Oscillator strength of a two-dimensional D ion in magnetic fields

In the present paper we determine the oscillator strength of two-dimensional (2D) D⁻ ions under the influence of a static magnetic field. The results are important for the analysis of the optical transitions observed in semiconductor quantum wells. We have applied the ab initio procedure Hyperspherical Adiabatic Approach, based on the use of hyperspherical coordinates. This approach uses an adiabatic separation of the total wave function that allows accurate energies determination from molecular-like potential curves. The convergence is obtained in a systematic way by the inclusion of non-adiabatic couplings without the need of adjustable parameters.     

Transmitted interference effect of double metallic nanoslits composed of a slit and a square-funnel slit

The transmitted interference characteristics for double metallic nanoslits, which are composed of a slit and a square funnel, are investigated by the finite-difference time-domain method. Two types of interference patterns, i.e., the periodic single peak or the periodic double peaks profile, are observed by varying the geometric parameters of the square-funnel slit. The fringe period crucially depends on the exit layer thickness of the square-funnel slit. The near-field field intensity can be enhanced three times that in symmetric slit-doublet structure by selecting specific parameters. We also find that surface plasmon waves can creep along the interface between metal and dielectric, even though the interface possesses orthogonal corners.     

Many body effects and internal transitions of confined excitons in GaAs and CdTe quantum wells

Many body effects contribute significantly to the energy states of electron-hole pairs confined in quantum wells in the presence of excess electrons. We present results of optically detected resonance spectroscopy of the internal transitions of photo-excited electron-hole pairs in the presence of excess electrons for GaAs QWs and CdTe QWs. Compared to the case of isolated negatively charged excitons, excess electrons produce a large blue shift of the internal transitions in modulation-doped GaAs quantum wells (QWs) for filling factor <2, and similar effects are found in CdTe QWs. For filling factor >2 no internal transitions are observed. These measurements demonstrate the strong effects of electron-electron correlations on the internal transitions of charged excitons in these quasi-2D systems and the importance of magnetic translation invariance. In the presence of excess electrons, the observed internal transitions are those of a magnetoplasmon bound to a mobile valence band hole.     

Optical transmission resonances tuned by external static magnetic field in an n-doped semiconductor grating with subwavelength slits

Transmission behavior of light through a grating consisting of n-doped semiconductor with subwavelength slits under the application of external static magnetic fields is investigated. As dielectric constant of n-doped semiconductor can be substantially altered by applied magnetic field, in the Voigt configuration and for TM-polarized illumination, two transmission resonance peaks associated with localized waveguide modes of slits are significantly shifted toward the lower frequency regime with the increase of the applied magnetic field. These characteristics can be assigned to a reduction of effective plasma frequency of n-doped semiconductor under the applied magnetic field. Our findings may provide possibility for achieving tunable transmission resonance spectrum.     

Resonant enhancement of inelastic light scattering in the fractional quantum Hall regime at ν=1/3

Strong resonant enhancements of inelastic light scattering from the long wavelength inter-Landau level magnetoplasmon and the intra-Landau level spin wave excitations are seen for the fractional quantum Hall state at ν=1/3. The energies of the sharp peaks (FWHM 0.2 meV) in the profiles of resonant enhancement of inelastic light scattering intensities coincide with the energies of photoluminescence bands assigned to negatively charged exciton recombination. To interpret the observed enhancement profiles, we propose three-step light scattering mechanisms in which the intermediate resonant transitions are to states with charged excitonic excitations.     

Donor bound excitons in wurtzite InGaN quantum dots: Effects of built-in electric fields

Within the framework of the effective-mass approximation and variational approach, we present calculations of the bound exciton binding energy, due to an ionized donor, in wurtzite In_xGa_1−xN/GaN strained quantum dots (QDs), considering three-dimensional confinement of the electron and hole in the QDs and the strong built-in electric field induced by the spontaneous and piezoelectric polarizations. Our results show that the position of the ionized donor, the strong built-in electric field, and the structural parameters of the QDs have a strong influence on the donor binding energy. The variation of this energy versus position of the donor ion is in double figures of milli-electron volt. Realistic cases, including the donor in the QD and in the surrounding barriers, are considered.     

