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Search Results: 1 - 10 of 10566 matches for " Sergio; Ulloa "
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Notarios y agricultores: Crecimiento y atraso en el campo mexicano, 1780-1920
Sergio Valerio Ulloa
Secuencia , 2011,
Spin injection and accumulation in inhomogeneous semiconductors
Dan Csontos,Sergio E. Ulloa
Physics , 2005, DOI: 10.1016/j.physe.2005.12.152
Abstract: We present a study of spin transport in charge and spin inhomogeneous semiconductor systems. In particular, we investigate the propagation of spin-polarized electrons through a boundary between two semiconductor regions with different doping concentrations. We use a theoretical and numerical method, presented in this paper, based on a self-consistent treatment of a two-component version of the Boltzmann transport equation. We show that space-charge effects strongly influence the spin transport properties, in particular giving rise to pronounced spin accumulation and spin density enhancement.
Anisotropic optical response of the diamond (111)-2x1 surface
Cecilia Noguez,Sergio E. Ulloa
Physics , 1995, DOI: 10.1103/PhysRevB.53.13138
Abstract: The optical properties of the 2$\times$1 reconstruction of the diamond (111) surface are investigated. The electronic structure and optical properties of the surface are studied using a microscopic tight-binding approach. We calculate the dielectric response describing the surface region and investigate the origin of the electronic transitions involving surface and bulk states. A large anisotropy in the surface dielectric response appears as a consequence of the asymmetric reconstruction on the surface plane, which gives rise to the zigzag Pandey chains. The results are presented in terms of the reflectance anisotropy and electron energy loss spectra. While our results are in good agreement with available experimental data, additional experiments are proposed in order to unambiguously determine the surface electronic structure of this interesting surface.
Extended states in disordered systems: role of off-diagonal correlations
Wei Zhang,Sergio E. Ulloa
Physics , 2004, DOI: 10.1103/PhysRevB.69.153203
Abstract: We study one-dimensional systems with random diagonal disorder but off-diagonal short-range correlations imposed by structural constraints. We find that these correlations generate effective conduction channels for finite systems. At a certain golden correlation condition for the hopping amplitudes, we find an extended state for an infinite system. Our model has important implications to charge transport in DNA molecules, and a possible set of experiments in semiconductor superlattices is proposed to verify our most interesting theoretical predictions.
Selection Rules for Transport Excitation Spectroscopy of Few-Electron Quantum Dots
Daniela Pfannkuche,Sergio E. Ulloa
Physics , 1995, DOI: 10.1103/PhysRevLett.74.1194
Abstract: Tunneling of electrons traversing a few-electron quantum dot is strongly influenced by the Coulomb interaction leading to Coulomb blockade effects and single-electron tunneling. We present calculations which demonstrate that correlations between the electrons cause a strong suppression of most of the energetically allowed tunneling processes involving excited dot states. The excitation of center-of-mass modes, in contrast, is unaffected by the Coulomb interaction. Therefore, channels connected to these modes dominate the excitation spectra in transport measurements.
Geometrical-confinement effects on excitons in quantum disks
Jakyoung Song,Sergio E. Ulloa
Physics , 1995, DOI: 10.1103/PhysRevB.52.9015
Abstract: Excitons confined to flat semiconductor quantum dots with elliptical cross section are considered as we study geometrical effects on exciton binding energy, electron-hole separation, and the resulting linear optical properties. We use numerical matrix diagonalization techniques with appropriately large and optimized basis sets in an effective-mass Hamiltonian approach. The linear optical susceptibilities of GaAs and InAs dots for several different size ratios are discussed and compared to experimental photoluminescence spectra obtained on GaAs/AlGaAs and InAs/GaAs quantum dots. For quantum dots with several nm in size, there is a strong blue shift of the luminescence due to geometrical confinement effects. Also, transition peaks are split and shifted towards higher energy, in comparison with dots with circular cross sections.
Spin polarization control by electric field gradients
Dan Csontos,Sergio E. Ulloa
Physics , 2006, DOI: 10.1103/PhysRevB.74.155207
Abstract: We show that the propagation of spin polarized carriers may be dramatically affected by {\em inhomogeneous} electric fields. Surprisingly, the spin diffusion length is found to strongly depend on the sign and magnitude of electric field \emph{gradients}, in addition to the previously reported dependence on the sign and magnitude of the electric field [Yu & Flatt\'e, Phys. Rev. B {\bf 66}, 201202(R) 2002; {\em ibid.} {\bf 66}, 235302 (2002)]. This indicates that purely electrical effects may play a crucial role in spin polarized injection, transport and detection in semiconductor spintronics. A generalized drift-diffusion equation that describes our findings is derived and verified by numerical calculations using the Boltzmann transport equation.
Landau level mixing by full spin-orbit interactions
Mahdi. Zarea,Sergio. E. Ulloa
Physics , 2005, DOI: 10.1103/PhysRevB.72.085342
Abstract: We study a two-dimensional electron gas in a perpendicular magnetic field in the presence of both Rashba and Dresselhaus spin-orbit interactions. Using a Bogoliubov transformation we are able to write an approximate formula for the Landau levels, thanks to the simpler form of the resulting Hamiltonian. The exact numerical calculation of the energy levels, is also made simpler by our formulation. The approximate formula and the exact numerical results show excellent agreement for typical semiconductors, especially at high magnetic fields. We also show how effective Zeeman coupling is modified by spin-orbit interactions.
Zeeman splitting of shallow donors in GaN
Francisco Mireles,Sergio E. Ulloa
Physics , 1998, DOI: 10.1063/1.123270
Abstract: The Zeeman splitting of the donor spectra in cubic- and hexagonal-GaN are studied using an effective mass theory approach. Soft-core pseudopotentials were used to describe the chemical shift of the different substitutional dopants. The donor ground states calculated range from 29.5 to 33.7 meV, with typically 1 meV higher binding in the hexagonal phase. Carbon is found to produce the largest donor binding energy. The ionization levels and excited states are in excellent agreement with Hall and optical measurements, and suggest the presence of residual C in recent experiments.
Acceptor binding energies in GaN and AlN
Francisco Mireles,Sergio E. Ulloa
Physics , 1998, DOI: 10.1103/PhysRevB.58.3879
Abstract: We employ effective mass theory for degenerate hole-bands to calculate the acceptor binding energies for Be, Mg, Zn, Ca, C and Si substitutional acceptors in GaN and AlN. The calculations are performed through the 6$\times $6 Rashba-Sheka-Pikus and the Luttinger-Kohn matrix Hamiltonians for wurtzite (WZ) and zincblende (ZB) crystal phases, respectively. An analytic representation for the acceptor pseudopotential is used to introduce the specific nature of the impurity atoms. The energy shift due to polaron effects is also considered in this approach. The ionization energy estimates are in very good agreement with those reported experimentally in WZ-GaN. The binding energies for ZB-GaN acceptors are all predicted to be shallower than the corresponding impurities in the WZ phase. The binding energy dependence upon the crystal field splitting in WZ-GaN is analyzed. Ionization levels in AlN are found to have similar `shallow' values to those in GaN, but with some important differences, which depend on the band structure parameterizations, especially the value of crystal field splitting used.
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