Search

The coupling of an electron gas confined by a harmonic potential and submitted to a strong magnetic field tilted in relation to the confining direction creates an excitation called here magneto-polariton. This work describes the origin of this excitation and calculates the energy eigenvalues for different tilted angles. It presents the reasons leading to the crossings and repulsions of the energy bands, the degeneracy of the states, the way the Fermi levels change with the magnetic field, the influence of an external electric field, and how the Hall resistance plateaus change with the change in the tilted angle. [ABSTRACT FROM AUTHOR]

This work is a numerical simulation of the bond percolation in an array of junctions and bifurcations mimicking a section of a graphene slab. We calculate the size distribution of graphene clusters as a filler of a polymer aiming to obtain the percolation threshold. We obtained the sigmoidal distribution of graphene clusters as a function of concentration of persistent conducting bonds creating these clusters. The probability density of this distribution shows a universal complementary Fermi–Dirac behavior as a signature of a topological response. Using a tight-binding model for the transmission from the source to the drain, we obtain a smooth transition from an insulator to a conductor through a dirty metal as the concentration of conductive bonds increases for small arrays. As the array size increases, the simulation shows a sharp non-metal-to-metal transition from a pure polymer into a pristine suspended graphene layer. [ABSTRACT FROM AUTHOR]

This work uses Computational Fluid Dynamics (CFD) to simulate the two-phase flow (oil and water) through a reservoir represented by a sandbox model. We investigated the influence in the flows of water having higher and lower mobilities than oil. To accomplish this, we also developed a dedicated solver, with the appropriated equations and representative models implemented in the open-source CFD OpenFOAM platform. In this solver, the black-oil model represented the oil. The results show that the Buckley–Leverett water-flood equation is a good approach for the three-dimensional flow. We observe that the water wall front is mixed to some extent with the oil and evolves obeying an exponential law. Water with mobility lower than oil is not common. However, in this case, the oil recovery is improved and the amount of injected water is reduced. The results comparing different mobilities show that a careful economic assessment should be performed before the field development. We have shown that the low water mobility can increase, as in this studied example, the water front saturation from 0.57 to 0.73, giving a substantial improvement in the oil recovery. The reservoir simulation can provide all process information needed to perform an economical assessment in an oil field exploration. [ABSTRACT FROM AUTHOR]

We investigate the spin-polarized transport of GaMnAs nanolayers in which a ferromagnetic order exists below a certain transition temperature. Our calculation for the self-averaged resistivity takes into account the existence of an impurity band determining the extended ('metallic' transport) or localized (hopping by thermal excitation) nature of the states at and near the Fermi level. Magnetic order and resistivity are inter-related due to the influence of the spin polarization of the impurity band and the effect of the Zeeman splitting on the mobility edge. We obtain, for a given range of Mn concentration and carrier density, a 'metallic' behavior in which the transport by extended carriers dominates at low temperature, and is dominated by the thermally excited localized carriers near and above the transition temperature. This gives rise to a conspicuous hump of the resistivity which has been experimentally observed and brings light onto the relationship between transport and magnetic properties of this material. [ABSTRACT FROM AUTHOR]

Presents a study which examined linear and nonlinear characteristics of superconductor superlattice perpendicular conduction. Linear mobilities from the balance-equation theory; Comparisons between theories and experiments; Conclusions.

The magnetic order resulting from an indirect exchange between magnetic moments in the metallic phase of a Ga[SUB1-x]Mn[SUBx]As bilayered structure is studied via Monte Carlo simulation. The coupling mechanism involves a perturbative calculation in second order of the interaction between the magnetic moments and carriers (holes). We take into account a possible polarization of the hole gas establishing, thus self-consistency between the magnetic order and the electronic structure. It leads to a ferromagnetic order even in the case of thin layers. This fact is analyzed in terms of the inter-and intralayer interactions. [ABSTRACT FROM AUTHOR]

The magnetic order in the diluted magnetic semiconductor barrier of double AlAs/GaAs:Mn quantum well structures is investigated by Monte Carlo simulations. A confinement adapted RKKY mechanism is implemented for indirect exchange between Mn ions mediated by holes. It is shown that, depending on the barrier width and the hole concentration, a ferromagnetic or a spin-glass order can be established. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The group chain U(8)⊃O(8)⊃SU(3) plays an important role in many particle systems whenever the fundamental particles have eight degrees of freedom. As a particular example, the systems of many gluons and pairs of quark-antiquark are discussed, which can be coupled to a flavor octet. In order to determine the explicit structure of states and decay probabilities, the calculation of the Clebsch-Gordan coefficients (CGC) of this group chain is indispensable. In this contribution, the polynomial states of the U(8) chain are constructed and also the isoscalar factors of the CGC’s. Tables of isoscalar factors are presented. The method shown serves as an example for higher rank groups. [ABSTRACT FROM AUTHOR]

