Tunneling conductance of graphene ferromagnet-insulator-superconductor junctions

We study the transport properties of a graphene ferromagnet-insulator superconductor (FIS) junction within the Blonder-Tinkham-Klapwijk formalism by solving spin-polarized Dirac-Bogoliubov-de-Gennes equation. We find that the retro and specular Andreev reflections in the graphene FIS junction are drastically modified in the presence of exchange interaction and that the spin-polarization ($P_T$) of tunneling current can be tuned from the positive to negative value by bias voltage ($V$). In the thin-barrier limit, the conductance $G$ of a graphene FIS junction oscillates as a function of barrier strength $\chi$. Both the amplitude and phase of the conductance oscillation varies with the exchange energy $E_{ex}$. For $E_{ex}<E_F$ (Fermi energy), the amplitude of oscillation decreases with $E_{ex}$. For $E_{ex}^{c}>E_{ex}>E_F$, the amplitude of oscillation increases with $E_{ex}$, where $E_{ex}^{c}=2E_{F}+U_{0}$ ($U_{0}$ is the applied electrostatic potential on the superconducting segment of the junction). For $E_{ex} > E_{ex}^{c}$, the amplitude of oscillation decreases with $E_{ex}$ again. Interestingly, a universal phase difference of $\pi/2$ in $\chi$ exists between the $G-\chi$ curves for $E_{ex}>E_F$ and $E_{ex}<E_F$. Finally, we find that the transitions between retro and specular Andreev reflections occur at $eV=|E_{F}-E_{ex}|$ and $eV=E_{ex}+E_{F}$, and hence the singular behavior of the conductance near these bias voltages results from the difference in transport properties between specular and retro Andreev reflections.Comment: Accepted for publication in Physical Review

Carrier-induced ferromagnetism in n-type ZnMnAlO and ZnCoAlO thin films at room temperature

The realization of semiconductors that are ferromagnetic above room temperature will potentially lead to a new generation of spintronic devices with revolutionary electrical and optical properties. Transition temperatures in doped ZnO are high but, particularly for Mn doping, the reported moments have been small. We show that by careful control of both oxygen deficiency and aluminium doping the ferromagnetic moments measured at room temperature in n-type ZnMnO and ZnCoO are close to the ideal values of 5mB and 3mB respectively. Furthermore a clear correlation between the magnetisation per transition metal ion and the ratio of the number of carriers to the number of transition metal donors was established as is expected for carrier induced ferromagnetism for both the Mn and Co doped films. The dependence of the magnetisation on carrier density is similar to that predicted for the transition temperature for a dilute magnetic semiconductor in which the exchange between the transition metal ions is through the free carriers.Comment: 14 pages pd

European Core Health Indicators - status and perspectives.

The European Core Health Indicators (ECHI) are a key source of comparable health information for the European Union (EU) and its Member States (MS). The ECHI shortlist contains 88 indicators which were developed by experts from MS and international organisations. Most indicators are derived from data sources at the EU's statistical office (Eurostat), the World Health Organisation (WHO) and the Organisation for Economic Co-operation and Development (OECD) and are available for most MS. The remaining indicators on the shortlist are at different stages of conceptual and/or methodological development. The indicators have been reviewed in the past against scientific developments, changes in data collections and emerging policy needs, yet not as part of a systematic and sustainable procedure. There is also no regular inventory of problems met by the MS in collecting the necessary data. Work package 4 of the BRIDGE Health project aimed at updating and improving the existing ECHI-indicator knowledge and expertise and at strengthening the scientific base that supports the effective development and use of health indicators for health policy evaluation and prioritization by the EU and its MS. The aim of this paper is to present a first overview of its outcomes and to explore issues concerning the ECHI data availability, content and policy relevance, update process and accessibility to stakeholders, in light of working towards a sustainable future

Calculations of giant magnetoresistance in Fe/Cr trilayers using layer potentials determined from {\it ab-initio} methods

