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Sökning: L773:1098 0121

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  • Aavikko, R., et al. (författare)
  • Clustering of vacancy defects in high-purity semi-insulating SiC
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 75:8, s. 085208
  • Tidskriftsartikel (refereegranskat)abstract
    • Positron lifetime spectroscopy was used to study native vacancy defects in semi-insulating silicon carbide. The material is shown to contain (i) vacancy clusters consisting of four to five missing atoms and (ii) Si-vacancy-related negatively charged defects. The total open volume bound to the clusters anticorrelates with the electrical resistivity in both as-grown and annealed materials. Our results suggest that Si-vacancy-related complexes electrically compensate the as-grown material, but migrate to increase the size of the clusters during annealing, leading to loss of resistivity. © 2007 The American Physical Society.
  • Abay, Simon, 1980-, et al. (författare)
  • Charge transport in InAs nanowire Josephson junctions
  • 2014
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 89
  • Tidskriftsartikel (refereegranskat)abstract
    • We present an extensive experimental and theoretical study of the proximity effect in InAs nanowires connected to superconducting electrodes. We fabricate and investigate devices with suspended gate-controlled nanowires and nonsuspended nanowires, with a broad range of lengths and normal-state resistances. We analyze the main features of the current-voltage characteristics: the Josephson current, excess current, and subgap current as functions of length, temperature, magnetic field, and gate voltage, and compare them with theory. The Josephson critical current for a short-length device, L = 30 nm, exhibits a record high magnitude of 800 nA at low temperature that comes close to the theoretically expected value. The critical current in all other devices is typically reduced compared to the theoretical values. The excess current is consistent with the normal resistance data and agrees well with the theory. The subgap current shows a large number of structures; some of them are identified as subharmonic gap structures generated by multiple Andreev reflection. The other structures, detected in both suspended and nonsuspended devices, have the form of voltage steps at voltages that are independent of either the superconducting gap or length of the wire. By varying the gate voltage in suspended devices, we are able to observe a crossover from typical tunneling transport at large negative gate voltage, with suppressed subgap current and negative excess current, to pronounced proximity junction behavior at large positive gate voltage, with enhanced Josephson current and subgap conductance as well as a large positive excess current.
  • Aberg, D., et al. (författare)
  • Contributions of point defects, chemical disorder, and thermal vibrations to electronic properties of Cd1-xZnxTe alloys
  • 2013
  • Ingår i: Physical Review B. - 1098-0121. ; 88:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a first-principles study based on density functional theory of thermodynamic and electronic properties of the most important intrinsic defects in the semiconductor alloy Cd1-xZnxTe with x < 0.13. The alloy is represented by a set of supercells with disorder on the Cd/Zn sublattice. Defect formation energies as well as electronic and optical transition levels are analyzed as a function of composition. We show that defect formation energies increase with Zn content with the exception of the neutral Te vacancy. This behavior is qualitatively similar to but quantitatively rather different from the effect of volumetric strain on defect properties in pure CdTe. Finally, the relative carrier scattering strengths of point defects, alloy disorder, and phonons are obtained. It is demonstrated that for realistic defect concentrations, carrier mobilities are limited by phonon scattering for temperatures above approximately 150 K.
  • Aberg, D., et al. (författare)
  • Electronic structure of LaBr3 from quasiparticle self-consistent GW calculations
  • 2012
  • Ingår i: Physical Review B. - 1098-0121. ; 85:12
  • Tidskriftsartikel (refereegranskat)abstract
    • Rare-earth-based scintillators in general and lanthanum bromide (LaBr3) in particular represent a challenging class of materials due to pronounced spin-orbit coupling and subtle interactions between d and f states that cannot be reproduced by standard density functional theory (DFT). Here a detailed investigation of the electronic band structure of LaBr3 using the quasiparticle self-consistent GW (QPscGW) method is presented. This parameter-free approach is shown to yield an excellent description of the electronic structure of LaBr3. Specifically, it is able to reproduce the band gap, the correct level ordering and spacing of the 4f and 5d states, as well as the spin-orbit splitting of La-derived states. The QPscGW results are subsequently used to benchmark several computationally less demanding techniques including DFT+U, hybrid exchange-correlation functionals, and the G(0)W(0) method. Spin-orbit coupling is included self-consistently at each QPscGW iteration and maximally localized Wannier functions are used to interpolate quasiparticle energies. The QPscGW results provide an excellent starting point for investigating the electronic structure of excited states, charge self-trapping, and activator ions in LaBr3 and related materials.
  • Abergel, David, et al. (författare)
  • Infrared absorption of closely aligned heterostructures of monolayer and bilayer graphene with hexagonal boron nitride
  • 2015
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - AMER PHYSICAL SOC. - 1098-0121. ; 92:11
  • Tidskriftsartikel (refereegranskat)abstract
    • We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for an almost completely full first miniband is necessary to extract meaningful information about the moire characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moire parameters.
