Spin-polarized transport through an Aharonov-Bohm interferometer with Rashba spin-orbit interaction

We study electron transport through an Aharonov-Bohm (AB) interferometer with a noninteracting quantum dot in each of its arms. Both a magnetic flux phi threading through the AB ring and the Rashba spin-orbit (SO) interaction inside the two dots are taken into account. Due to the existence of the SO interaction, the electrons flowing through different arms of the AB ring will acquire a spin-dependent phase factor in the tunnel-coupling strengths. This phase factor, as well as the influence of the magnetic flux, will induce various interesting interference phenomena. We show that the conductance and the local density of states can become spin polarized by tuning the magnetic flux and the Rashba interaction strength. Under certain circumstances, a pure spin-up or spin-down conductance can be obtained when a spin-unpolarized current is injected from the external leads. Therefore, the electron spin can be manipulated by adjusting the Rashba spin-orbit strength and the structure parameters. (c) 2006 American Institute of Physics.

Magnetotransport in two-dimensional electron gas: The interplay between spin-orbit interaction and Zeeman splitting

We investigate theoretically the interplay between Zeeman splitting, Rashba spin-orbit interaction (RSOI), and Dresselhaus spin-orbit interaction (DSOI) and its influence on the magnetotransport property of two-dimensional electron gas (2DEG) at low temperature. Our theoretical results show that the nodes of the beating patterns of the magnetoresistivity rho(xx) for 2DEG with RSOI or DSOI alone depend sensitively on the total spin splitting induced by these three spin splitting mechanisms. It is interesting to find that the eigenstates in the presence of RSOI alone are connected with those in the presence of DSOI alone but with opposite Zeeman splitting by a time-reversal transformation. Consequently, the magnetoresistivities exhibit exactly the same oscillation patterns for these two cases. For strong RSOI or DSOI alone, the magneto-oscillation of rho(xx) shows two distinct periods. For 2DEG with both RSOI and DSOI, the beating patterns vanish for equal RSOI and DSOI strengths and vanishing Zeeman splitting. They will appear again, however, when Zeeman splitting or the difference between RSOI and DSOI strengths increases.

Optical spectra and exciton state in vertically stacked self-assembled quantum discs

We study the oscillator strengths of the optical transitions of the vertically stacked self-assembled InAs quantum discs. The oscillator strengths change evidently when the two quantum discs are far apart from each other. A vertically applied electric held affects the oscillator strengths severely, while the oscillator strengths change slowly as the radius of one disc increases. We also studied the excitonic energy of the system, including the Coulomb interaction. The excitonic energy increases with the increasing radius of one disc, but decreases as a vertically applied electric field increases.

Electrolyte electroreflectance spectroscopy studies on the interfacial behavior of the near-surface In0.15Ga0.85As/GaAs quantum well electrode vertical bar non-aqueous electrolyte

Electrolyte electroreflectance spectra of the near-surface strained-layer In0.15Ga0.85As/GaAs double single-quantum-well electrode have been studied at different biases in non-aqueous solutions of ferrocene and acetylferrocene. The optical transitions, the Franz-Keldysh oscillations (FKOs) and the quantum confined Stark effects (QCSE) of In0.15Ga0.85As/GaAs quantum well electrodes are analyzed. Electric field strengths at the In0.15Ga0.85As/GaAs interface are calculated in both solutions by a fast Fourier transform analysis of FKOs. A dip is exhibited in the electric field strength versus bias (from 0 to 1.2 V) curve in ferrocene solution. A model concerning the interfacial tunneling transfer of electrons is used to explain the behavior of the electric field. (C) 2001 Elsevier Science B.V. All rights reserved.

Theoretical analysis and experimental study of Fourier transformation of Franz-Keldysh oscillations in GaAs

Fourier transformation (FT) has been used in the theoretical line shape analysis of Franz-Keldysh oscillations (FKOs) in detail by numerical simulations. FKOs from the surface-intrinsic-n(+) GaAs structure were obtained in photoreflectance (PR) measurements with various modulation light intensities and with different strengths of bias light illumination, which were used to change the static electric field in the intrinsic layer of the sample. The FT spectra of the PR spectra, including the real part, imaginary part, and the modulus, were very consistent with the theoretical line shapes. The ratio of the square root of the reduced mass (root mu (L)/root mu (H)) and the ratio of transition strength of the electron heavy hole to the electron light hole were obtained from the PT spectra. In addition, the electric field in the intrinsic layer of the sample without and with bias illumination and the modulation field induced by photomodulation were also obtained. (C) 2000 American Institute of Physics. [S0021-8979(00)02123-X].

