Shell effects in quantum dots in tilted magnetic fields

The motion of a few electrons in a three-dimensional harmonic oscillator potential under the influence of a homogeneous magnetic field of arbitrary direction is studied. The ground state of the Fermi system is obtained by minimizing the total energy with regard to the confining frequencies. From this a dependence of the equilibrium shape on the electron number, the magnetic field parameters and the slab thickness is found.     

Dependence of the lowest impurity state on alloy composition in GaAs-(Al,Ga)As quantum dots

The ground state donor binding energy is estimated using the simple first order perturbation method for a GaAs-Al _x Ga_1−x As spherical quantum dot. The calculated energy is computed as a function of Al-concentration. Donor binding energy is found to be quite sensitive to Al-concentration (x), specifically for smallx. Furthermore, the binding energy is found to be highest for the smallest and the center-doped dot indicating the strongest confinement in those cases.     

Formation and charge control of a quantum dot by etched trenches and multiple gates

We have fabricated a GaAs/InGaAs/AlGaAs-based single-electron transistor (SET) formed by etched trenches and multiple gates. Clear Coulomb-blockade oscillations have been observed when the gate biases are scanned. By self-consistently solving three-dimensional Schr?dinger and Poisson equations, we have studied the energy-band structure and the carrier distribution of our SET. General agreement between numerical simulation results and measurement data has been obtained, thus indicating the effectiveness of our SET-device design as well as the necessity of a complete three-dimensional quantum-mechanical simulation. Received: 18 October 2001 / Accepted: 6 January 2002 / Published online: 20 March 2002     

Quantum dots in aperiodic order

We study numerically with a Green-function technique one-dimensional arrays of quantum dots with two different models. The arrays are ordered according to the Fibonacci, the Thue–Morse, and the Rudin–Shapiro sequences. As a comparison, results from a periodically ordered chain and also from a random chain are included. The focus is on how the conductance (calculated within the Landauer–Büttiker formalism) depends on the Fermi level. In the first model, we find that in some cases rather small systems (≈60 dots) behave in the same manner as very large systems (>16,000 dots) and this makes it possible in these cases to interpret our results for the small systems in terms of the spectral properties of the infinite systems. In particular, we find that it is possible to see some consequences of the singular continuous spectra that some of the systems possess, at least for temperatures up to 100 mK. In the second model, we study the phenomenon ohmic addition, i.e. when the resistances of the constrictions add up to the total resistance. It results that of the systems studied, it is only the Rudin–Shapiro system that has this behaviour for large structures, while the resistances of the Fibonacci and the Thue–Morse systems might reach a limiting value (as a periodic system does).     

Binding energy of an exciton bound to ionized donors in quantum dots

Binding energies for an exciton (X) trapped in the two-dimensional quantum dot by a positive ion located on the z axis at a distance d from the dot plane are calculated by using the method of few-body physics. This configuration is called a barrier (D⁺,X) center. The dependence of the binding energy of the ground state of the barrier (D⁺,X) center on the dot radius for a few values of the distance d between the fixed positive ion on the z axis and the dot plane is obtained. We find that when d<0.2nm the barrier (D⁺,X) center does not form a bound state.     

Electronic structure of single-wall carbon nanotubes studied by resonant inelastic X-ray scattering

F in the nanotubes, indicating that metallic nanotubes are present in the sample. Received: 2 April 1998     

The self-consistent calculation of Si δ-doped GaAs structures

In this study, we report results of a self-consistent calculation obtained for the sub-band structure of Si δ-doped GaAs material by using a new alternative method. We will discuss the influence of the δ-doping concentration and the δ-layerthickness on the sub-band structure for a non-uniform distribution, which is taken as different from the known Gaussian distribution. The confining potential, the sub-band energies, the sub-band occupations, and the Fermi energy have been calculated by solving the Schr?dinger and Poisson equations by using the Airy functions self-consistently. Received: 4 December 2000 / Accepted: 31 January 2001 / Published online: 26 April 2001     

Electronic conductance of a multi-channel point contact in a magnetic field

We present the numerical results of the electronic conductanceG of a quantum wire with a multichannel point contact structure in a perpendicular external magnetic fieldH at zero temperature, based on the rigorous quantum mechanics of a two-dimensional noninteracting electron gas. Computational results show the approximate quantization of the electronic conductance. WheH is weak,Ginteger multiples of 2e ²/h; and whenH is trong, Ginteger multiples of 2ne ²/h, wheren is the number of channels in the point contact structure of the quantum wire. Quantum leaps take place whenH±2m ^* E _F /[e(2j+1)], wherej is either zero or a positive integer small enough for the external magnetic fieldH to be strong, andm ^* is the effective mass of an electron in the device. To our knowledge, no report on this quantization of electronic conductance has been published. Oscillations are manifest in theGH curves for comparatively narrow channels because of the quantum size effect.     

