12,890 research outputs found
On Inflation and Variation of the Strong Coupling Constant
Variation of constants in the very early universe can generate inflation. We
consider a scenario where the strong coupling constant was changing in time and
where the gluon condensate underwent a phase transition ending the inflation.Comment: 12 pages, 1 figure, accepted for publication in International Journal
of Modern Physics
Measurement of the complex Faraday angle in thin-film metals and high temperature superconductors
A sensitive polarization modulation technique uses photoelastic modulation
and hetrodyne detection to simultaneously measure the Faraday rotation and
induced ellipticity in light transmitted by semiconducting and metallic
samples. The frequencies measured are in the mid-infrared and correspond to the
spectral lines of a CO2 laser. The measured temperature range is continuous and
extends from 35 to 330K. Measured samples include GaAs and Si substrates, gold
and copper films, and YBCO and BSCCO high temperature superconductors.Comment: 12 pages of text, 6 figures, fixed typos in formulas, added figur
Classical Rotons in Cold Atomic Traps
We predict the emergence of a roton minimum in the dispersion relation of
elementary excitations in cold atomic gases in the presence of diffusive light.
In large magneto-topical traps, multiple-scattering of light is responsible for
the collective behavior of the system, which is associated to an effective
Coulomb-like interaction between the atoms. In optically thick clouds, the
re-scattered light undergoes diffusive propagation, which is responsible for a
stochastic short-range force acting on the atoms. We show that the dynamical
competition between these two forces results on a new polariton mode, which
exhibits a roton minimum. Making use of Feynman's formula for the static
structure factor, we show that the roton minimum is related to the appearance
of long-range order in the system.Comment: 5 pages, 3 figure
Gravity Waves as a Probe of Hubble Expansion Rate During An Electroweak Scale Phase Transition
Just as big bang nucleosynthesis allows us to probe the expansion rate when
the temperature of the universe was around 1 MeV, the measurement of gravity
waves from electroweak scale first order phase transitions may allow us to
probe the expansion rate when the temperature of the universe was at the
electroweak scale. We compute the simple transformation rule for the gravity
wave spectrum under the scaling transformation of the Hubble expansion rate. We
then apply this directly to the scenario of quintessence kination domination
and show how gravity wave spectra would shift relative to LISA and BBO
projected sensitivities.Comment: 28 pages, 2 figures
Generalized Landau-Pollak Uncertainty Relation
The Landau-Pollak uncertainty relation treats a pair of rank one projection
valued measures and imposes a restriction on their probability distributions.
It gives a nontrivial bound for summation of their maximum values. We give a
generalization of this bound (weak version of the Landau-Pollak uncertainty
relation). Our generalization covers a pair of positive operator valued
measures. A nontrivial but slightly weak inequality that can treat an arbitrary
number of positive operator valued measures is also presented.Comment: Simplified the proofs. To be published in Phys.Rev.
Critical currents, flux-creep activation energy and potential barriers for the vortex motion from the flux creep experiments
We present an experimental study of thermally activated flux creep in a
superconducting ring-shaped epitaxial YBCO film as well as a new way of
analyzing the experimental data. The measurements were made in a wide range of
temperatures between 10 and 83 K. The upper temperature limit was dictated by
our experimental technique and at low temperatures we were limited by a
crossover to quantum tunneling of vortices. It is shown that the experimental
data can very well be described by assuming a simple thermally activated
hopping of vortices or vortex bundles over potential barriers, whereby the
hopping flux objects remain the same for all currents and temperatures. The new
procedure of data analysis also allows to establish the current and temperature
dependencies of the flux-creep activation energy U, as well as the temperature
dependence of the critical current Ic, from the flux-creep rates measured at
different temperatures. The variation of the activation energy with current,
U(I/Ic), is then used to reconstruct the profile of the potential barriers in
real space.Comment: 12 pages, 13 Postscript figures, Submitted to Physical Review
Nanomechanical properties of few-layer graphene membranes
We have measured the mechanical properties of few-layer graphene and graphite
flakes that are suspended over circular holes. The spatial profile of the
flake's spring constant is measured with an atomic force microscope. The
bending rigidity of and the tension in the membranes are extracted by fitting a
continuum model to the data. For flakes down to eight graphene layers, both
parameters show a strong thickness-dependence. We predict fundamental resonance
frequencies of these nanodrums in the GHz range based on the measured bending
rigidity and tension.Comment: 9 pages, 3 figures, This article has been accepted by Appl. Phys.
Lett. After it is published, it will be found at http://apl.aip.org
Two-Particle Schroedinger Equation Animations of Wavepacket-Wavepacket Scattering (revised)
A simple and explicit technique for the numerical solution of the
two-particle, time-dependent Schr\"{o}dinger equation is assembled and tested.
The technique can handle interparticle potentials that are arbitrary functions
of the coordinates of each particle, arbitrary initial and boundary conditions,
and multi-dimensional equations. Plots and animations are given here and on the
World Wide Web of the scattering of two wavepackets in one dimension.Comment: 13 pages, 8 figures, animations at
http://nacphy.physics.orst.edu/ComPhys/PACKETS
Nonlocal extension of the dispersive-optical-model to describe data below the Fermi energy
Present applications of the dispersive-optical-model analysis are restricted
by the use of a local but energy-dependent version of the generalized
Hartree-Fock potential. This restriction is lifted by the introduction of a
corresponding nonlocal potential without explicit energy dependence. Such a
strategy allows for a complete determination of the nucleon propagator below
the Fermi energy with access to the expectation value of one-body operators
(like the charge density), the one-body density matrix with associated natural
orbits, and complete spectral functions for removal strength. The present
formulation of the dispersive optical model (DOM) therefore allows the use of
elastic electron-scattering data in determining its parameters. Application to
Ca demonstrates that a fit to the charge radius leads to too much
charge near the origin using the conventional assumptions of the functional
form of the DOM. A corresponding incomplete description of high-momentum
components is identified, suggesting that the DOM formulation must be extended
in the future to accommodate such correlations properly. Unlike the local
version, the present nonlocal DOM limits the location of the deeply-bound hole
states to energies that are consistent with (\textit{e,e}\textit{p})
and (\textit{p,2p}) data.Comment: 14 pages, 10 figures, submitted to Physical Review
Static response of Fermi liquids with tensor interactions
We use Landau's theory of a normal Fermi liquid to derive expressions for the
static response of a system with a general tensor interaction that conserves
the total spin and the total angular momentum of the quasiparticle-quasihole
pair. The magnetic susceptibility is calculated in detail, with the inclusion
of the center of mass tensor and cross vector terms in addition to the exchange
tensor one. We also introduce a new parametrization of the tensor Landau
parameters which significantly reduces the importance of high angular harmonic
contributions. For nuclear matter and neutron matter we find that the two most
important effects of the tensor interaction are to give a contribution from
multipair states and to renormalize the magnetic moments. Response to a weak
probe may be calculated using similar methods, replacing the magnetic moments
with the matrix elements of the weak charges
- …