Exciton states and optical spectra in CdSe nanocrystallite quantum dots

By using the hole effective-mass Hamiltonian for semiconductors with the wurtzite structure, we have studied the exciton states and optical spectra in CdSe nanocrystallite quantum dots. The intrinsic asymmetry of the hexagonal lattice structure and the effect of spin-orbital coupling (SOC) on the hole states are investigated. It is found that the strong SOC limit is a good approximation for hole states. The selection rules and oscillator strengths for optical transitions between the conduction- and valence-band states are obtained. The Coulomb interaction of exciton states is also taken into account. In order to identify the exciton states, we use the approximation of eliminating the coupling of Gamma(6)(X, Y) with Gamma(1)(Z) states. The results are found to account for most of the important features of the experimental photoluminescence excitation spectra of Norris ct nl. However, if the interaction between Gamma(6)(X, Y) and Gamma(1)(Z) states is ignored, the optically passive P-x state cannot become the ground hole state for small CdSe quantum dots of radius less than 30 Angstrom. It is suggested that the intrinsic asymmetry of the hexagonal lattice structure and the coupling of Gamma(6)(X,Y) with Gamma(1)(Z) states are important for understanding the "dark exciton" effect.

Influence of precipitates on GaN epilayer quality

GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive S-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in < 11-20 > directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. (C) 2000 Elsevier Science S.A, All rights reserved.

Fracture properties of PECVD silicon nitride thin films by long rectangular memrane bulge test

Bulge test combined with a refined load-deflection model for long rectangular membrane was applied to determine the mechanical and fracture properties of PECVD silicon nitride (SiNx) thin films. Plane-strain modulus E-ps prestress s(0), and fracture strength s(max) of SiNx thin films deposited both on bare Si substrate and on SiO2-topped Si substrate were extracted. The SiNx thin films on different substrates possess similar values of E-ps and s(0) but quite different values of s(max). The statistical analysis of fracture strengths were performed by Weibull distribution function and the fracture origins were further predicted.

THEORY OF THE ELECTRONIC-STRUCTURE OF POROUS SI

A theoretical model for the electronic structure of porous Si is presented. Three geometries of porous Si (wire with square cross section, pore with square cross section, and pore with circular cross section) along both the [001] and [110] directions are considered. It is found that the confinement geometry affects decisively the ordering of conduction-band states. Due to the quantum confinement effect, there is a mixing between the bulk X and GAMMA states, resulting in finite optical transition matrix elements, but smaller than the usual direct transition matrix elements by a factor of 10(-3). We found that the strengths of optical transitions are sensitive to the geometry of the structure. For (001) porous Si the structure with circular pores has much stronger optical transitions compared to the other two structures and it may play an important role in the observed luminescence. For this structure the energy difference between the direct and the indirect conduction-band minima is very small. Thus it is possible to observe photoluminescence from the indirect minimum at room temperature. For (110) porous Si of similar size of cross section the energy gap is smaller than that of (001) porous Si. The optical transitions for all three structures of (110) porous Si tend to be much stronger along the axis than perpendicular to the axis.

Spectroscopic properties of anisotropic absorption in a neodymium-doped YAlO3 laser crystal

Absorption spectra of YAlO3:Nd for the three crystallographic axes are investigated at room temperature, The spectral strengths indicate that the absorption coefficient of YAlO3:Nd is anisotropic. The anisotropy of the local electric field acting on the rare-earth ion in a laser crystal is considered, An extended Judd-Ofelt theory is applied to calculate the absorption cross sections and oscillator strengths of the electric-dipole transitions in the different principal directions. Three groups of the phenomenological parameters are derived from a least-squares-fitting procedure. We also analyze theoretically the anisotropy of the optical absorption of YAlO3:Nd crystal in detail. (C) 1997 American Institute of Physics.

Influence of precipitates on GaN epilayer quality

GaN epilayers grown on pre-nitridated (0001) sapphire substrates by metallorganic vapor phase epitaxy were investigated by wavelength dispersive X-ray spectroscopy and energy dispersive S-ray spectroscopy. Precipitates were observed to mainly consist of O impurity whose strengths were weaker than surrounding matrix. The precipitates were larger in size and distributed more sparsely and inhomogeneously in < 11-20 > directions of the epilayers grown on substrates pre-nitridated for longer periods. The larger precipitates often joined to cracks in the TEM specimens. The crack formation seems to be attributed to the compressive stress concentration at edge angles of the larger precipitates. Yellow luminescence of the epilayers was imaged by cathodoluminescence. The distribution similarity between the cathodoluminescence and the precipitates suggested that the precipitates were responsible for the yellow luminescence band. (C) 2000 Elsevier Science S.A, All rights reserved.