Capacitance analysis for a metal–insulator– semiconductor structure with an ultra-thin oxide layer

Applied Physics A - We have studied theoretically the capacitance characteristics of a metal–insulator–semiconductor structure with an ultra-thin oxide layer by self-consistently...     

Branching of electron current and quantum effects in two-dimensional dislocation barrier of n-type semiconductor

The current flow through the dislocation wall of the bicrystal (or intergrain) boundary in semiconductor is considered. It is known that the broken bonds along the dislocations make up the acceptor levels (being offset by energy ?_d from the bottom of the conductance band) whose population by electrons is defined by the bulk donor concentration n_D, the ?_d-value and the dislocation spacing D_y. At low enough n_D-value (n_D<e²/?_dD_y⁴), electrons captured at the dislocation acceptors produce the periodic 2D-lattice with two comparable periods of the order of n_s^−1/2 where n_s∼(?_dN_D/e²)^1/2 is 2D-density of captured electrons. Charges of those electrons result in the formation of the dislocation potential barrier whose height (with mean value in order of ?_d) is periodically modulated in the wall plane. Naturally, the electron current prefers to flow along the paths with the lowest barrier height, i.e. through the saddle points of the 3D-potential relief positioned in the dislocation wall plane. At low enough temperatures that leads to separation of the electron current into numerous branches. Provided both 2D-lattice periods of the captured charges are smaller as compared with the electron mean free path, all those narrow and short branches make up a filamentary 1D-conductors with the ballistic transport. As this takes place, effects associated with the 1D-conductance quantization define the whole system resistance and could be observed experimentally. In particular, exponentially strong temperature dependence of the whole system resistance vanishes.     

Effect of spin-flip scattering on electrical transport in magnetic tunnel junctions

By means of the nonequilibrium Green function technique, the effect of spin-flip scatterings on the spin-dependent electrical transport in ferromagnet–insulator–ferromagnet (FM–I–FM) tunnel junctions is investigated. It is shown that Jullière's formula for the tunnel conductance must be modified when including the contribution from the spin-flip scatterings. It is found that the spin-flip scatterings could lead to an angular shift of the tunnel conductance, giving rise to the junction resistance not being the largest when the orientations of magnetizations in the two FM electrodes are antiparallel, which may offer an alternative explanation for such a phenomenon observed previously in experiments in some FM–I–FM junctions. The spin-flip assisted tunneling is also observed.     

The weak electroproduction of Σ in electron–proton scattering

We obtain differential cross sections for the reaction e⁻+p→Σ⁰+ν_e, for incident electron energies from 0.5 GeV to 6.0 GeV. This calculation is phenomenologically based and makes use of SU(3) and SU(2) relations. We obtain contributions of the individual form factors to the differential cross section and show that the vector current form factor dominates in the region of observability. We also obtain reaction rates for a standard set of conditions. Finally we compare this case with that for Λ production and discuss what might be learned from this reaction concerning the structure of the weak, strangeness changing, hadronic current.     

Equilibrium of a gravitating plasma in a dipolar magnetic field

The equilibria of plasma in a dipolar magnetic field under the gravitational influence of a massive body (a star or black hole) and a self gravitating plasma are considered. Analytical solutions are found that can be useful for understanding the physics of plasma flows in accretion disks and star formation.     

Heavy flavour contributions to the deep inelastic scattering sum rules

We calculate the effect of the less singular terms at small x on the evolution of the coefficient function in φ³ theory in six dimensions, which result from a complete solution of the ladder equation. Scale-invariant next-to-leading order contributions are also studied. We show that the small-x approximation does not deliver the dominant contributions.     

Critical currents and the critical state in superconductors with magnetic ions

The critical state model is used to derive equations that relate the additional magnetic moment (ΔM) produced by the flux pinning to the critical current density (J_c) measured in transport measurements. The equations derived for conventional superconductors can be used for superconductors that contain magnetic ions, if ΔM is replaced by ΔM/(1 + χ′(H)) where χ′(H) is the differential susceptibility. In the critical state, the field gradient has contributions from both the macroscopic supercurrents and the Amperian currents from the magnetic ions. Magnetic measurements are sensitive to both contributions. Transport measurements only characterise the macroscopic supercurrents. For superconductors which contain rare-earth elements, the J_c values calculated using hysteretic magnetisation measurements without including the term χ′(H), can be in error by factors of 7.     

