31,373 research outputs found
Unconventional scanning tunneling conductance spectra for graphene
We compute the tunneling conductance of graphene as measured by a scanning
tunneling microscope (STM) with a normal/superconducting tip. We demonstrate
that for undoped graphene with zero Fermi energy, the first derivative of the
tunneling conductance with respect to the applied voltage is proportional to
the density of states of the STM tip. We also show that the shape of the STM
spectra for graphene doped with impurities depends qualitatively on the
position of the impurity atom in the graphene matrix and relate this
unconventional phenomenon to the pseudopsin symmetry of the Dirac
quasiparticles in graphene. We suggest experiments to test our theory.Comment: 6 pages, 3 figure
Magnetoelastic Effects in Iron Telluride
Iron telluride doped lightly with selenium is known to undergo a first order
magneto-structural transition before turning superconducting at higher doping.
We study the effects of magneto-elastic couplings on this transition using
symmetry considerations. We find that the magnetic order parameters are coupled
to the uniform monoclinic strain of the unit cell with one iron per cell, as
well as to the phonons at high symmetry points of the Brillouin zone. In the
magnetic phase the former gives rise to monoclinic distortion while the latter
induces dimerization of the ferromagnetic iron chains due to alternate
lengthening and shortening of the nearest-neighbour iron-iron bonds. We compare
this system with the iron arsenides and propose a microscopic magneto-elastic
Hamiltonian which is relevant for all the iron based superconductors. We argue
that this describes electron-lattice coupling in a system where
electron-electron interaction is crucial.Comment: 5 pages, 2 figure
Hamilton-Jacobi method for Domain Walls and Cosmologies
We use Hamiltonian methods to study curved domain walls and cosmologies. This
leads naturally to first order equations for all domain walls and cosmologies
foliated by slices of maximal symmetry. For Minkowski and AdS-sliced domain
walls (flat and closed FLRW cosmologies) we recover a recent result concerning
their (pseudo)supersymmetry. We show how domain-wall stability is consistent
with the instability of adS vacua that violate the Breitenlohner-Freedman
bound. We also explore the relationship to Hamilton-Jacobi theory and compute
the wave-function of a 3-dimensional closed universe evolving towards de Sitter
spacetime.Comment: 18 pages; v2: typos corrected, one ref added, version to appear in
PR
Subprime mortgages and the housing bubble
This paper explores the link between the house-price expectations of mortgage lenders and the extent of subprime lending. It argues that bubble conditions in the housing market are likely to spur subprime lending, with favorable price expectations easing the default concerns of lenders and thus increasing their willingness to extend loans to risky borrowers. Since the demand created by subprime lending feeds back onto house prices, such lending also helps to fuel an emerging housing bubble. The paper, however, focuses on the reverse causal linkage, where subprime lending is a consequence rather than a cause of bubble conditions. These ideas are illustrated in a theoretical model, and empirical work tests for a connection between price expectations and the extent of subprime lending.Subprime mortgage ; Global financial crisis
An Inverse Problem for Localization Operators
A classical result of time-frequency analysis, obtained by I. Daubechies in
1988, states that the eigenfunctions of a time-frequency localization operator
with circular localization domain and Gaussian analysis window are the Hermite
functions. In this contribution, a converse of Daubechies' theorem is proved.
More precisely, it is shown that, for simply connected localization domains, if
one of the eigenfunctions of a time-frequency localization operator with
Gaussian window is a Hermite function, then its localization domain is a disc.
The general problem of obtaining, from some knowledge of its eigenfunctions,
information about the symbol of a time-frequency localization operator, is
denoted as the inverse problem, and the problem studied by Daubechies as the
direct problem of time-frequency analysis. Here, we also solve the
corresponding problem for wavelet localization, providing the inverse problem
analogue of the direct problem studied by Daubechies and Paul.Comment: 18 pages, 1 figur
An exploration of the experiences and utility of functional electrical stimulation for foot drop in people with multiple sclerosis
Purpose: Functional electrical stimulation (FES) is effective in improving walking in people with multiple sclerosis (MS) with foot drop. There is limited research exploring people’s experiences of using this device. This study aims to explore the utility, efficacy, acceptability, and impact on daily life of the device in people with MS.
Methods: An interpretative phenomenological approach was employed. Ten participants who had used FES for 12 months were interviewed. Transcripts were analysed, and emergent themes identified.
Results: Nine participants continued to use the device. Three relevant super-ordinate themes were identified; impact of functional electrical stimulation, sticking with functional electrical stimulation, and autonomy and control. Participants reported challenges using the device; however, all reported positive physical and psychological benefits. Intrinsic and external influences such as; access to professional help, the influence of others, an individual’s ability to adapt, and experiences using the device, influenced their decisions to continue with the device. A thematic model of these factors was developed.
Conclusions: This study has contributed to our understanding of people with MS experiences of using the device and will help inform prescribing decisions and support the continued, appropriate use of FES over the longer term
Benzene formation in the inner regions of protostellar disks
Benzene (c-C6H6) formation in the inner 3 AU of a protostellar disk can be
efficient, resulting in high abundances of benzene in the midplane region. The
formation mechanism is different to that found in interstellar clouds and in
protoplanetary nebulae, and proceeds mainly through the reaction between allene
(C3H4) and its ion. This has implications for PAH formation, in that some
fraction of PAHs seen in the solar system could be native rather than inherited
from the interstellar medium.Comment: 9 pages, 2 colour figures, to be published in the Astrophysical
Journal Letter
Different types of X-ray bursts from GRS 1915+105 and their origin
We report the X-ray observations of the Galactic X-ray transient source GRS
1915+105 with the PPCs of the Indian X-ray Astronomy Experiment(IXAE) onboard
the Indian satellite IRS-P3 during 1997 June - August, which have revealed the
presence of four types of intense X-ray bursts. All the observed bursts have a
slow exponential rise, a sharp linear decay, and they can broadly be put in two
classes: irregular and quasi-regular bursts in one class, and regular bursts in
another class. The regular bursts are found to have two distinct time scales
and they persist over extended durations. There is a strong correlation between
the preceding quiescent time and the burst duration for the quasi-regular and
irregular bursts. No such correlation is found for the regular bursts. The
ratio of average flux during the burst time to the average flux during the
quiescent phase is high and variable for the quasi- regular and irregular
bursts while it is low and constant for the regular bursts. We suggest that the
peculiar bursts that we have seen are charact- eristic of the change of state
of the source. The source can switch back and forth between the low-hard state
and the high-soft state near critical accretion rates in a very short time
scale. A test of the model is presented using the publicly available 13-60 keV
RXTE/PCA data for irregular and regular bursts concurrent with our
observations.Comment: 13 pages, 12 figures, Accepted in APJ, emulateapj style use
- …