Dislocation nucleation governed softening and maximum strength in nano-twinned metals

In conventional metals, there is plenty of space for dislocations-line defects whose motion results in permanent material deformation-to multiply, so that the metal strengths are controlled by dislocation interactions with grain boundaries(1,2) and other obstacles(3,4). For nano-structured materials, in contrast, dislocation multiplication is severely confined by the nanometre-scale geometries so that continued plasticity can be expected to be source-controlled. Nano-grained polycrystalline materials were found to be strong but brittle(5-9), because both nucleation and motion of dislocations are effectively suppressed by the nanoscale crystallites. Here we report a dislocation-nucleation-controlled mechanism in nano-twinned metals(10,11) in which there are plenty of dislocation nucleation sites but dislocation motion is not confined. We show that dislocation nucleation governs the strength of such materials, resulting in their softening below a critical twin thickness. Large-scale molecular dynamics simulations and a kinetic theory of dislocation nucleation in nano-twinned metals show that there exists a transition in deformation mechanism, occurring at a critical twin-boundary spacing for which strength is maximized. At this point, the classical Hall-Petch type of strengthening due to dislocation pile-up and cutting through twin planes switches to a dislocation-nucleation-controlled softening mechanism with twin-boundary migration resulting from nucleation and motion of partial dislocations parallel to the twin planes. Most previous studies(12,13) did not consider a sufficient range of twin thickness and therefore missed this strength-softening regime. The simulations indicate that the critical twin-boundary spacing for the onset of softening in nano-twinned copper and the maximum strength depend on the grain size: the smaller the grain size, the smaller the critical twin-boundary spacing, and the higher the maximum strength of the material.

Theoretical calculation of photoionization cross sections of B-like ions: N2+, O3+ and F4+

There can be found some notable discrepancies with regard to the resonance structures when R-matrix calculations from the Opacity Project and other sources are compared with recent absolute experimental measurements of Bizau et al [Astron. Astrophts. 439 387 (2005)] for B-like ions N2+, O3+ and F4+. We performed close-coupling calculations based on the R-matrix formalism for the photoionizations of ions mentioned above both for the ground states and first excited states in the near threshold regions. The present results are compared with experimental ones given by Bizau et al and earlier theoretical ones. Excellent agreement is obtained between our theoretical results and the experimental photoionization cross sections. The present calculations show a significant improvement over the previous theoretical results.

Theoretical study on electron impact excitation of highly charged Ne-like ions

The electron impact excitation (EIE) cross sections from the ground state to all of the 2s(2)2p(5)3l and 2s2p(6)3l(l=s, p, d) states along the Ne-like isoelectronic sequence of ions (Z = 50-57) have been calculated by using the multiconfiguration Dirac-Fock package GRASP92 and the fully relativistic distorted-wave program REIE06. In the calculations, the relativistic effects and electron correlation effects are considered systematically. Based on those calculations, the EIE cross sections along the Ne-like isoelectronic sequence of ions for different incident electron energies are discussed, and some important conclusions are drawn. We also study the influence of the correlation effects on the values of the 3C/3D line-intensity ratio [3C: (2p(1/2)3d(3/2))(1) -> 2s(2)2p(6) S-1(0), 3D: (2p(3/2)3d(5/2))(1) -> 2s(2)2p(6) S-1(0)] along the Ne-like sequence. A comparison is made between the present results and previous theoretical calculations and experimental results for the EIE cross sections in Ba-46 (+) ions, and a good agreement is obtained.

Iron XII to XXII spectra measurements between 250 and 400 angstrom

This paper reports laboratory measurements of the spectrum of the most abundant transition Fe ions in the universe. Spectrum of iron of low-lying excited states has been observed in the wavelength range 250-400 angstrom at Chinese Institute of Atomic Energy on the HI-13 tandem accelerator with beams of 130 MeV. A number of spectral lines have been mainly classified as transitions of magnesium-, sodium-, neon-, fluorine-like ions. A total of 54 lines have been measured. Most of them have been mainly ascribed to 3s3p(k)-3p(k+1), 3s(2)3p(k)-3s3p(k+1), 2p(5)3s-2p(5)3p, 3s3d-3p3d and 2p(5)3p-2p(5)3d resonance transitions. These spectral lines have been identified, among which 21 are new and accurately measured. The analysis of the spectra was based on a comparison with other experimental results and calculated values. (c) 2010 Elsevier Ltd. All rights reserved.