of the
buckling-modes and the influence of the spin-fluctuations should be
investigated.Comment: 19 pages, 9 Postscript figures, Late
Theory of the magnetoeletric effect in a lightly doped high-Tc cuprate
In a recent study Viskadourakis et al. discovered that extremely underdoped
La_2CuO_(4+x) is a relaxor ferroelectric and a magnetoelectric material at low
temperatures. It is further observed that the magnetoelectric response is
anisotropic for different directions of electric polarization and applied
magnetic field. By constructing an appropriate Landau theory, we show that a
bi-quadratic magnetoelectric coupling can explain the experimentally observed
polarization dependence on magnetic field. This coupling leads to several novel
low-temperature effects including a feedback enhancement of the magnetization
below the ferroelectric transition, and a predicted magnetocapacitive effect.Comment: 5 pages, 4 figure
0-pi transitions in Josephson junctions with antiferromagnetic interlayers
We show that the dc Josephson current through
superconductor-antiferromagnet-superconductor (S/AF/S) junctions manifests a
remarkable atomic scale dependence on the interlayer thickness. At low
temperatures the junction is either a 0- or pi-junction depending on whether
the AF interlayer consists of an even or odd number of atomic layers. This is
associated with different symmetries of the AF interlayers in the two cases. In
the junction with odd AF interlayers an additional pi-0 transition can take
place as a function of temperature. This originates from the interplay of
spin-split Andreev bound states. Experimental implications of these theoretical
findings are discussed.Comment: 4 pages, 2 figure
Conductance calculations for quantum wires and interfaces: mode matching and Green functions
Landauer's formula relates the conductance of a quantum wire or interface to
transmission probabilities. Total transmission probabilities are frequently
calculated using Green function techniques and an expression first derived by
Caroli. Alternatively, partial transmission probabilities can be calculated
from the scattering wave functions that are obtained by matching the wave
functions in the scattering region to the Bloch modes of ideal bulk leads. An
elegant technique for doing this, formulated originally by Ando, is here
generalized to any Hamiltonian that can be represented in tight-binding form. A
more compact expression for the transmission matrix elements is derived and it
is shown how all the Green function results can be derived from the mode
matching technique. We illustrate this for a simple model which can be studied
analytically, and for an Fe|vacuum|Fe tunnel junction which we study using
first-principles calculations.Comment: 14 pages, 5 figure
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