Intermixing behavior in InGaAs/InGaAsP multiple quantum wells with dielectric and InGaAs capping layers

The influence of the InGaAs capping layer on the intermixing behavior of dielectric-capped In_0.53Ga_0.47 As/In_0.81Ga_0.19As_0.37P_0.63 multiple quantum wells (MQWs) was investigated by measuring the change in the photoluminescence spectra after rapid thermal annealing. The magnitude of the energy shift in the transition energy from the first electronic sub-band to the first heavy- and light-hole sub-bands of the MQWs is large when SiO₂ and InGaAs hybrid capping layers are employed, but it is rather small when Si₃N₄ and InGaAs hybrid capping layers are employed. This result indicates that the InGaAs capping layer holds promise for applications involved in the fabrication of integrated photonic devices, but only when it is incorporated with the SiO₂ capping layer. The reason why the InGaAs capping layer behaves differently under the SiO₂ and Si₃N₄ capping layers is also discussed. Received: 4 December 1999 / Accepted: 26 September 2000 / Published online: 10 January 2001     

Local structure of porous silicon studied by means of X-ray emission spectroscopy

2,3 X-ray emission spectra of porous silicon (P-Si) and of spark-processed silicon (sp-Si). Both types of Si-structure display strong photoluminescence in the visible range of the spectrum. Porous samples were prepared by anodization of n^-- and p⁺-Si-wafers. Whereas for the P-Si processed from p⁺-Si the presence of some amorphous silicon is detected, the X-ray emission spectra of porous Si prepared from n^--Si display a higher content of SiO₂. For spark-processed Si the Si L_2,3 X-ray emission spectra reveal a much stronger degree of oxidation which extends to depths larger than 10000 Å. Furthermore, the chemical state of silicon atoms of sp-Si measured at the center of the processed area is close to that of silicon dioxide, and it has an influence on the photoluminescence energy. Specifically, green photoluminescent sp-Si shows a higher degree of oxidation than the blue luminescent specimen. However, the depth of oxidation consistently decreases in areas with weak or no PL. Possible origins of the observed photoluminescence are discussed. Accepted: 6 March 1997     

Second-harmonic spectroscopy of electronic structure of Si/SiO2 multiple quantum wells

Size effects in the resonant nonlinear optical response of amorphous Si/SiO₂ multiple quantum wells (MQW) are studied by second-harmonic generation (SHG) spectroscopy in a spectral interval of second-harmonic photon energies from 2.5 to 3.4 eV. The sensitivity of SHG spectroscopy to thickness-dependent electronic structure (sub-band energy position and density of states line shape) of MQW is demonstrated. A monotonic red shift of central energies of SHG resonances by 120 m eV upon increase of the well thickness from 2.5 to 10 ? is observed. This is interpreted as a size dependence of the position of singularities in the combined density of states for a 2D gas of electrons moving in an effective potential well. It is shown that, for agreement with experiment, the simplest (rectangular) shape of the well should be modified in order to take into account the lattice-potential distortion at the interfaces. Received: 16 October 2001 / Revised version: 16 April 2002 / Published online: 6 June 2002     

Electronic band structures of low-dimensional adsorbate systems: Rare-gas adsorption on transition metals

Angle-resolved photoemission data are dis-cussed for five different Xe adlayers which exhibit electronic structures of different dimensionalities. Xe adsorption on Ni (110)-(1 × 2)-3Hand the (×) R30^° Xe layer on Ru (001) reveal two-dimensional (2D) Xe-derived band structures that are characteristic for hexagonal rare-gas layers. Different Xe 5p dispersion widths on Ni and on Ru are found due to the difference in the Xe-Xe nearest-neighbor distance. For three rare-gas systems (two different Xe coverages on hydrogen-modified Pt (110)-(1 × 2)-H and Kr step decoration on a Pt (997) surface) true one-dimensional (1D) band structures are found. For Xe step adsorption on Pt (997), electronic localized (0D) behavior is observed due to an enlarged Xe-Xe separation. The qualitative differences of the band structures in the case of 2D, 1D and 0D rare-gas systems are demonstrated and are explained by the different dimensionalities of the various structures. Received: 3 August 2000 / Accepted: 4 August 2000 / Published online: 7 March 2001