Two-dimensional electron gas in a periodic magnetic field

We study the energy spectrum and electronic properties of a two-dimensional (2D) spinless electron gas in a periodic magnetic field which has the symmetry of a triangular lattice. We show that the energy bands depend strongly on the value of the magnetic field. For large field the low-energy electrons are localized on closed rings where the magnetic field vanishes. This results in the appearance of persistent currents around these rings. We also calculate the intrinsic Hall conductivity, which is quantized when the Fermi level is in a gap.     

Analytic and numeric Green’s functions for a two-dimensional electron gas in an orthogonal magnetic field

We have derived closed analytic expressions for the Green’s function of an electron in a two-dimensional electron gas threaded by a uniform perpendicular magnetic field, also in the presence of a uniform electric field and of a parabolic spatial confinement. A workable and powerful numerical procedure for the calculation of the Green’s functions for a large infinitely extended quantum wire is considered exploiting a lattice model for the wire, the tight-binding representation for the corresponding matrix Green’s function, and the Peierls phase factor in the Hamiltonian hopping matrix element to account for the magnetic field. The numerical evaluation of the Green’s function has been performed by means of the decimation-renormalization method, and quite satisfactorily compared with the analytic results worked out in this paper. As an example of the versatility of the numerical and analytic tools here presented, the peculiar semilocal character of the magnetic Green’s function is studied in detail because of its basic importance in determining magneto-transport properties in mesoscopic systems.     

First-principles calculations of magnetic and electronic structures around surfaces and interfaces of magnetic multi-layered structure

Electronic and magnetic structures of ferromagnetic (FM)/non-magnetic (NM) and FM/antiferromagnetic (AF) bi-layer systems are calculated by the first principles approach. For the FM/NM system, we focus on the Co/Cu multi-layered structure whose interfacial layer is assumed to have a mixed composition of Co and Cu atoms, and show a possibility that Co atoms at the interface play a significant role as the spin-dependent scattering potentials. In the FM/AF system, we consider Fe or Co monolayer as FM layer and MnNi as AF layers. It is predicted that the Mn moments adjacent to FM layer are forced to align the FM moments, and those of under layer go gradually to anti-parallel alignment as in the bulk MnNi.     

Itinerant electron transport in microscopically inhomogeneous magnetic fields

We report on magnetoresistance measurements in thin nickel films modulated by a periodic magnetic field emanating from micromagnetic arrays fabricated at the film surface. By increasing the strength of the magnetic potential using nickel and dysprosium micromagnets, we are able to quench the anisotropic magnetoresistance (AMR) in the film.     

Intersubband spin-orbit coupling and spin splitting in symmetric quantum wells

In semiconductors with inversion asymmetry, spin-orbit coupling gives rise to the well-known Dresselhaus and Rashba effects. If one considers quantum wells with two or more conduction subbands, an additional, intersubband-induced spin-orbit term appears whose strength is comparable to the Rashba coupling, and which remains finite for symmetric structures. We show that the conduction band spin splitting due to this intersubband spin-orbit coupling term is negligible for typical III-V quantum wells.     

Coupled chains in electric and magnetic fields revisited

Coupled chains in electric and magnetic fields are discussed in terms of interplays between periodicity conditions and the factorization of the wavefunction in the wavenumber representation. Proceeding in this manner yields a quickly tractable matching condition providing the quantization rule to the alternative derivation of complex energy bands. Besides achieving a deeper understanding of Wannier-Stark ladders in terms of complex resonance energies, we now have the opportunity to establish related energy levels by resorting to a suitable conversion technique. This amounts to perform an immediate integration of such energy bands over the Brillouin-zone. Such energy levels exhibit, this time, a safe dependence on the chain parameter. Dynamic localization effects, conserved currents and stripe conductivity formulae have also been discussed.     

Microwave radiation induced magneto-oscillations in the longitudinal and transverse resistance of a two-dimensional electron gas

We confirm the existence of magneto-resistance oscillations in a microwave-irradiated two-dimensional electron gas, first reported in a series of papers by Zudov et al. [Phys. Rev. B 64 (2001) 201311] and Mani et al. [Nature (London) 420 (2002) 646]. In our experiments, on a sample with a moderate mobility, the microwave induced oscillations are observed not only in the longitudinal but also in the transverse-resistance (Hall resistance). The phase of the oscillations is such that the decrease (increase) in the longitudinal resistance is accompanied by an increase (decrease) in the absolute value of the Hall resistance. We believe that these new results provide valuable new information to better understand the origin of this interesting phenomenon.     

