Helioseismology and the solar age

The problem of measuring the solar age by means of helioseismology hasbeen recently revisited by Guenther & Demarque (1997) and by Weiss & Schlattl (1998). Different best values for $t_{\rm seis}$ and different assessment of the uncertainty resulted from these two works. We show that depending on the way seismic data are used, one may obtain the value $t_{\rm seis}\approx 4.6$ Gy, close to the age of the oldest meteorites, $t_{\rm met}=4.57$ Gy, like in the first paper, or above 5 Gy like in the second paper. The discrepancy in the seismic estimates of the solar age may be eliminated by assuming higher than the standard metal abundance and/or an upward revision of the opacities in the solar radiative interior.We argue that the most accurate and robust seismic measure of the solar age are the small frequency separations, $D_{\ell,n}=\nu_{l,n}-\nu_{\ell+1,n-1}$, for spherical harmonic degrees $\ell=0,2$ and radial orders $n\gg\ell$.The seismic age inferred by minimization of the sum of squared differences between the model and the solar small separations is $t_{\rm seis}=4.66\pm0.11$, a number consistent with meteoritic data.Our analysis supports earlier suggestions of using small frequency separations as stellar age indicators.Comment: 8 pages + 4 ps figures included, LaTeX file with l-aa.sty, submitted to Astronomy and Astrophysic

Extracting convergent surface energies from slab calculations

The formation energy of a solid surface can be extracted from slab calculations if the bulk energy per atom is known. It has been pointed out previously that the resulting surface energy will diverge with slab thickness if the bulk energy is in error, in the context of calculations which used different methods to study the bulk and slab systems. We show here that this result is equally relevant for state-of-the-art computational methods which carefully treat bulk and slab systems in the same way. Here we compare different approaches, and present a solution to the problem that eliminates the divergence and leads to rapidly convergent and accurate surface energies.Comment: 3 revtex pages, 1 figure, in print on J. Phys. Cond. Mat

Ligand-protein interactions in lysozyme investigated through a dual-resolution model

A fully atomistic modelling of biological macromolecules at relevant length- and time-scales is often cumbersome or not even desirable, both in terms of computational effort required and it a posteriori analysis. This difficulty can be overcome with the use of multi-resolution models, in which different regions of the same system are concurrently described at different levels of detail. In enzymes, computationally expensive atomistic detail is crucial in the modelling of the active site in order to capture e.g. the chemically subtle process of ligand binding. In contrast, important yet more collective properties of the remainder of the protein can be reproduced with a coarser description. In the present work, we demonstrate the effectiveness of this approach through the calculation of the binding free energy of hen egg white lysozyme (HEWL) with the inhibitor di-N-acetylchitotriose. Particular attention is posed to the impact of the mapping, i.e. the selection of atomistic and coarse-grained residues, on the binding free energy. It is shown that, in spite of small variations of the binding free energy with respect to the active site resolution, the separate contributions coming from different energetic terms (such as electrostatic and van der Waals interactions) manifest a stronger dependence on the mapping, thus pointing to the existence of an optimal level of intermediate resolution

First-principles prediction of structure, energetics, formation enthalpy, elastic constants, polarization, and piezoelectric constants of AlN, GaN, and InN: comparison of local and gradient-corrected density-functional theory

A number of diverse bulk properties of the zincblende and wurtzite III-V nitrides AlN, GaN, and InN, are predicted from first principles within density functional theory using the plane-wave ultrasoft pseudopotential method, within both the LDA (local density) and GGA (generalized gradient) approximations to the exchange-correlation functional. Besides structure and cohesion, we study formation enthalpies (a key ingredient in predicting defect solubilities and surface stability), spontaneous polarizations and piezoelectric constants (central parameters for nanostructure modeling), and elastic constants. Our study bears out the relative merits of the two density functional approaches in describing diverse properties of the III-V nitrides (and of the parent species N$_2$, Al, Ga, and In), and leads us to conclude that the GGA approximation, associated with high-accuracy techniques such as multiprojector ultrasoft pseudopotentials or modern all-electron methods, is to be preferred in the study of III-V nitrides.Comment: RevTeX 6 pages, 12 tables, 0 figure

Free-carrier screening of polarization fields in wurtzite GaN/InGaN laser structures

The free-carrier screening of macroscopic polarization fields in wurtzite GaN/InGaN quantum wells lasers is investigated via a self-consistent tight-binding approach. We show that the high carrier concentrations found experimentally in nitride laser structures effectively screen the built-in spontaneous and piezoelectric polarization fields, thus inducing a ``field-free'' band profile. Our results explain some heretofore puzzling experimental data on nitride lasers, such as the unusually high lasing excitation thresholds and emission blue-shifts for increasing excitation levels.Comment: RevTeX 4 pages, 4 figure

Universal divergenceless scaling between structural relaxation and caged dynamics in glass-forming systems

On approaching the glass transition, the microscopic kinetic unit spends increasing time rattling in the cage of the first neighbours whereas its average escape time, the structural relaxation time $\tau_\alpha$, increases from a few picoseconds up to thousands of seconds. A thorough study of the correlation between $\tau_\alpha$ and the rattling amplitude, expressed by the Debye-Waller factor (DW), was carried out. Molecular-dynamics (MD) simulations of both a model polymer system and a binary mixture were performed by varying the temperature, the density $\rho$, the potential and the polymer length to consider the structural relaxation as well as both the rotational and the translation diffusion. The simulations evidence the scaling between the $\tau_\alpha$ and the Debye-Waller factor. An analytic model of the master curve is developed in terms of two characteristic length scales pertaining to the distance to be covered by the kinetic unit to reach a transition state. The model does not imply $\tau_\alpha$ divergences. The comparison with the experiments supports the numerical evidence over a range of relaxation times as wide as about eighteen orders of magnitude. A comparison with other scaling and correlation procedures is presented. The study suggests that the equilibrium and the moderately supercooled states of the glassformers possess key information on the huge slowing-down of their relaxation close to the glass transition. The latter, according to the present simulations, exhibits features consistent with the Lindemann melting criterion and the free-volume model.Comment: 8 pages, 11 figure

Spontaneous polarization and piezoelectric constants of III-V nitrides

The spontaneous polarization, dynamical Born charges, and piezoelectric constants of the III-V nitrides AlN, GaN, and InN are studied ab initio using the Berry phase approach to polarization in solids. The piezoelectric constants are found to be up 10 times larger than in conventional III-V's and II-VI's, and comparable to those of ZnO. Further properties at variance with those of conventional III-V compounds are the sign of the piezoelectric constants (positive as in II-VI's) and the very large spontaneous polarization.Comment: RevTeX 4 pages, improved upon revie