8,073 research outputs found
First-principles calculation of the piezoelectric tensor d of III-V nitrides
We report direct first-principles density-functional calculations of the
piezoelectric tensor \tensor{d} relating polarization to applied stress for
the binary compounds AlN, GaN, and InN. The values of \tensor{d} are rather
sensitive to the choice of the exchange-correlation functional, and results are
presented for both the local-density and gradient approximations. A comparison
with experiment and with values predicted indirectly from the elastic and
e-piezoconstant tensors is also presented.Comment: RevTeX 3 pages, no figure
Russo's formula for random interlacements
In this paper we obtain a couple of explicit expressions for the derivative
of the probability of an increasing event in the random interlacements model.
The event is supported in a finite subset of the lattice, and the derivative is
with respect to the intensity parameter of the model.Comment: 14 pages, 3 figures, added section, accepted for publication in the
Journal of Statistical Physic
The phase diagrams of iron-based superconductors: theory and experiments
Phase diagrams play a primary role in the understanding of materials
properties. For iron-based superconductors (Fe-SC), the correct definition of
their phase diagrams is crucial because of the close interplay between their
crystallo-chemical and magnetic properties, on one side, and the possible
coexistence of magnetism and superconductivity, on the other. The two most
difficult issues for understanding the Fe-SC phase diagrams are: 1) the origin
of the structural transformation taking place during cooling and its
relationship with magnetism; 2) the correct description of the region where a
crossover between the magnetic and superconducting electronic ground states
takes place. Hence a proper and accurate definition of the structural, magnetic
and electronic phase boundaries provides an extremely powerful tool for
material scientists. For this reason, an exact definition of the thermodynamic
phase fields characterizing the different structural and physical properties
involved is needed, although it is not easy to obtain in many cases. Moreover,
physical properties can often be strongly dependent on the occurrence of
micro-structural and other local-scale features (lattice micro-strain, chemical
fluctuations, domain walls, grain boundaries, defects), which, as a rule, are
not described in a structural phase diagram. In this review, we critically
summarize the results for the most studied 11-, 122- and 1111-type compound
systems, providing a correlation between experimental evidence and theory
Stability of Ge-related point defects and complexes in Ge-doped SiO_2
We analyze Ge-related defects in Ge-doped SiO_2 using first-principles
density functional techniques. Ge is incorporated at the level of ~ 1 mol % and
above. The growth conditions of Ge:SiO_2 naturally set up oxygen deficiency,
with vacancy concentration increasing by a factor 10^5 over undoped SiO_2, and
O vacancies binding strongly to Ge impurities. All the centers considered
exhibit potentially EPR-active states, candidates for the identification of the
Ge(n) centers. Substitutional Ge produces an apparent gap shrinking via its
extrinsic levels.Comment: RevTeX 4 pages, 2 ps figure
CB damping of primordial gravitational waves and the fine-tuning of the CB temperature anisotropy
Damping of primordial gravitational waves due to the anisotropic stress
contribution owing to the cosmological neutrino background (CB) is
investigated in the context of a radiation-to-matter dominated Universe.
Besides its inherent effects on the gravitational wave propagation, the
inclusion of the CB anisotropic stress into the dynamical equations also
affects the tensor mode contribution to the anisotropy of the cosmological
microwave background (CB) temperature. Given that the fluctuations of
the CB temperature in the (ultra)relativistic regime are driven by a
multipole expansion, the mutual effects on the gravitational waves and on the
CB are obtained through a unified prescription for a
radiation-to-matter dominated scenario. The results are confronted with some
preliminary results for the radiation dominated scenario. Both scenarios are
supported by a simplified analytical framework, in terms of a scale independent
dynamical variable, , that relates cosmological scales, , and the
conformal time, . The background relativistic (hot dark) matter
essentially works as an effective dispersive medium for the gravitational waves
such that the damping effect is intensified for the Universe evolving to the
matter dominated era. Changes on the temperature variance owing to the
inclusion of neutrino collision terms into the dynamical equations result into
spectral features that ratify that the multipole expansion coefficients
's die out for .Comment: 24 pages, 8 figure
On uniform closeness of local times of Markov chains and i.i.d. sequences
In this paper we consider the field of local times of a discrete-time Markov
chain on a general state space, and obtain uniform (in time) upper bounds on
the total variation distance between this field and the one of a sequence of
i.i.d. random variables with law given by the invariant measure of that
Markov chain. The proof of this result uses a refinement of the soft local time
method of [11].Comment: 42 pages, 2 figures, coupling construction of Section 4 corrected,
results unchange
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