118 research outputs found
Complete Next to Leading Order QCD Corrections to the Photon Structure Functions and
We present the complete NLO QCD analysis of the photon structure functions
and for a real photon target. In
particular we study the heavy flavor content of the structure functions which
is due to two different production mechanisms, namely collisions of a virtual
photon with a real photon, and with a parton. We observe that the charm
contributions are noticeable for as well as
in the x-region studied.Comment: Latex 34 pages, 24 figures, uuencoded, attached at end, ITP-SB-93-46,
FERMILAB-Pub-93/240-T, SMU HEP 93-1
Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d
Coherent time-domain spectroscopy is used to measure the screening and
dissipation of high-frequency electromagnetic fields in a set of underdoped
Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a
phase-fluctuation driven transition from the superconductor to normal state,
with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of
vortex-pair unbinding.Comment: Nature, Vol. 398, 18 March 1999, pg. 221 4 pages with 4 included
figure
Determination of the Coherence Length and the Cooper-Pair Size in Unconventional Superconductors by Tunnelling Spectroscopy
The main purpose of the paper is to discuss a possibility of the
determination of the values of the coherence length and the Cooper-pair size in
unconventional superconductors by using tunnelling spectroscopy. In the mixed
state of type-II superconductors, an applied magnetic field penetrates the
superconductor in the form of vortices which form a regular lattice. In
unconventional superconductors, the inner structure of a vortex core has a
complex structure which is determined by the order parameter of the
superconducting state and by the pairing wavefunction of the Cooper pairs. In
clean superconductors, the spatial variations of the order parameter and the
pairing wavefunction occur over the distances of the order of the coherence
length and the Cooper-pair size, respectively. Therefore, by performing
tunnelling spectroscopy along a line passing through a vortex core, one is
able, in principle, to estimate the values of the coherent length and the
Cooper-pair size.Comment: 13 pages, including 17 figure
Reduction of the Superfluid Density in the Vortex-Liquid Phase of Bi2Sr2CaCu2Oy
In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was
measured in the mixed state at 40.8 GHz.The surface reactance, which is
proportional to the real part of the effective penetration depth, increased
rapidly just above the first-order vortex-lattice melting transition field and
the second magnetization peak field.This increase is ascribed to the decrease
in the superfluid density rather than the loss of pinning.This result indicates
that the vortex melting transition changes the electronic structure as well as
the vortex structure.Comment: 4 pages, 4 figures, accepted for publication in Phys. Rev. Let
Anisotropic s-wave superconductivity in MgB_2
It has recently been observed that MgB_2 is a superconductor with a high
transition temperature. Here we propose a model of anisotropic s-wave
superconductivity which consistently describes the observed properties of this
compound, including the thermodynamic and optical response in sintered MgB_2
wires. We also determine the shape of the quasiparticle density of states and
the anisotropy of the upper critical field and the superfluid density which
should be detectable once single-crystal samples become available.Comment: RevTex, 10 pages with 4 eps figure
Gentle Perturbations of the Free Bose Gas I
It is demonstrated that the thermal structure of the noncritical free Bose
Gas is completely described by certain periodic generalized Gaussian stochastic
process or equivalently by certain periodic generalized Gaussian random field.
Elementary properties of this Gaussian stochastic thermal structure have been
established. Gentle perturbations of several types of the free thermal
stochastic structure are studied. In particular new models of non-Gaussian
thermal structures have been constructed and a new functional integral
representation of the corresponding euclidean-time Green functions have been
obtained rigorously.Comment: 51 pages, LaTeX fil
Nonthermal nature of incipient extremal black holes
We examine particle production from spherical bodies collapsing into extremal
Reissner-Nordstr\"om black holes. Kruskal coordinates become ill-defined in the
extremal case, but we are able to find a simple generalization of them that is
good in this limit. The extension allows us to calculate the late-time
worldline of the center of the collapsing star, thus establishing a
correspondence with a uniformly accelerated mirror in Minkowski spacetime. The
spectrum of created particles associated with such uniform acceleration is
nonthermal, indicating that a temperature is not defined. Moreover, the
spectrum contains a constant that depends on the history of the collapsing
object. At first sight this points to a violation of the no-hair theorems;
however, the expectation value of the stress-energy-momentum tensor is zero and
its variance vanishes as a power law at late times. Hence, both the no-hair
theorems and the cosmic censorship conjecture are preserved. The power-law
decay of the variance is in distinction to the exponential fall-off of a
nonextremal black hole. Therefore, although the vanishing of the stress
tensor's expectation value is consistent with a thermal state at zero
temperature, the incipient black hole does not behave as a thermal object at
any time and cannot be regarded as the thermodynamic limit of a nonextremal
black hole, regardless of the fact that the final product of collapse is
quiescent.Comment: 13 pages, 2 epsf figures, RevTeX 3. Minor changes, version published
in PR
G1 Cosmologies with Gravitational and Scalar Waves
I present here a new algorithm to generate families of inhomogeneous massless
scalar field cosmologies. New spacetimes, having a single isometry, are
generated by breaking the homogeneity of massless scalar field models
along one direction. As an illustration of the technique I construct
cosmological models which in their late time limit represent perturbations in
the form of gravitational and scalar waves propagating on a non-static
inhomogeneous background. Several features of the obtained metrics are
discussed, such as their early and late time limits, structure of singularities
and physical interpretation.Comment: 24 pages, 2 figure
Emergence of Skyrme crystal in Gross-Neveu and 't Hooft models at finite density
We study two-dimensional, large field theoretic models (Gross-Neveu
model, 't Hooft model) at finite baryon density near the chiral limit. The same
mechanism which leads to massless baryons in these models induces a breakdown
of translational invariance at any finite density. In the chiral limit baryonic
matter is characterized by a spatially varying chiral angle with a wave number
depending only on the density. For small bare quark masses a sine-Gordon kink
chain is obtained which may be regarded as simplest realization of the Skyrme
crystal for nuclear matter. Characteristic differences between confining and
non-confining models are pointed out.Comment: 27 pages, 11 figures, added reference, corrected sig
Energy Release During Disk Accretion onto a Rapidly Rotating Neutron Star
The energy release L_s on the surface of a neutron star (NS) with a weak
magnetic field and the energy release L_d in the surrounding accretion disk
depend on two independent parameters that determine its state (for example,
mass M and cyclic rotation frequency f) and is proportional to the accretion
rate. We derive simple approximation formulas illustrating the dependence of
the efficiency of energy release in an extended disk and in a boundary layer
near the NS surface on the frequency and sense of rotation for various NS
equations of state. Such formulas are obtained for the quadrupole moment of a
NS, for a gap between its surface and a marginally stable orbit, for the
rotation frequency in an equatorial Keplerian orbit and in the marginally
stable circular orbit, and for the rate of NS spinup via disk accretion. In the
case of NS and disk counterrotation, the energy release during accretion can
reach . The sense of NS rotation is a factor that strongly
affects the observed ratio of nuclear energy release during bursts to
gravitational energy release between bursts in X-ray bursters. The possible
existence of binary systems with NS and disk counterrotation in the Galaxy is
discussed. Based on the static criterion for stability, we present a method of
constructing the dependence of gravitational mass M on Kerr rotation parameter
j and on total baryon mass (rest mass) m for a rigidly rotating neutron star.
We show that all global NS characteristics can be expressed in terms of the
function M(j, m) and its derivatives.Comment: 42 pages, 12 figures, to appear in Astronomy Letters, 2000, v.26,
p.69
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