The ab initio full-potential linearized augmented plane-wave method explicitly designed for the slab geometry was employed to elucidate the physical origin of the layer potentials for the trilayers nFe/3Cr/nFe(001), where n is the number of Fe monolayers. The thickness of the transition-metal ferromagnet has been ranged from $n=1$ up to n=8 while the spacer thickness was fixed to 3 monolayers. The calculated potentials were inserted in the Fuchs-Sondheimer formalism in order to calculate the giant magnetoresistance (GMR) ratio. The predicted GMR ratio was compared with the experiment and the oscillatory behavior of the GMR as a function of the ferromagnetic layer thickness was discussed in the context of the layer potentials. The reported results confirm that the interface monolayers play a dominant role in the intrinsic GMR.Comment: 17 pages, 7 figures, 3 tables. accepted in J. Phys.: Cond. Matte

Operationally Efficient Propulsion System Study (OEPPS)

A description is presented, through view graphs, of the problems encountered in today's launch vehicles and how these problems have adversely affected the ability to achieve serviceability, reliability, and operability. The need is emphasized to recognize and understand the operations problems and the effort that must be made to avoid them in future designs. Technology areas that will enhance operations requirements are also presented

Grain boundary effects on magnetotransport in bi-epitaxial films of La0.7_{0.7}Sr0.3_{0.3}MnO3_3

The low field magnetotransport of La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) films grown on SrTiO$_3$ substrates has been investigated. A high qualtity LSMO film exhibits anisotropic magnetoresistance (AMR) and a peak in the magnetoresistance close to the Curie temperature of LSMO. Bi-epitaxial films prepared using a seed layer of MgO and a buffer layer of CeO$_2$ display a resistance dominated by grain boundaries. One film was prepared with seed and buffer layers intact, while a second sample was prepared as a 2D square array of grain boundaries. These films exhibit i) a low temperature tail in the low field magnetoresistance; ii) a magnetoconductance with a constant high field slope; and iii) a comparably large AMR effect. A model based on a two-step tunneling process, including spin-flip tunneling, is discussed and shown to be consistent with the experimental findings of the bi-epitaxial films.Comment: REVTeX style; 14 pages, 9 figures. Figure 1 included in jpeg format (zdf1.jpg); the eps was huge. Accepted to Phys. Rev.

Vortex lattices in strong type-II superconducting two-dimensional strips

We show how to calculate semi-analytically the dense vortex state in strong type-II superconducting nanostructures. For the specific case of a strip, we find vortex lattice solutions which also incorporate surface superconductivity. We calculate the energy cost to displace individual vortex rows parallel to the surfaces and find that this energy oscillates with the magnetic field. Remarkably, we also find that, at a critical field $H^*$ below $H_{c2}$, this ''shear'' energy becomes strictly zero for the surface rows due to an unexpected mismatch with the bulk lattice.Comment: Title, abstract, and some text paragraphs have been rewritte

Development and Application of 3-Dimensional Transmission Electron Microscopy (3D-TEM) for the Characterization of Metal-Zeolite Catalyst Systems

With electron tomography (3D-TEM) a 3D-reconstruction is calculated from a series of TEM images taken at a tilt angle range (tilting range) of +70° to -70°. The reconstruction can be visualized with contour surfaces that give information about the surface of the sample as well as with slices through the reconstruction that give detailed information on the interior of the sample. Electron tomography gives much more information than Scanning Electron Microscopy (SEM), since SEM gives only information about the surface of a sample. As a case study, the imaging of silver clusters on zeolite NaY is given. The reconstruction shows silver particles at the external surface as well as a silver particle in a mesopore of the zeolite crystallite. It is concluded that 3D-TEM comprises a breakthrough in the characterization of nano-structured solid catalysts

Theory of Umklapp-assisted recombination of bound excitons in Si:P

We present the calculations for the oscillator strength of the recombination of excitons bound to phosphorous donors in silicon. We show that the direct recombination of the bound exciton cannot account for the experimentally measured oscillator strength of the no-phonon line. Instead, the recombination process is assisted by an umklapp process of the donor electron state. We make use of the empirical pseudopotential method to evaluate the Umklapp-assisted recombination matrix element in second-order perturbation theory. Our result is in excellent agreement with the experiment. We also present two methods to improve the optical resolution of the optical detection of the spin state of a single nucleus in silicon.Comment: 9 pages, 6 EPS figures, Revtex