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- Ahuja, Rajeev, et al.
(författare)
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Electronic and optical properties of lead iodide
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 92:12, s. 7219-7224
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Tidskriftsartikel (refereegranskat)abstract
- The electronic properties and the optical absorption of lead iodide (PbI2) have been investigated experimentally by means of optical absorption and spectroscopic ellipsometry, and theoretically by a full-potential linear muffin-tin-orbital method. PbI2 has been recognized as a very promising detector material with a large technological applicability. Its band-gap energy as a function of temperature has also been measured by optical absorption. The temperature dependence has been fitted by two different relations, and a discussion of these fittings is given.
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- Peres, N. M. R., et al.
(författare)
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Phase diagram and magnetic collective excitations of the Hubbard model for graphene sheets and layers
- 2004
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Ingår i: Physical Review B Condensed Matter. - 0163-1829 .- 1095-3795. ; 70:19, s. 1-12
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Tidskriftsartikel (refereegranskat)abstract
- We discuss the magnetic phases of the Hubbard model for the honeycomb lattice both in two and three spatial dimensions. A ground state phase diagram is obtained depending on the interaction strength U and electronic density n. We find a first order phase transition between ferromagnetic regions where the spin is maximally polarized (Nagaoka ferromagnetism) and regions with smaller magnetization (weak ferromagnetism). When taking into account the possibility of spiral states, we find that the lowest critical U is obtained for an ordering momentum different from zero. The evolution of the ordering momentum with doping is discussed. The magnetic excitations (spin waves) in the antiferromagnetic insulating phase are calculated from the random-phase approximation for the spin susceptibility. We also compute the spin fluctuation correction to the mean field magnetization by virtual emission/absorption of spin waves. In the large U limit, the renormalized magnetization agrees qualitatively with the Holstein-Primakoff theory of the Heisenberg antiferromagnet, although the latter approach produces a larger renormalization.
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- Ahuja, Rajeev, et al.
(författare)
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Optical properties of SiGe alloys
- 2003
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 93:7, s. 3832-3836
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of Si1-xGex have been investigated theoretically using a full-potential linear muffin-tin-orbital method. We present the density-of-states as well as the real and imaginary parts of the dielectric function. The calculated dielectric function was found to be in good agreement with the spectroscopic ellipsometry measurements by J. Bahng , J. Phys.: Condens. Matter 13, 777 (2001), and we obtained a static dielectric constant of epsilon(0)=12.19+2.45x in the Si rich regime (xless than or equal to0.5).
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- Araujo, C. M., et al.
(författare)
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Electrical resistivity, MNM transition and band-gap narrowing of cubic GaN : Si
- 2002
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Ingår i: Microelectronics Journal. - 0026-2692. ; 33:4, s. 365-369
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Tidskriftsartikel (refereegranskat)abstract
- The electrical resistivity of the Si-donor cubic GaN is investigated theoretically at low temperature. The critical impurity concentration, N-c, for the metal-nonmetal transition is estimated in three different ways: from using the generalized Drude approach (GDA) for the resistivity; from the vanishing of the chemical potential calculated using the dielectric function model with a Lorentz-Lorenz correction; from finding the crossing point between the energy in the insulating and metallic states. The bandgap narrowing (BGN) has been determined theoretically and experimentally above the MNM transition, The experimental data have been obtained with photoluminescence measurements. Theoretical and experimental results are in rough agreement in the range of impurity concentration of interest.
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- Araujo, C. M., et al.
(författare)
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Spin-dependent conductance in nonmagnetic InGaAs asymmetric double barrier devices
- 2004
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Ingår i: Brazilian journal of physics. - 0103-9733 .- 1678-4448. ; 34:2B, s. 632-634
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Tidskriftsartikel (refereegranskat)abstract
- The spin dependence of the conductance of an asymmetric double-barrier InGaAs device is studied within the multiband k(.)p and envelope function approximations. The spin-dependent transmission probability for electrons across the structure is obtained using transfer matrices and the low bias conductance per unit area is calculated as a function of the Fermi energy (or doping) in the contacts. The possibility to obtain spin polarized currents in such devices is demonstrated, however, the resulting degree of polarization is rather small (a few percent) in the specific InGaAs structures considered here.
