256 research outputs found
Topology Change in (2+1)-Dimensional Gravity
In (2+1)-dimensional general relativity, the path integral for a manifold
can be expressed in terms of a topological invariant, the Ray-Singer torsion of
a flat bundle over . For some manifolds, this makes an explicit computation
of transition amplitudes possible. In this paper, we evaluate the amplitude for
a simple topology-changing process. We show that certain amplitudes for spatial
topology change are nonvanishing---in fact, they can be infrared
divergent---but that they are infinitely suppressed relative to similar
topology-preserving amplitudes.Comment: 19 pages of text plus 4 pages of figures, LaTeX (using epsf),
UCD-11-9
Ultrafast dynamics in the presence of antiferromagnetic correlations in electron-doped cuprate LaCeCuO
We used femtosecond optical pump-probe spectroscopy to study the photoinduced
change in reflectivity of thin films of the electron-doped cuprate
LaCeCuO (LCCO) with dopings of x0.08 (underdoped) and
x0.11 (optimally doped). Above T, we observe fluence-dependent
relaxation rates which onset at a similar temperature that transport
measurements first see signatures of antiferromagnetic correlations. Upon
suppressing superconductivity with a magnetic field, it is found that the
fluence and temperature dependence of relaxation rates is consistent with
bimolecular recombination of electrons and holes across a gap (2)
originating from antiferromagnetic correlations which comprise the pseudogap in
electron-doped cuprates. This can be used to learn about coupling between
electrons and high-energy () excitations in these
compounds and set limits on the timescales on which antiferromagnetic
correlations are static
Analytical Study of Certain Magnetohydrodynamic-alpha Models
In this paper we present an analytical study of a subgrid scale turbulence
model of the three-dimensional magnetohydrodynamic (MHD) equations, inspired by
the Navier-Stokes-alpha (also known as the viscous Camassa-Holm equations or
the Lagrangian-averaged Navier-Stokes-alpha model). Specifically, we show the
global well-posedness and regularity of solutions of a certain MHD-alpha model
(which is a particular case of the Lagrangian averaged
magnetohydrodynamic-alpha model without enhancing the dissipation for the
magnetic field). We also introduce other subgrid scale turbulence models,
inspired by the Leray-alpha and the modified Leray-alpha models of turbulence.
Finally, we discuss the relation of the MHD-alpha model to the MHD equations by
proving a convergence theorem, that is, as the length scale alpha tends to
zero, a subsequence of solutions of the MHD-alpha equations converges to a
certain solution (a Leray-Hopf solution) of the three-dimensional MHD
equations.Comment: 26 pages, no figures, will appear in Journal of Math Physics;
corrected typos, updated reference
On the flow map for 2D Euler equations with unbounded vorticity
In Part I, we construct a class of examples of initial velocities for which
the unique solution to the Euler equations in the plane has an associated flow
map that lies in no Holder space of positive exponent for any positive time. In
Part II, we explore inverse problems that arise in attempting to construct an
example of an initial velocity producing an arbitrarily poor modulus of
continuity of the flow map.Comment: http://iopscience.iop.org/0951-7715/24/9/013/ for published versio
Doping-dependent nodal Fermi velocity in Bi-2212 revealed by high-resolution ARPES
The improved resolution of laser-based angle-resolved photoemission
spectroscopy (ARPES) allows reliable access to fine structures in the spectrum.
We present a systematic, doping-dependent study of a recently discovered
low-energy kink in the nodal dispersion of Bi2Sr2CaCu2O8+d (Bi-2212), which
demonstrates the ubiquity and robustness of this kink in underdoped Bi-2212.
The renormalization of the nodal velocity due to this kink becomes stronger
with underdoping, revealing that the nodal Fermi velocity is non-universal, in
contrast to assumed phenomenology. This is used together with laser-ARPES
measurements of the gap velocity, v2, to resolve discrepancies with thermal
conductivity measurements.Comment: Submitted to Phys. Rev. Let
Dynamics of Open Bosonic Quantum Systems in Coherent State Representation
We consider the problem of decoherence and relaxation of open bosonic quantum
systems from a perspective alternative to the standard master equation or
quantum trajectories approaches. Our method is based on the dynamics of
expectation values of observables evaluated in a coherent state representation.
We examine a model of a quantum nonlinear oscillator with a density-density
interaction with a collection of environmental oscillators at finite
temperature. We derive the exact solution for dynamics of observables and
demonstrate a consistent perturbation approach.Comment: 7 page
Global well-posedness issues for the inviscid Boussinesq system with Yudovich's type data
The present paper is dedicated to the study of the global existence for the
inviscid two-dimensional Boussinesq system. We focus on finite energy data with
bounded vorticity and we find out that, under quite a natural additional
assumption on the initial temperature, there exists a global unique solution.
None smallness conditions are imposed on the data. The global existence issues
for infinite energy initial velocity, and for the B\'enard system are also
discussed.Comment: 12 page
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