553 research outputs found
BlackMax: A black-hole event generator with rotation, recoil, split branes and brane tension
We present a comprehensive black-hole event generator, BlackMax, which
simulates the experimental signatures of microscopic and Planckian black-hole
production and evolution at the LHC in the context of brane world models with
low-scale quantum gravity. The generator is based on phenomenologically
realistic models free of serious problems that plague low-scale gravity, thus
offering more realistic predictions for hadron-hadron colliders. The generator
includes all of the black-hole graybody factors known to date and incorporates
the effects of black-hole rotation, splitting between the fermions, non-zero
brane tension and black-hole recoil due to Hawking radiation (although not all
simultaneously).
The generator can be interfaced with Herwig and Pythia.Comment: 32 pages, 61 figures, webpage
http://www-pnp.physics.ox.ac.uk/~issever/BlackMax/blackmax.htm
Magnetic Coherence as a Universal Feature of Cuprate Superconductors
Recent inelastic neutron scattering (INS) experiments on
LaSrCuO have established the existence of a {\it magnetic
coherence effect}, i.e., strong frequency and momentum dependent changes of the
spin susceptibility, , in the superconducting phase. We show, using the
spin-fermion model for incommensurate antiferromagnetic spin fluctuations, that
the magnetic coherence effect establishes the ability of INS experiments to
probe the electronic spectrum of the cuprates, in that the effect arises from
the interplay of an incommensurate magnetic response, the form of the
underlying Fermi surface, and the opening of the d-wave gap in the fermionic
spectrum. In particular, we find that the magnetic coherence effect observed in
INS experiments on LaSrCuO requires that the Fermi surface be
closed around up to optimal doping. We present several predictions
for the form of the magnetic coherence effect in YBaCuO in
which an incommensurate magnetic response has been observed in the
superconducting state.Comment: 9 pages, 12 figures; extended version of Phys. Rev B, R6483 (2000
Transport Anomalies and the Role of Pseudogap in the "60-K Phase" of YBa_{2}Cu_{3}O_{7-\delta}
We report the result of our accurate measurements of the a- and b-axis
resistivity, Hall coefficient, and the a-axis thermopower in untwinned
YBa_{2}Cu_{3}O_{y} single crystals in a wide range of doping. It is found that
both the a-axis resistivity and the Hall conductivity show anomalous
dependences on the oxygen content y in the "60-K phase" below the pseudogap
temperature T^*. The complete data set enables us to narrow down the possible
pictures of the 60-K phase, with which we discuss a peculiar role of the
pseudogap in the charge transport.Comment: 4 pages, 4 figures, accepted for publication in PR
A Theory of the Longitudinal and Hall Conductivities of the Cuprate Superconductors
We establish the applicability to transport phenomena in the cuprate
superconductors of a nearly antiferromagnetic Fermi liquid (NAFL) description
of the magnetic interaction between planar quasiparticles by using it to obtain
the temperature dependent resistivity and Hall conductivity seen experimentally
in the normal state. Following a perturbative calculation of the anisotropic
(as one goes around the Fermi surface) quasiparticle lifetimes which are the
hallmark of a NAFL, we obtain simple approximate expressions for the
longitudinal, , and Hall, , conductivities which
reflect the magnetic crossovers seen experimentally as one varies the doping
level and temperature. We present a simple phenomenological model for the
variation in mean free path around the Fermi surface, and use this to extract
from experiments on and quasiparticle lifetimes in
the hot (strongly coupled quasiparticle) and cold (weakly coupled
quasiparticle) regions of the Fermi surface which are consistent with the
perturbation theory estimates. We improve upon the latter by carrying out
direct numerical (non-variational) solutions of the Boltzmann equation for
representative members of the YBaCuO and
LaSrCuO systems, with results for transport properties in
quantitative agreement with experiment. Using the same numerical approach we
study the influence of CuO chains on the a-b plane anisotropy and find results
in agreement with experimental findings in YBaCuO.Comment: 49 pages + 24 PostScript figure
NMR and Neutron Scattering Experiments on the Cuprate Superconductors: A Critical Re-Examination
We show that it is possible to reconcile NMR and neutron scattering
experiments on both LSCO and YBCO, by making use of the Millis-Monien-Pines
mean field phenomenological expression for the dynamic spin-spin response
function, and reexamining the standard Shastry-Mila-Rice hyperfine Hamiltonian
for NMR experiments. The recent neutron scattering results of Aeppli et al on
LSCO (x=14%) are shown to agree quantitatively with the NMR measurements of
and the magnetic scaling behavior proposed by Barzykin and Pines.
