Origin of the efficient light emission from inversion domain boundaries in GaN

Intentionally-produced inversion domain boundaries in GaN have been reported to be highly efficient recombination centers. Here I report a rationale for this phenomenon based on ab initio density-functional calculations. I also propose a model, based on the existence of polarization in GaN, of the observation that a domain boundary acts as a rectifying junction under voltage applied between the two opposite-polarity surfaces.Comment: RevTeX 4 pages, 4 eps figure

The Increasing Inequality in Income distribution: A Note

In the last 20 years the within countries income inequality has continuously increased. This is a global phenomenon which is observable both advanced and developing countries. Excessive income and wealth inequalities played a role in the genesis of the recent financial crisis and may impair the recovery of the world economy. The long term trend of rising inequalities is the result of different forces. On the one side technological change modified the demand for labour in favour of skilled workers widening the skill premium in wages. From the other side, globalization in trade and finance have contributed to the problem. In particular, fast financial liberalization seems to be a major source of increased inequalities. The huge expansion of financial flows in an international environment lacking adequate international regulatory and supervisory mechanisms means the problems of global economic instability and growing inequality cannot be solved at the national level. New supranational rules and cooperative solutions are called for.Inequality, globalization, technical change

Vibrational stability of graphene under combined shear and axial strains

We study the vibrational properties of graphene under combined shear and uniaxial tensile strain using density-functional perturbation theory. Shear strain always causes rippling instabilities with strain-dependent direction and wavelength; armchair strain contrasts this instability, enabling graphene stability in a large range of combined strains. A complementary description based on membrane elasticity theory nicely clarifies the competition of shear-induced instability and uniaxial tension. We also report the large strain-induced shifts of the split components of the G optical phonon line, which may serve as a shear diagnostic. As to the electronic properties, we find that conical intersections move away from the Brillouin zone border under strain, and they tend to coalesce at large strains, making the opening of gaps difficult to assess. By a detailed search, we find that even at large strains, only small gaps in the tens-of-meV range open at the former Dirac points.Comment: 6 pages, 11 figure

Multiferroicity in V-doped PbTiO3_{3}

We report \emph{ab initio} predictions on the proper multiferroic (ferromagnetic, insulating and ferroelectric) character of PbTiO$_{3}$ doped with vanadium. V impurities coupled ferromagnetically carry a magnetization of 1 $\mu_{\rm B}$ each. The coupling is expected to be strong, since the paramagnetic solution is higher by 150 meV/vanadium, and no stable antiferromagnetic solution was found. The electronic gap in the doped system is about 0.2-0.3 eV in GGA, hence the system is properly multiferroic. V doping increases the spontaneous polarization in PbTiO$_{3}$, with an approximate percentual rate of 0.7 $\mu$C/cm$^{2}$.Comment: 4 pages, 3 figures, 1 tabl

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