10,493 research outputs found
First principles study of intrinsic point defects in hexagonal barium titanate
Density functional theory (DFT) calculations have been used to study the nature of intrinsic defects in the hexagonal polymorph of barium titanate. Defect formation energies are derived for multiple charge states and due consideration is given to finite-size effects (elastic and electrostatic) and the band gap error in defective cells. Correct treatment of the chemical potential of atomic oxygen means that it is possible to circumvent the usual errors associated with the inaccuracy of DFT calculations on the oxygen dimer. Results confirm that both mono- and di-vacancies exist in their nominal charge states over the majority of the band gap. Oxygen vacancies are found to dominate the system in metal-rich conditions with face sharing oxygen vacancies being preferred over corner sharing oxygen vacancies. In oxygen-rich conditions, the dominant vacancy found depends on the Fermi level. Binding energies also show the preference for metal-oxygen di-vacancy formation. Calculated equilibrium concentrations of vacancies in the system are presented for numerous temperatures. Comparisons are drawn with the cubic polymorph as well as with previous potential-based simulations and experimental results
Resistance of superconducting nanowires connected to normal metal leads
We study experimentally the low temperature resistance of superconducting
nanowires connected to normal metal reservoirs. We find that a substantial
fraction of the nanowires is resistive, down to the lowest temperature
measured, indicative of an intrinsic boundary resistance due to the
Andreev-conversion of normal current to supercurrent. The results are
successfully analyzed in terms of the kinetic equations for diffusive
superconductors
Modeling the non-recycled Fermi gamma-ray pulsar population
We use Fermi Gamma-ray Space Telescope detections and upper limits on
non-recycled pulsars obtained from the Large Area Telescope (LAT) to constrain
how the gamma-ray luminosity L depends on the period P and the period
derivative \dot{P}. We use a Bayesian analysis to calculate a best-fit
luminosity law, or dependence of L on P and \dot{P}, including different
methods for modeling the beaming factor. An outer gap (OG) magnetosphere
geometry provides the best-fit model, which is L \propto P^{-a} \dot{P}^{b}
where a=1.36\pm0.03 and b=0.44\pm0.02, similar to but not identical to the
commonly assumed L \propto \sqrt{\dot{E}} \propto P^{-1.5} \dot{P}^{0.5}. Given
upper limits on gamma-ray fluxes of currently known radio pulsars and using the
OG model, we find that about 92% of the radio-detected pulsars have gamma-ray
beams that intersect our line of sight. By modeling the misalignment of radio
and gamma-ray beams of these pulsars, we find an average gamma-ray beaming
solid angle of about 3.7{\pi} for the OG model, assuming a uniform beam. Using
LAT-measured diffuse fluxes, we place a 2{\sigma} upper limit on the average
braking index and a 2{\sigma} lower limit on the average surface magnetic field
strength of the pulsar population of 3.8 and 3.2 X 10^{10} G, respectively. We
then predict the number of non-recycled pulsars detectable by the LAT based on
our population model. Using the two-year sensitivity, we find that the LAT is
capable of detecting emission from about 380 non-recycled pulsars, including
150 currently identified radio pulsars. Using the expected five-year
sensitivity, about 620 non-recycled pulsars are detectable, including about 220
currently identified radio pulsars. We note that these predictions
significantly depend on our model assumptions.Comment: 26 pages, 10 figures, Accepted by ApJ on 8 September 201
Colloidal brazil nut effect in sediments of binary charged suspensions
Equilibrium sedimentation density profiles of charged binary colloidal
suspensions are calculated by computer simulations and density functional
theory. For deionized samples, we predict a colloidal ``brazil nut'' effect:
heavy colloidal particles sediment on top of the lighter ones provided that
their mass per charge is smaller than that of the lighter ones. This effect is
verifiable in settling experiments.Comment: 4 pages, 4 figure
Altitude Limits for Rotating Vector Model Fitting of Pulsar Polarization
Traditional pulsar polarization sweep analysis starts from the point dipole
rotating vector model (RVM) approximation. If augmented by a measurement of the
sweep phase shift, one obtains an estimate of the emission altitude
(Blaskiewicz, Cordes, & Wasserman). However, a more realistic treatment of
field line sweepback and finite altitude effects shows that this estimate
breaks down at modest altitude ~ 0.1R_{LC}. Such radio emission altitudes turn
out to be relevant to the young energetic and millisecond pulsars that dominate
the \gamma-ray population. We quantify the breakdown height as a function of
viewing geometry and provide simple fitting formulae that allow observers to
correct RVM-based height estimates, preserving reasonable accuracy to R ~
0.3R_{LC}. We discuss briefly other observables that can check and improve
height estimates
Exposure of undergraduates to authentic GP teaching and subsequent entry to GP training: a quantitative study of UK medical schools.