The interplay between disorder and spin polarization in a GaMnAs thin layer results into spin-polarized impurity hole bands. A figure of merit is defined to label the nature of the sample as metallic or nonmetallic. It is shown that samples with the highest figures of merit have a ratio between the extended hole density and the Mn concentration near 0.2, in agreement with the ratio of 0.1–0.25 known to occur among samples produced with the highest Curie temperatures. Both the nonmetal-to-metal and the metal-to-nonmetal transitions experimentally observed in the ferromagnetic regime are obtained as the Mn concentration increases. [ABSTRACT FROM AUTHOR]

Ferromagnetism in Mn-doped ZnO is investigated by Monte Carlo simulations assuming indirect exchange interaction via two different competing mechanisms: (i) antiferromagnetic superexchange and (ii) an oscillating carrier mediated interaction. The calculations are performed for p- and n-type samples. The Mn ion is taken as a substitutional impurity and its concentrations varied from 3% to 20%. The carrier concentrations (holes or electrons) were tested in the range of 1×1016–1×1020 cm-3. These simulations showed paramagnetic, ferromagnetic, and spin-glass behaviors for the p-type samples depending on the Mn and hole concentrations. On the other hand, no phase transition was observed for the n-type samples, whatever the Mn and electron concentrations used in the simulations. [ABSTRACT FROM AUTHOR]

A self-consistent calculation of the density of states and the spectral density function is performed in a two-dimensional spin-polarized hole system based on a multiple-scattering approximation. Using parameters corresponding to GaMnAs thin layers, a wide range of Mn concentrations and hole densities have been explored to understand the nature, localized or extended, of the spin-polarized holes at the Fermi level for several values of the average magnetization of the Mn system. The authors show that, for a certain interval of Mn and hole densities, an increase on the magnetic order of the Mn ions come together with a change of the nature of the states at the Fermi level. This fact provides a delocalization of spin-polarized extended states antialigned to the average Mn magnetization and a higher spin polarization of the hole gas. This nonmetal-to-metal transition caused by the increase of Mn concentration once the ferromagnetic phase is reached was observed experimentally since the first samples were produced. These results are consistent with the occurrence of ferromagnetism with relatively high transition temperatures observed in some thin film samples and multilayered structures of this material. [ABSTRACT FROM AUTHOR]

The occurrence of inhomogeneous spin-density distribution in multilayered ferromagnetic diluted magnetic semiconductor nanostructures leads to strong dependence of the spin-polarized transport properties on these systems. The spin-dependent mobility, conductivity, and resistivity in (Ga,Mn)As/GaAs, (Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a function of temperature, scaled by the average magnetization of the diluted magnetic semiconductor layers. An increase of the resistivity near the transition temperature is obtained. The authors observed that the spin-polarized transport properties change strongly among the three materials. [ABSTRACT FROM AUTHOR]

The authors suggest geometrical arrangements in (Zn,Co)O multilayered structures which maximize the spin polarization densities. The theoretical argument is supported by electronic structure calculations based on a two-band effective mass model adapted to describe these systems and solved self-consistently with the aid of Poisson’s equation. The exchange correlation, the strain terms, and the magnetic interactions are all included in the Hamiltonian. From their results a set of parameters can be determined to maximize the presence of spin-polarized charge distributions inside or outside the nonmagnetic layers, which enhances the transition temperature to ferromagnetism or the carrier mobility in the system. [ABSTRACT FROM AUTHOR]

The search for materials tailored for spintronic devices led recently to the study of III–V diluted magnetic semiconductors (DMS). These materials are found to be closer to room-temperature ferromagnetism than their II–VI counterparts. Following this pathway, heterostructures based on III–V DMS were found to have higher Curie temperature. In this work, we propose geometrical arrangements that maximize spin separation combining strained and DMS III–V layers. [ABSTRACT FROM AUTHOR]

The spin-dependent mobility for the lateral transport of the hole gas in a GaMnAs/GaAs heterostructure containing several metallic-like ferromagnetic layers is calculated. The electronic structure is obtained self-consistently taking into account the direct Coulomb Hartree and exchange-correlation terms, besides the sp–d exchange interaction with the Mn magnetic moments.© 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Guiana dolphins produce whistles with a higher frequency and less complexity than most other delphinid species. The present study used a recording system with sampling rate of 192 kHz to describe the high-frequency whistles of Sotalia guianensis in Guanabara Bay, Rio de Janeiro. Eleven acoustic parameters (start, end, minimum, maximum, delta, center and peak frequency, duration, and frequency at 14, 12, and 34 of duration) were measured for all whistles. Whistles with a fundamental frequency up to 66.7 kHz were reported, thereby expanding the known frequency range of this species.