40,170 research outputs found
Minimal Model for Disorder-induced Missing Moment of Inertia in Solid He
The absence of a missing moment inertia in clean solid He suggests that
the minimal experimentally relevant model is one in which disorder induces
superfluidity in a bosonic lattice. To this end, we explore the relevance of
the disordered Bose-Hubbard model in this context. We posit that a clean array
He atoms is a self-generated Mott insulator, that is, the He atoms
constitute the lattice as well as the `charge carriers'. With this assumption,
we are able to interpret the textbook defect-driven supersolids as excitations
of either the lower or upper Hubbard bands. In the experiments at hand,
disorder induces a closing of the Mott gap through the generation of mid-gap
localized states at the chemical potential. Depending on the magnitude of the
disorder, we find that the destruction of the Mott state takes place for
either through a Bose glass phase (strong disorder) or through a direct
transition to a superfluid (weak disorder). For , disorder is always
relevant. The critical value of the disorder that separates these two regimes
is shown to be a function of the boson filling, interaction and the momentum
cut off. We apply our work to the experimentally observed enhancement He
impurities has on the onset temperature for the missing moment of inertia. We
find quantitative agreement with experimental trends.Comment: 9 pages, 5 figures: Extended version of previous paper in which the
pase diagram for the disordered Bose-Hubbard model is computed using
mean-field theory and one-loop RG. The criterion for the Bose glass is
derived explicitly. (a few typos are corrected
Improved Coefficient and Variance Estimation in Stable First-Order Dynamic Regression Models
In dynamic regression models the least-squares coefficient estimators are biased in finite samples, and so are the usual estimators for the disturbance variance and for the variance of the coefficient estimators. By deriving the expectation of the initial terms in an expansion of the usual expression for the asymptotic coefficient variance estimator and by comparing these with an approximation to the true variance we find an approximation to the bias in variance estimation from which a bias corrected estimator for the variance readily follows. This is also achieved for a bias corrected coefficient estimator and allows to compare analytically the second-order approximation to the mean squared error of the least-squares estimator and its counterpart for the first-order bias corrected coefficient estimator. Two rather strong results on efficiency gains through bias correction for AR(1) models follow. Illustrative simulation results on the magnitude of bias in coefficient and variance estimation and on the scope for effective bias correction and efficiency improvement are presented for some relevant particular cases of the ARX(1) class of models.
Lorentz Violation and Synchrotron Radiation
We consider the radiation emitted by an ultrarelativistic charged particle
moving in a magnetic field, in the presence of an additional Lorentz-violating
interaction. In contrast with prior work, we treat a form of Lorentz violation
that is represented by a renormalizable operator. Neglecting the radiative
reaction force, the particle's trajectory can be determined exactly. The
resulting orbit is generally noncircular and does not lie in the place
perpendicular to the magnetic field. We do not consider any Lorentz violation
in the electromagnetic sector, so the radiation from the accelerated charge can
be determined by standard means, and the radiation spectrum will exhibit a
Lorentz-violating directional dependence. Using data on emission from the Crab
nebula, we can set a bound on a particular combination of Lorentz-violating
coefficients at the level.Comment: 14 page
Excitation of the molecular gas in the nuclear region of M82
We present high-resolution HIFI spectroscopy of the nucleus of the archetypical starburst galaxy M 82. Six ^(12)CO lines, 2 ^(13)CO lines and 4 fine-structure lines have been detected. Besides showing the effects of the overall velocity structure of the nuclear region, the line profiles also indicate the presence of multiple components with different optical depths, temperatures, and densities in the observing beam. The data have been interpreted using a grid of PDR models. It is found that the majority of the molecular gas is in low density (n = 10^(3.5) cm^(-3)) clouds, with column densities of N_H = 10^(21.5) cm^(-2) and a relatively low UV radiation field (G_0 = 10^2). The remaining gas is predominantly found in clouds with higher densities (n = 10^5 cm^(-3)) and radiation fields (G_0 = 10^(2.75)), but somewhat lower column densities (N_H = 10^(21.2) cm^(-2)). The highest J CO lines are dominated by a small (1% relative surface filling) component, with an even higher density (n = 10^6 cm^(-3)) and UV field (G_0 = 10^(3.25)). These results show the strength of multi-component modelling for interpretating the integrated properties of galaxies
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