  • Aboelfotoh, M O, et al. (författare)
  • Schottky-barrier behavior of metals on n- and p-type 6H-SiC
  • 2003
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121. ; 67:7
  • Tidskriftsartikel (refereegranskat)abstract
    • The Schottky-barrier height of a number of metals (Ti, Ni, Cu, and Au) on n- and p-type Si-terminated 6H-SiC has been measured in the temperature range 150-500 K. It is found that the barrier height to n-type 6H-SiC does not exhibit a temperature dependence, while for p-type 6H-SiC the change in the barrier height with temperature follows very closely the change in the indirect energy gap in 6H-SiC. These results are inconsistent with models of Schottky-barrier formation based on the concept of a charge neutrality level. Furthermore, the present results cannot be reconciled with a defect pinning mechanism, contrary to the conclusions of earlier studies on III-V compound semiconductors. We suggest that chemical bonding at the metal-semiconductor interface plays an important role in determining the Schottky-barrier height.
  • Abou-Hamad, E., et al. (författare)
  • Structural properties of carbon nanotubes derived from (13)C NMR
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - College Park, Md. : American Physical Society. - 1098-0121. ; 84:16, s. 165417
  • Tidskriftsartikel (refereegranskat)abstract
    • We present a detailed experimental and theoretical study on how structural properties of carbon nanotubes can be derived from 13C NMR investigations. Magic angle spinning solid state NMR experiments have been performed on single-and multiwalled carbon nanotubes with diameters in the range from 0.7 to 100 nm and with number of walls from 1 to 90. We provide models on how diameter and the number of nanotube walls influence NMR linewidth and line position. Both models are supported by theoretical calculations. Increasing the diameter D, from the smallest investigated nanotube, which in our study corresponds to the inner nanotube of a double-walled tube to the largest studied diameter, corresponding to large multiwalled nanotubes, leads to a 23.5 ppm diamagnetic shift of the isotropic NMR line position d. We show that the isotropic line follows the relation d = 18.3/D + 102.5 ppm, where D is the diameter of the tube and NMR line position d is relative to tetramethylsilane. The relation asymptotically tends to approach the line position expected in graphene. A characteristic broadening of the line shape is observed with the increasing number of walls. This feature can be rationalized by an isotropic shift distribution originating from different diamagnetic shielding of the encapsulated nanotubes together with a heterogeneity of the samples. Based on our results, NMR is shown to be a nondestructive spectroscopic method that can be used as a complementary method to, for example, transmission electron microscopy to obtain structural information for carbon nanotubes, especially bulk samples.
  • Abrikosov, I. A., et al. (författare)
  • Competition between Magnetic Structures in the Fe-Rich FCC FeNi Alloys
  • 2007
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - American Physical Society. - 1098-0121. ; 76:1, s. 014434
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the results of a systematic ab initio study of the magnetic structure of Fe rich fcc FeNi binary alloys for Ni concentrations up to 50 at. %. Calculations are carried out within density-functional theory using two complementary techniques, one based on the exact muffin-tin orbital theory within the coherent potential approximation and another one based on the projector augmented-wave method. We observe that the evolution of the magnetic structure of the alloy with increasing Ni concentration is determined by a competition between a large number of magnetic states, collinear as well as noncollinear, all close in energy. We emphasize a series of transitions between these magnetic structures, in particular we have investigated a competition between disordered local moment configurations, spin spiral states, the double layer antiferromagnetic state, and the ferromagnetic phase, as well as the ferrimagnetic phase with a single spin flipped with respect to all others. We show that the latter should be particularly important for the understanding of the magnetic structure of the Invar alloys.
  • Achermann, M, et al. (författare)
  • Carrier and field dynamics around nanoscale Schottky contacts investigated by ultrafast near-field optics
  • 2002
  • Ingår i: Physical Review B (Condensed Matter and Materials Physics). - American Physical Society. - 1098-0121. ; 65:4
  • Tidskriftsartikel (refereegranskat)abstract
    • We present femtosecond-resolved optical near-field pump-probe measurements of spatiotemporal carrier dynamics around a single nanoscale tungsten (W) disk embedded in GaAs. In these samples, Schottky contacts are formed at the W/GaAs interface. The experimental results are modeled by a selfconsistent treatment of the drift-diffusion equation for the carriers and Poisson's equation for the built-in electric field. At lower optically excited carrier densities, we observe that the built-in field suppresses electron transport towards and trapping into the metal particles. In this regime, an accumulation of carriers is seen at the edge of the depletion region of the Schottky contacts. The calculation reveals that the formation of a self-induced dynamic potential well is the origin of this result. In the high-density regime, efficient carrier transport towards and trapping into the W nanoparticle take place, resulting from the screening of the built-in field. These results allow us to describe measurements of the carrier dynamics in annealed low-temperature grown GaAs and demonstrate that the coupling of the carrier and field dynamics can substantially affect carrier trapping in metal-semiconductor composite materials.
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