21,614 research outputs found
Effective inter-band coupling in MgB2 due to anharmonic phonons
We investigate the origin of the inter-band coupling in MgB2 by focusing on
its unusual phononic features, namely, the strong anharmonicity of the phonons
and the presence of both linear and quadratic electron-phonon interactions of
the Su-Schrieffer-Heeger (SSH) type. The bare electronic Hamiltonian has two
bands with intra- and inter-band hopping, which lead to two decoupled
hybridized bands. The phonon Hamiltonian including the anharmonic terms is
diagonalized approximately by a squeezing transformation, which causes the
softening of the phonon frequency. The linear SSH coupling amplitude is
reduced, consistently with the estimates from first-principle calculations.
Additionally, the quadratic coupling generates an effective phonon-induced
interaction between the hybridized bands, which is non-vanishing even in the
limit of vanishing inter-bare-band hopping amplitude.Comment: 11 page
A model Hamiltonian for MgB2 which takes into account its unusual phononic features
Taking as a starting point the results of LDA calculations, which show that
in MgB2 the phonons have a strong quartic anharmonicity and that the
bond-stretching electron-phonon interaction (EPI) has both a linear and a large
quadratic component, we propose a model Hamiltonian which succesfully matches a
number of experimental evidences. We relate the single critical temperature for
both superconducting gaps to a phonon-induced inter-band coupling whose
amplitude increases with temperature. We also obtain phonon frequencies and
linewidths depending on the band filling, as well as band energies and
hybridization amplitudes depending on the phonon number.Comment: 19 pages, no figures, accepted on The European Physical Journal
Generalizations of the Familywise Error Rate
Consider the problem of simultaneously testing null hypotheses H_1,...,H_s.
The usual approach to dealing with the multiplicity problem is to restrict
attention to procedures that control the familywise error rate (FWER), the
probability of even one false rejection. In many applications, particularly if
s is large, one might be willing to tolerate more than one false rejection
provided the number of such cases is controlled, thereby increasing the ability
of the procedure to detect false null hypotheses. This suggests replacing
control of the FWER by controlling the probability of k or more false
rejections, which we call the k-FWER. We derive both single-step and stepdown
procedures that control the k-FWER, without making any assumptions concerning
the dependence structure of the p-values of the individual tests. In
particular, we derive a stepdown procedure that is quite simple to apply, and
prove that it cannot be improved without violation of control of the k-FWER. We
also consider the false discovery proportion (FDP) defined by the number of
false rejections divided by the total number of rejections (defined to be 0 if
there are no rejections). The false discovery rate proposed by Benjamini and
Hochberg [J. Roy. Statist. Soc. Ser. B 57 (1995) 289-300] controls E(FDP).
Here, we construct methods such that, for any \gamma and \alpha,
P{FDP>\gamma}\le\alpha. Two stepdown methods are proposed. The first holds
under mild conditions on the dependence structure of p-values, while the second
is more conservative but holds without any dependence assumptions.Comment: Published at http://dx.doi.org/10.1214/009053605000000084 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Role of the particle's stepping cycle in an asymmetric exclusion process: A model of mRNA translation
Messenger RNA translation is often studied by means of statistical-mechanical
models based on the Asymmetric Simple Exclusion Process (ASEP), which considers
hopping particles (the ribosomes) on a lattice (the polynucleotide chain). In
this work we extend this class of models and consider the two fundamental steps
of the ribosome's biochemical cycle following a coarse-grained perspective. In
order to achieve a better understanding of the underlying biological processes
and compare the theoretical predictions with experimental results, we provide a
description lying between the minimal ASEP-like models and the more detailed
models, which are analytically hard to treat. We use a mean-field approach to
study the dynamics of particles associated with an internal stepping cycle. In
this framework it is possible to characterize analytically different phases of
the system (high density, low density or maximal current phase). Crucially, we
show that the transitions between these different phases occur at different
parameter values than the equivalent transitions in a standard ASEP, indicating
the importance of including the two fundamental steps of the ribosome's
biochemical cycle into the model.Comment: 9 pages, 9 figure
The effect of stellar migration on Galactic chemical evolution: a heuristic approach
In the last years, stellar migration in galactic discs has been the subject
of several investigations. However, its impact on the chemical evolution of the
Milky Way still needs to be fully quantified. In this paper, we aim at imposing
some constraints on the significance of this phenomenon by considering its
influence on the chemical evolution of the Milky Way thin disc. We do not
investigate the physical mechanisms underlying the migration of stars. Rather,
we introduce a simple, heuristic treatment of stellar migration in a detailed
chemical evolution model for the thin disc of the Milky Way, which already
includes radial gas flows and reproduces several observational constraints for
the solar vicinity and the whole Galactic disc. When stellar migration is
implemented according to the results of chemo-dynamical simulations by Minchev
et. al. (2013) and finite stellar velocities of 1 km s are taken into
account, the high-metallicity tail of the metallicity distribution function of
long-lived thin-disc stars is well reproduced. By exploring the velocity space,
we find that the migrating stars must travel with velocities in the range 0.5
-2 km s to properly reproduce the high-metallicity tail of the
metallicity distribution. We confirm previous findings by other authors that
the observed spread in the age-metallicity relation of solar neighbourhood
stars can be explained by the presence of stars which originated at different
Galactocentric distances, and we conclude that the chemical properties of stars
currently observed in the solar vicinity do suggest that stellar migration is
present to some extent.Comment: Accepted for publication by Ap
Two years of monitoring Supergiant Fast X-ray Transients with Swift
We present two years of intense Swift monitoring of three SFXTs, IGR
J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007).
Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed
power laws with by hard photon indices (G~1-2). Their outburst broad-band
(0.3-150 keV) spectra can be fit well with models typically used to describe
the X-ray emission from accreting NSs in HMXBs. We assess how long each source
spends in each state using a systematic monitoring with a sensitive instrument.
These sources spend 3-5% of the total in bright outbursts. The most probable
flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to
luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of
magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19,
39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively.
We present a complete list of BAT on-board detections further confirming the
continued activity of these sources. This demonstrates that true quiescence is
a rare state, and that these transients accrete matter throughout their life at
different rates. X-ray variability is observed at all timescales and
intensities we can probe. Superimposed on the day-to-day variability is
intra-day flaring which involves variations up to one order of magnitude that
can occur down to timescales as short as ~1ks, and whichcan be explained by the
accretion of single clumps composing the donor wind with masses
M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table
On Optimality of Stepdown and Stepup Multiple Test Procedures
Consider the multiple testing problem of testing k null hypotheses, where the
unknown family of distributions is assumed to satisfy a certain monotonicity
assumption. Attention is restricted to procedures that control the familywise
error rate in the strong sense and which satisfy a monotonicity condition.
Under these assumptions, we prove certain maximin optimality results for some
well-known stepdown and stepup procedures.Comment: Published at http://dx.doi.org/10.1214/009053605000000066 in the
Annals of Statistics (http://www.imstat.org/aos/) by the Institute of
Mathematical Statistics (http://www.imstat.org
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