The reconciliation of the relaxation rates with the degree of
incommensuration in the spin fluctuation spectrum seen in neutron experiments
is achieved by introducing a new transferred hyperfine coupling between
oxygen nuclei and their next nearest neighbor spins; this leads to a
near-perfect cancellation of the influence of the incommensurate spin
fluctuation peaks on the oxygen relaxation rates of LSCO. The inclusion of the
new term also leads to a natural explanation, within the one-component
model, the different temperature dependence of the anisotropic oxygen
relaxation rates for different field orientations, recently observed by
Martindale . The measured significant decrease with doping of the
anisotropy ratio, in LSCO system, from
for to for LSCO (x=15%) is made compatible with the
doping dependence of the shift in the incommensurate spin fluctuation peaks
measured in neutron experiments, by suitable choices of the direct and
transferred hyperfine coupling constants and B.Comment: 24 pages in RevTex, 9 figures include
Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRuSb based on a Realistic Tight-binding Model with Strong Correlation
The filled-skutterudite compound CeRuSb shows a pseudo-gap
structure in the optical conductivity spectra similar to the Kondo insulators,
but metallic behavior below 80 K. The resistivity shows a large peak at 80 K,
and the Seebeck coefficient is positive and also shows a large peak at nearly
the same temperature. In order to explain all these features, a simplified
tight-binding model, which captures the essential features of the band
calculation, is proposed. Using this model and introducing the correlation
effect within the framework of the dynamical mean field approximation and the
iterative perturbation theory, the temperature dependences of the optical
conductivity, resistivity and the Seebeck coefficient are calculated, which can
explain the experiments.Comment: 4 pages, 6 figure
Andreev Bound States and Self-Consistent Gap Functions for SNS and SNSNS Systems
Andreev bound states in clean, ballistic SNS and SNSNS junctions are
calculated exactly and by using the Andreev approximation (AA). The AA appears
to break down for junctions with transverse dimensions chosen such that the
motion in the longitudinal direction is very slow. The doubly degenerate states
typical for the traveling waves found in the AA are replaced by two standing
waves in the exact treatment and the degeneracy is lifted.
A multiple-scattering Green's function formalism is used, from which the
states are found through the local density of states. The scattering by the
interfaces in any layered system of ballistic normal metals and clean
superconducting materials is taken into account exactly. The formalism allows,
in addition, for a self-consistent determination of the gap function. In the
numerical calculations the pairing coupling constant for aluminum is used.
Various features of the proximity effect are shown
Homogeneity, Flatness and "Large" Extra Dimensions
We consider a model in which the universe is the direct product of a
(3+1)-dimensional Friedmann, Robertson-Walker (FRW) space and a compact
hyperbolic manifold (CHM). Standard Model fields are confined to a point in the
CHM (i.e. to a brane). In such a space, the decay of massive Kaluza-Klein modes
leads to the injection of any initial bulk entropy into the observable (FRW)
universe. Both Kolmogoro-Sinai mixing due to the non-integrability of flows on
CHMs and the large statistical averaging inherent in the collapse of the
initial entropy onto the brane smooth out any initial inhomogeneities in the
distribution of matter and of 3-curvature on any slice of constant 3-position.
If, as we assume, the initial densities and curvatures in each fundamental
correlation volume are drawn from some universal underlying distributions
independent of location within the space, then these smoothing mechanisms
effectively reduce the density and curvature inhomogeneities projected onto the
FRW. This smoothing is sufficient to account for the current homogeneity and
flatness of the universe. The fundamental scale of physics can be \gsim 1TeV.
All relevant mass and length scales can have natural values in fundamental
units. All large dimensionless numbers, such as the entropy of the universe,
are understood as consequences of the topology of spacetime which is not
explained. No model for the origin of structure is proffered.Comment: minor changes, matches version published in Phys. Rev. Let
Measuring anisotropic scattering in the cuprates
A simple model of anisotropic scattering in a quasi two-dimensional metal is
studied. Its simplicity allows an analytic calculation of transport properties
using the Boltzmann equation and relaxation time approximation. We argue that
the c-axis magnetoresistance provides the key test of this model of transport.
We compare this model with experiments on overdoped Tl-2201 and find reasonable
agreement using only weak scattering anisotropy. We argue that optimally doped
Tl-2201 should show strong angular-dependent magnetoresistance within this
model and would provide a robust way of determining the in-plane scattering
anisotropy in the cuprates.Comment: 12 pages, 8 figures, typset in REVTeX 4. Version 2; added references
and corrected typo
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