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- da Silva, A. F., et al.
(författare)
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Influence of Si doping on optical properties of wurtzite GaN
- 2001
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Ingår i: Journal of Physics. - : IOP Publishing. - 0953-8984 .- 1361-648X. ; 13:40, s. 8891-8899
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Tidskriftsartikel (refereegranskat)abstract
- The band gap shift (BGS) of Si-doped wurtzite GaN for impurity concentrations spanning the insulating to the metallic regimes has been investigated at low temperature. The critical impurity concentration for the metal-non-metal transition is estimated from the generalized Drude approach for the resistivity to be about 1.0 x 10(18) cm(-3). The calculations for the BGS were carried out within a framework of the random phase approximation, taking into account the electron-electron, electron-optical phonon, and electron-ion interactions. In the wake of very recent photoluminescence measurements, we have shown and discussed the possible transitions involved in the experimental results.
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| 10. |
- da Silva, A. F., et al.
(författare)
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Optical properties of oxide compounds PbO, SnO2 and TiO2
- 2004
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Ingår i: Physica Scripta. - 0031-8949 .- 1402-4896. ; T109, s. 180-183
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Tidskriftsartikel (refereegranskat)abstract
- The optical properties of the oxide compounds PbO and SnO2 have been investigated experimentally by transmission spectroscopy measurements and theoretically by a full-potential linearized augmented plane wave (FPLAPW) method. The dielectric functions of the rutile TiO2 has also been calculated. The calculated band-gap energies as well as the optical absorption were found to be in a very good qualitatively agreement with the experimental results.
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- Fernandez, J. R. L., et al.
(författare)
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Electrical resistivity and band-gap shift of Si-doped GaN and metal-nonmetal transition in cubic GaN, InN and AlN systems
- 2001
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Ingår i: Journal of Crystal Growth. - 0022-0248 .- 1873-5002. ; 231:3, s. 420-427
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Tidskriftsartikel (refereegranskat)abstract
- The critical impurity concentration N-c of the metal-nonmetal (MNM) transition for the cubic GaN, InN and AIN systems. is calculated using the following two different criteria: vanishing of the donor binding energy and the crossing point between the energies in the metallic and insulating phases. A dielectric function model with a Lorentz-Lorenz correction is used for the insulating phase. The InN presents an order of magnitude increase in N-c as compared to the other two systems. The electrical resistivity of the Si-donor system GaN is investigated theoretically and experimentally from room temperature down to 10K. It presents a metallic character above a certain high impurity concentration identified as N-c. The samples were grown by plasma assisted molecular beam epitaxy (MBE) on GaAs (0 0 1) substrate. The model calculation is carried out from a recently proposed generalized Drude approach (GDA) presenting a very good estimation for the metallic region. The band-gap shift (BGS) of Si-doped GaN has also been investigated above the MNM transition where this shift is observed. Theoretical and experimental results have a rough agreement in a range of impurity concentration of interest.
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| 14. |
- Persson, Clas, et al.
(författare)
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Optical and reduced band gap in n- and p-type GaN and AlN
- 2002
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Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 92:6, s. 3207-3216
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Tidskriftsartikel (refereegranskat)abstract
- We present a full band calculation of the doping-induced energy shifts of the conduction-band minimum and the valence-band maximum for n- and p-type GaN and AlN. Both wurtzite and zinc-blende structures have been considered. The resulting optical and reduced band-gap energies are presented as functions of the ionized impurity concentration in the heavily doped regime. The computational method is based on a zero-temperature Green's function formalism within the random phase approximation and with the local-field correction of Hubbard. The calculation goes beyond the spherical approximation of the energy bands by using energy dispersions and overlap integrals from a first-principle, full-potential band-structure calculation. Inclusion of the spin-orbit interaction is crucial for describing the uppermost valence bands properly, and we show that the nonparabolicity of the valence bands influences the energy shifts strongly, especially the shift of the optical band gap. With the full band structure, we can explain the results of photoluminescence measurements by Yoshikawa [J. Appl. Phys. 86, 4400 (1999)].
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