Effect of n+-GaAs thickness and doping density on spin injection of GaMnAs/n+-GaAs Esaki tunnel junction

We investigated the influence of n+-GaAs thickness and doping density of GaMnAs/n+-GaAs Esaki tunnel junction on the efficiency of the electrical electron spin injection. We prepared seven samples of GaMnAs/n+-GaAs tunnel junctions with different n+-GaAs thickness and doping density grown on identical p-AlGaAs/p-GaAs/n-AlGaAs light emitting diode (LED) structures. Electroluminescence (EL) polarization of the surface emission was measured under the Faraday configuration with external magnetic field. All samples have the bias dependence of the EL polarization, and higher EL polarization is obtained in samples in which n+-GaAs is completely depleted at zero bias. The EL polarization is found to be sensitive to the bias condition for both the (Ga,Mn)As/n+-GaAs tunnel junction and the LED structure.Comment: 4pages, 4figures, 1table, To appear in Physica

Markov property and strong additivity of von Neumann entropy for graded quantum systems

It is easy to verify the equivalence of the quantum Markov property and the strong additivity of entropy for graded quantum systems as well. However, the structure of Markov states for graded systems is different from that for tensor product systems. For three-composed graded systems there are U(1)-gauge invariant Markov states whose restriction to the pair of marginal subsystems is non-separable.Comment: 14 pages, to appear J. Math. Phy

Transition temperature of ferromagnetic semiconductors: a dynamical mean field study

We formulate a theory of doped magnetic semiconductors such as Ga$_{1-x}$Mn$_x$As which have attracted recent attention for their possible use in spintronic applications. We solve the theory in the dynamical mean field approximation to find the magnetic transition temperature $T_c$ as a function of magnetic coupling strength $J$ and carrier density $n$. We find that $T_c$ is determined by a subtle interplay between carrier density and magnetic coupling.Comment: 4 pages, 4 figure

Clustering induced suppression of ferromagnetism in diluted magnets

Ferromagnetism in diluted magnets in the compensated regime p << x is shown to be suppressed by the formation of impurity spin clusters. The majority bulk spin couplings are shown to be considerably weakened by the preferential accumulation of holes in spin clusters, resulting in low-energy magnon softening and enhanced low-temperature decay of magnetic order. A locally self-consistent magnon renormalization analysis of spin dynamics shows that although strong intra-cluster correlations tend to prolong global order, T_c is still reduced compared to the ordered case.Comment: published version, 5 pages, 4 figure

Global versus Local Ferromagnetism in a Model for Diluted Magnetic Semiconductors Studied with Monte Carlo Techniques

A model recently introduced for diluted magnetic semiconductors by Berciu and Bhatt (PRL 87, 107203 (2001)) is studied with a Monte Carlo technique, and the results are compared to Hartree-Fock calculations. For doping rates close to the experimentally observed metal-insulator transition, a picture dominated by ferromagnetic droplets formed below a T* scale emerges. The moments of these droplets align as the temperature is lowered below a critical value Tc<T*. Our Monte Carlo investigations provide critical temperatures considerably smaller than Hartree-Fock predictions. Disorder does not seem to enhance ferromagnetism substantially. The inhomogeneous droplet state should be strongly susceptible to changes in doping and external fields.Comment: 4 pages, 4 figure

Theory of Ferromagnetism in Diluted Magnetic Semiconductor Quantum Wells

We present a mean field theory of ferromagnetism in diluted magnetic semiconductor quantum wells. When subband mixing due to exchange interactions between quantum well free carriers and magnetic impurities is neglected, analytic result can be obtained for the dependence of the critical temperature and the spontaneous magnetization on the distribution of magnetic impurities and the quantum well width. The validity of this approximate theory has been tested by comparing its predictions with those from numerical self-consistent field calculations. Interactions among free carriers, accounted for using the local-spin-density approximation, substantially enhance the critical temperature. We demonstrate that an external bias potential can tune the critical temperature through a wide range.Comment: 4 pages, 3 figures, submitted to Phys. Rev.

Gamma Rays From The Galactic Center and the WMAP Haze

Recently, an analysis of data from the Fermi Gamma Ray Space Telescope has revealed a flux of gamma rays concentrated around the inner ~0.5 degrees of the Milky Way, with a spectrum that is sharply peaked at 2-4 GeV. If interpreted as the products of annihilating dark matter, this signal implies that the dark matter consists of particles with a mass between 7.3 and 9.2 GeV annihilating primarily to charged leptons. This mass range is very similar to that required to accommodate the signals reported by CoGeNT and DAMA/LIBRA. In addition to gamma rays, the dark matter is predicted to produce energetic electrons and positrons in the Inner Galaxy, which emit synchrotron photons as a result of their interaction with the galactic magnetic field. In this letter, we calculate the flux and spectrum of this synchrotron emission assuming that the gamma rays from the Galactic Center originate from dark matter, and compare the results to measurements from the WMAP satellite. We find that a sizable flux of hard synchrotron emission is predicted in this scenario, and that this can easily account for the observed intensity, spectrum, and morphology of the "WMAP Haze".Comment: 5 pages, 4 figure