BACKGROUND: It has been suggested that the quantity of exposure to general practice teaching at medical school is associated with future choice of a career as a GP. AIM: To examine the relationship between general practice exposure at medical school and the percentage of each school's graduates appointed to a general practice training programme after foundation training (postgraduate years 1 and 2). DESIGN AND SETTING: A quantitative study of 29 UK medical schools. METHOD: The UK Foundation Programme Office (UKFPO) destination surveys of 2014 and 2015 were used to determine the percentage of graduates of each UK medical school who were appointed to a GP training programme after foundation year 2. The Spearman rank correlation was used to examine the correlation between these data and the number of sessions spent in placements in general practice at each medical school. RESULTS: A statistically significant association was demonstrated between the quantity of authentic general practice teaching at each medical school and the percentage of its graduates who entered GP training after foundation programme year 2 in both 2014 (correlation coefficient [r] 0.41,P= 0.027) and 2015 (r 0.3,P= 0.044). Authentic general practice teaching here is described as teaching in a practice with patient contact, in contrast to non-clinical sessions such as group tutorials in the medical school. DISCUSSION: The authors have demonstrated, for the first time in the UK, an association between the quantity of clinical GP teaching at medical school and entry to general practice training. This study suggests that an increased use of, and investment in, undergraduate general practice placements would help to ensure that the UK meets its target of 50% of medical graduates entering general practice
Magnetic Photon Splitting: Computations of Proper-time Rates and Spectra
The splitting of photons in the presence of an intense magnetic field has
recently found astrophysical applications in polar cap models of gamma-ray
pulsars and in magnetar scenarios for soft gamma repeaters. Numerical
computation of the polarization-dependent rates of this third order QED process
for arbitrary field strengths and energies below pair creation threshold is
difficult: thus early analyses focused on analytic developments and simpler
asymptotic forms. The recent astrophysical interest spurred the use of the
S-matrix approach by Mentzel, Berg and Wunner to determine splitting rates. In
this paper, we present numerical computations of a full proper-time expression
for the rate of splitting that was obtained by Stoneham, and is exact up to the
pair creation threshold. While the numerical results derived here are in accord
with the earlier asymptotic forms due to Adler, our computed rates still differ
by as much as factors of 3 from the S-matrix re-evaluation of Wilke and Wunner,
reflecting the extreme difficulty of generating accurate S-matrix numerics for
fields below about \teq{4.4\times 10^{13}}Gauss. We find that our proper-time
rates appear very accurate, and exceed Adler's asymptotic specializations
significantly only for photon energies just below pair threshold and for
supercritical fields, but always by less than a factor of around 2.6. We also
provide a useful analytic series expansion for the scattering amplitude valid
at low energies.Comment: 13 pages, AASTeX format, including 3 eps figures, ApJ in pres
Full polar cap cascade scenario: -ray and X-ray luminosities from spin-powered pulsars
We modify polar cap cascade picture to include the ICS of the higher
generation pairs. In such a ``full-cascade'' scenario, not only the
perpendicular portion of the energy of the pairs goes to high energy radiation
via SR, but the parallel portion of the energy of the pairs can also contribute
to high energy emission via ICS with the soft thermal photons from either the
full neutron star surface or the hot polar cap. An important output of such a
scenario is that the soft tail of the ICS spectrum can naturally result in a
non-thermal X-ray component which can contribute to the luminosities observed
by ROSAT and ASCA. Here we present an analytic description of such a full polar
cap cascade scenario within the framework of Harding & Muslimov acceleration
model. We present the theoretical predictions of the -ray luminosities,
the thermal and non-thermal X-ray luminosities for the known spin-powered X-ray
pulsars. Our results show that the observed different dependences of the high
energy luminosities on the pulsar spin-down luminosities, i.e., and , are well
reproduced. Our model predicts that the {\em pulsed} soft X-rays in the ROSAT
band from most of the millisecond pulsars might be of thermal origin if there
is no strong multipole field components near the surfaces of these pulsars.Comment: 23 pages, emulateapj style, final version to appear in the
Astrophysical Journa
Comment on ``Cosmological Gamma Ray Bursts and the Highest Energy Cosmic Rays''
In a letter with the above title, published some time ago in PRL, Waxman made
the interesting suggestion that cosmological gamma ray bursts (GRBs) are the
source of the ultra high energy cosmic rays (UHECR). This has also been
proposed independently by Milgrom and Usov and by Vietri. However, recent
observations of GRBs and their afterglows and in particular recent data from
the Akeno Great Air Shwoer Array (AGASA) on UHECR rule out extragalactic GRBs
as the source of UHECR.Comment: Comment on a letter with the above title published by E. Waxman in
PRL 75, 386 (1995). Submitted for publication in PRL/Comment
Superfluid Friction and Late-time Thermal Evolution of Neutron Stars
The recent temperature measurements of the two older isolated neutron stars
PSR 1929+10 and PSR 0950+08 (ages of and yr,
respectively) indicate that these objects are heated. A promising candidate
heat source is friction between the neutron star crust and the superfluid it is
thought to contain. We study the effects of superfluid friction on the
long-term thermal and rotational evolution of a neutron star. Differential
rotation velocities between the superfluid and the crust (averaged over the
inner crust moment of inertia) of rad s for PSR
1929+10 and rad s for PSR 0950+08 would account for their
observed temperatures. These differential velocities could be sustained by
pinning of superfluid vortices to the inner crust lattice with strengths of
1 MeV per nucleus. Pinned vortices can creep outward through thermal
fluctuations or quantum tunneling. For thermally-activated creep, the coupling
between the superfluid and crust is highly sensitive to temperature. If pinning
maintains large differential rotation ( rad s), a feedback
instability could occur in stars younger than yr causing
oscillations of the temperature and spin-down rate over a period of . For stars older than yr, however, vortex creep occurs
through quantum tunneling, and the creep velocity is too insensitive to
temperature for a thermal-rotational instability to occur. These older stars
could be heated through a steady process of superfluid friction.Comment: 26 pages, 1 figure, submitted to Ap
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