Non-linear macroscopic polarization in III-V nitride alloys

We study the dependence of macroscopic polarization on composition and strain in wurtzite III-V nitride ternary alloys using ab initio density-functional techniques. The spontaneous polarization is characterized by a large bowing, strongly dependent on the alloy microscopic structure. The bowing is due to the different response of the bulk binaries to hydrostatic pressure, and to internal strain effects (bond alternation). Disorder effects are instead minor. Deviations from parabolicity (simple bowing) are of order 10 % in the most extreme case of AlInN alloy, much less at all other compositions. Piezoelectric polarization is also strongly non-linear. At variance with the spontaneous component, this behavior is independent of microscopic alloy structure or disorder effects, and due entirely to the non-linear strain dependence of the bulk piezoelectric response. It is thus possible to predict the piezoelectric polarization for any alloy composition using the piezoelectricity of the parent binaries.Comment: RevTex 7 pages, 7 postscript figures embedde

Accurate calculation of polarization-related quantities in semiconductors

We demonstrate that polarization-related quantities in semiconductors can be predicted accurately from first-principles calculations using the appropriate approach to the problem, the Berry-phase polarization theory. For III-V nitrides, our test case, we find polarizations, polarization differences between nitride pairs, and piezoelectric constants quite close to their previously established values. Refined data are nevertheless provided for all the relevant quantities.Comment: RevTeX 4 pages, no figure

GRB 970228 Within the EMBH Model

We consider the gamma-ray burst of 1997 February 28 (GRB 970228) within the ElectroMagnetic Black Hole (EMBH) model. We first determine the value of the two free parameters that characterize energetically the GRB phenomenon in the EMBH model, that is to say the dyadosphere energy, $E_{dya}=5.1\times10^{52}$ ergs, and the baryonic remnant mass $M_{B}$ in units of $E_{dya}$, $B=M_{B}c^{2}/E_{dya}=3.0\times10^{-3}$. Having in this way estimated the energy emitted during the beam-target phase, we evaluate the role of the InterStellar Medium (ISM) number density (n$_{ISM}$) and of the ratio ${\cal R}$ between the effective emitting area and the total surface area of the GRB source, in reproducing the observed profiles of the GRB 970228 prompt emission and X-ray (2-10 keV energy band) afterglow. The importance of the ISM distribution three-dimensional treatment around the central black hole is also stressed in this analysis.Comment: 4 pages, 1 figure, to appear in the Proceedings of the Los Alamos "Gamma Ray Burst Symposium" in Santa Fe, New Mexico, September 8-12 2003 (AIP Conf. Ser.), CHAPTER: GRB Connection to Supernova

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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

Effective absorbing column density in the gamma-ray burst afterglow X-ray spectra

We investigate the scaling relation between the observed amount of absorption in the X-ray spectra of Gamma Ray Burst (GRB) afterglows and the absorber redshift. Through dedicated numerical simulations of an ideal instrument, we establish that this dependence has a power law shape with index 2.4. However, for real instruments, this value depends on their low energy cut-off, spectral resolution and on the detector spectral response in general. We thus provide appropriate scaling laws for specific instruments. Finally, we discuss the possibility to measure the absorber redshift from X-ray data alone. We find that 10^5-10^6 counts in the 0.3-10 keV band are needed to constrain the redshift with 10% accuracy. As a test case we discuss the XMM-Newton observation of GRB 090618 at z=0.54. We are able to recover the correct redshift of this burst with the expected accuracy.Comment: MNRAS accepted. 6 figures. 3 table

A complete sample of bright Swift short Gamma-Ray Bursts

We present a carefully selected sample of short gamma-ray bursts (SGRBs) observed by the Swift satellite up to June 2013. Inspired by the criteria we used to build a similar sample of bright long GRBs (the BAT6 sample), we selected SGRBs with favorable observing conditions for the redshift determination on ground, ending up with a sample of 36 events, almost half of which with a redshift measure. The redshift completeness increases up to about 70% (with an average redshift value of z = 0.85) by restricting to those events that are bright in the 15-150 keV Swift Burst Alert Telescope energy band. Such flux-limited sample minimizes any redshift-related selection effects, and can provide a robust base for the study of the energetics, redshift distribution and environment of the Swift bright population of SGRBs. For all the events of the sample we derived the prompt and afterglow emission in both the observer and (when possible) rest frame and tested the consistency with the correlations valid for long GRBs. The redshift and intrinsic X-ray absorbing column density distributions we obtain are consistent with the scenario of SGRBs originated by the coalescence of compact objects in primordial binaries, with a possible minor contribution (~10%-25%) of binaries formed by dynamical capture (or experiencing large natal kicks). This sample is expected to significantly increase with further years of Swift activity.Comment: 17 pages, 8 figures, 7 tables. Accepted for publication in MNRA

Common effect of chemical and external pressures on the magnetic properties of RECoPO (RE = La, Pr)

We report a detailed investigation of RECoPO (RE = La, Pr) and LaCoAsO materials performed by means of muon spin spectroscopy. Zero-field measurements show that the electrons localized on the Pr$^{3+}$ ions do not play any role in the static magnetic properties of the compounds. Magnetism at the local level is indeed fully dominated by the weakly-itinerant ferromagnetism from the Co sublattice only. The increase of the chemical pressure triggered by the different ionic radii of La$^{3+}$ and Pr$^{3+}$, on the other hand, plays a crucial role in enhancing the value of the magnetic critical temperature and can be mimicked by the application of external hydrostatic pressure up to 24 kbar. A sharp discontinuity in the local magnetic field at the muon site in LaCoPO at around 5 kbar suggests a sizeable modification in the band structure of the material upon increasing pressure. This scenario is qualitatively supported by \emph{ab-initio} density-functional theory calculations.Comment: 13 pages, 10 figure

GRB orphan afterglows in present and future radio transient surveys

Orphan Afterglows (OA) are slow transients produced by Gamma Ray Bursts seen off-axis that become visible on timescales of days/years at optical/NIR and radio frequencies, when the prompt emission at high energies (X and gamma rays) has already ceased. Given the typically estimated jet opening angle of GRBs theta_jet ~ 3 deg, for each burst pointing to the Earth there should be a factor ~ 700 more GRBs pointing in other directions. Despite this, no secure OAs have been detected so far. Through a population synthesis code we study the emission properties of the population of OA at radio frequencies. OAs reach their emission peak on year-timescales and they last for a comparable amount of time. The typical peak fluxes (which depend on the observing frequency) are of few micro Jy in the radio band with only a few OA reaching the mJy level. These values are consistent with the upper limits on the radio flux of SN Ib/c observed at late times. We find that the OA radio number count distribution has a typical slope -1.7 at high fluxes and a flatter (-0.4) slope at low fluxes with a break at a frequency-dependent flux. Our predictions of the OA rates are consistent with the (upper) limits of recent radio surveys and archive searches for radio transients. Future radio surveys like VAST/ASKAP at 1.4 GHz should detect ~ 3x10^-3 OA deg^-2 yr-1, MeerKAT and EVLA at 8.4 GHz should see ~ 3x10^-1 OA deg-2 yr-1. The SKA, reaching the micro Jy flux limit, could see up to ~ 0.2-1.5 OA deg^-2 yr^-1. These rates also depend on the duration of the OA above a certain flux limit and we discuss this effect with respect to the survey cadence.Comment: (10 pages, 5 figures, 1 table) Accepted for publication by PAS