641 research outputs found
Absence of hole pairing in a simple t-J model on the Shastry-Sutherland lattice
The Shastry-Sutherland model is a two-dimensional frustrated spin model whose
ground state is a spin gap state. We study this model doped with one and two
holes on a 32-site lattice using exact diagonalization. When t'>0, we find that
the diagonal dimer order that exists at half-filling are retained at these
moderate doping levels. No other order is found to be favored on doping. The
holes are strongly repulsive unless the hopping terms are unrealistically
small. Therefore, the existence of a spin gap at half-filling does not
guarantee hole-pairing in the present case
IceCube's In-Ice Radio Extension: Status and Results
In 2006-2010, several Radio Frequency (RF) detectors and calibration
equipment were deployed as part of the IceCube array at depths between 5 to
1400 meters in preparation for a future large scale GZK neutrino detector.
IceCube's deep holes and well-established data handling system provide a unique
opportunity for deep-ice RF detection studies at the South-Pole. We will
present verification and calibration results as well as a status-review of
ongoing analyses such as ice-properties, RF noise and reconstruction
algorithms.Comment: 4 pages, 6 figures, to appear in the proceedings of the Acoustic and
Radio EeV Neutrino detection Activities (ARENA) 2010 conferenc
Phonon Universal Transmission Fluctuations and Localization in Semiconductor Superlattices with a Controlled Degree of Order
We study both analytically and numerically phonon transmission fluctuations
and localization in partially ordered superlattices with correlations among
neighboring layers. In order to generate a sequence of layers with a varying
degree of order we employ a model proposed by Hendricks and Teller as well as
partially ordered versions of deterministic aperiodic superlattices. By
changing a parameter measuring the correlation among adjacent layers, the
Hendricks- Teller superlattice exhibits a transition from periodic ordering,
with alterna- ting layers, to the phase separated opposite limit; including
many intermediate arrangements and the completely random case. In the partially
ordered versions of deterministic superlattices, there is short-range order
(among any conse- cutive layers) and long range disorder, as in the N-state
Markov chains. The average and fluctuations in the transmission, the
backscattering rate, and the localization length in these multilayered systems
are calculated based on the superlattice structure factors we derive
analytically. The standard deviation of the transmission versus the average
transmission lies on a {\it universal\/} curve irrespective of the specific
type of disorder of the SL. We illustrate these general results by applying
them to several GaAs-AlAs superlattices for the proposed experimental
observation of phonon universal transmission fluctuations.Comment: 16-pages, Revte
Theory of coherent acoustic phonons in InGaN/GaN multi-quantum wells
A microscopic theory for the generation and propagation of coherent LA
phonons in pseudomorphically strained wurzite (0001) InGaN/GaN multi-quantum
well (MQW) p-i-n diodes is presented. The generation of coherent LA phonons is
driven by photoexcitation of electron-hole pairs by an ultrafast Gaussian pump
laser and is treated theoretically using the density matrix formalism. We use
realistic wurzite bandstructures taking valence-band mixing and strain-induced
piezo- electric fields into account. In addition, the many-body Coulomb
ineraction is treated in the screened time-dependent Hartree-Fock
approximation. We find that under typical experimental conditions, our
microscopic theory can be simplified and mapped onto a loaded string problem
which can be easily solved.Comment: 20 pages, 17 figure
Charged Higgs boson contribution to scattering from low to ultrahigh energy in Higgs triplet model
We study the scattering from low to ultrahigh energy in the
framework of Higgs Triplet Model (HTM). We add the contribution of charged
Higgs boson exchange to the total cross section of the scattering. We obtain
the upper bound in this
process from low energy experiment. We show that by using the upper bound
obtained, the charged Higgs contribution can give enhancements to the total
cross section with respect to the SM prediction up to 5.16% at
eV and maximum at and would help to determine the
feasibility experiments to discriminate between SM and HTM at current available
facilities.Comment: 6 pages, 6 figure
The Triple Spar Campaign: Implementation and Test of a Blade Pitch Controller on a Scaled Floating Wind Turbine Model
Have mirror micrometeorites been detected?
Slow-moving ( km/s) 'dark matter particles' have allegedly been
discovered in a recent experiment. We explore the possibility that these slow
moving dark matter particles are small mirror matter dust particles originating
from our solar system. Ways of further testing our hypothesis, including the
possibility of observing these dust particles in cryogenic detectors such as
NAUTILUS, are also discussed.Comment: Few changes, about 8 pages lon
Superconductors with Magnetic Impurities: Instantons and Sub-gap States
When subject to a weak magnetic impurity potential, the order parameter and
quasi-particle energy gap of a bulk singlet superconductor are suppressed.
According to the conventional mean-field theory of Abrikosov and Gor'kov, the
integrity of the energy gap is maintained up to a critical concentration of
magnetic impurities. In this paper, a field theoretic approach is developed to
critically analyze the validity of the mean field theory. Using the
supersymmetry technique we find a spatially homogeneous saddle-point that
reproduces the Abrikosov-Gor'kov theory, and identify instanton contributions
to the density of states that render the quasi-particle energy gap soft at any
non-zero magnetic impurity concentration. The sub-gap states are associated
with supersymmetry broken field configurations of the action. An analysis of
fluctuations around these configurations shows how the underlying supersymmetry
of the action is restored by zero modes. An estimate of the density of states
is given for all dimensionalities. To illustrate the universality of the
present scheme we apply the same method to study `gap fluctuations' in a normal
quantum dot coupled to a superconducting terminal. Using the same instanton
approach, we recover the universal result recently proposed by Vavilov et al.
Finally, we emphasize the universality of the present scheme for the
description of gap fluctuations in d-dimensional superconducting/normal
structures.Comment: 18 pages, 9 eps figure
To wet or not to wet: that is the question
Wetting transitions have been predicted and observed to occur for various
combinations of fluids and surfaces. This paper describes the origin of such
transitions, for liquid films on solid surfaces, in terms of the gas-surface
interaction potentials V(r), which depend on the specific adsorption system.
The transitions of light inert gases and H2 molecules on alkali metal surfaces
have been explored extensively and are relatively well understood in terms of
the least attractive adsorption interactions in nature. Much less thoroughly
investigated are wetting transitions of Hg, water, heavy inert gases and other
molecular films. The basic idea is that nonwetting occurs, for energetic
reasons, if the adsorption potential's well-depth D is smaller than, or
comparable to, the well-depth of the adsorbate-adsorbate mutual interaction. At
the wetting temperature, Tw, the transition to wetting occurs, for entropic
reasons, when the liquid's surface tension is sufficiently small that the free
energy cost in forming a thick film is sufficiently compensated by the fluid-
surface interaction energy. Guidelines useful for exploring wetting transitions
of other systems are analyzed, in terms of generic criteria involving the
"simple model", which yields results in terms of gas-surface interaction
parameters and thermodynamic properties of the bulk adsorbate.Comment: Article accepted for publication in J. Low Temp. Phy
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