The dynamics of excitons and trions in resonant tunneling diodes

The aim of this work is to study the dynamic formation and dissociation of trions and excitons in double barrier resonant tunneling diodes. We propose a system of rate equations that takes into account the formation, dissociation and annihilation of these complexes inside the quantum well. From the solutions of the coupled equations, we are able to study the modulation of excitons and trions formation in the device as a function of the applied bias. The results of our model agree qualitatively with the experiments showing the viability of these rate equations system to study the dynamics of complex systems.     

Spin-dependent electron transport in a type II GaInAsSb/p-InAs heterojunction doped with Mn in quantized magnetic fields

We report a study of spin-related magnetotransport properties of a type II broken-gap heterostructure formed by InAs substrate bulky doped with Mn and δ-Mn-doped GaInAsSb epilayer. Planar and vertical quantum magnetotransport in a 2D-electron-hole system at the single type II broken-gap InAs/GaInAsSb heterointerface was investigated in high magnetic fields under the quantum Hall regime up to 15 T at low temperature (T=1.5 K). The I-V characteristics near the dielectric phase boundary show the step-like behavior that corresponds to the quantum conductance in a disordered 2D structure through the extended edge states of the nearest Landau level closest to the Fermi level. The value of these steps is determined by the orientation of the 2D-electron spin at the Landau level and the magnetic moment of Mn in the δ-layer.     

Selective suppression of extinction within the plasmon resonance of gold nanoparticles

We present extinction measurements on rectangular two-dimensional arrays of gold nanoparticles on a dielectric waveguide. The spectra exhibit spectrally narrow bands of suppressed extinction within the particle–plasmon resonance, resulting from destructive interference between the incident light field and the excited waveguide modes. The dependence of the spectral position of these high-transmission bands on different waveguide modes is investigated in detail. Received: 3 July 2001 / Published online: 10 October 2001     

Korringa relaxation time of magnetic ion system near a two-dimensional electron gas

Spin relaxation of Mn ions in a (Cd,Mn)Te quantum well with quasi-two-dimensional carriers (Q2DEG) is investigated. The mechanism of energy transfer is spin-flip scattering of Mn spin with electrons making transitions between spin subbands accompanied by a change in the Mn spin. A calculation of the spin-flip scattering rate shows that the Mn spin relaxation rate is proportional to the coupling constant squared, the density of states squared, and the electron temperature, the so called Korringa relaxation rate. It was found that for small Mn ion concentration, the relaxation time ≈10−7-10−6s is in a good agreement with experimental results. Moreover, the relaxation rate scales with L⁻², L being the well width, and it can be enhanced over its value in bulk.     

Low-dimensional electron gas at semiconductor surfaces

In recent years, it has become possible to create well-ordered semiconductor surfaces with metallic surface states by using self-assembly of metal atoms. Since these states lie in the band gap of the semiconductor, they completely decouple from the substrate. In addition to two-dimensional structures it is possible to obtain arrays of one-dimensional atomic chains, which may be viewed as the ultimate nanowires. The dimensionality can be varied systematically by using vicinal surfaces with variable step spacing. Angle-resolved photoemission and scanning tunnelling spectroscopy reveal surprising features, such as a fractional band filling, nanoscale phase separation into doped and undoped chain segments, and a spin-splitting at a non-magnetic surface. Prospects for one-dimensional electron gas physics in atomic chains are discussed.     

Anomalous electric-field dependence of excitonic Fano-resonance spectra in semiconductor quantum-wells

Optical absorption spectra due to Fano resonance (FR) of an exciton in a quantum well with an external electric field perpendicular to the layer plane are presented, based on multi-channel scattering calculations incorporating a hole-subband mixing effect. Peak values of the calculated FR spectra exhibit anomalous field-dependent changes. These cannot be accounted for by the commonly-known quantum-confined Stark effect (QCSE) that has been applied exclusively to bound state spectra. This behavior, ascribable to correlation between Fano couplings and the QCSE, is revealed just in high-resolution spectra, otherwise the field-dependence results in nothing but the same as that of the bound-state spectra.     

Electronic properties of zero-dimensional finite-sized nanographene

In this work, we use the tight-binding model to study the low-energy electronic properties of zero-dimensional finite-sized nanographene subject to the influence of an electric field. State energies and energy spacings are found to oscillate significantly with the field strength. The state energies and band gaps also rely upon the type of the nanographene. The electric field will modify state energies, alter energy gaps, and induce the complete energy gap modulations. The band gap of the type-IV nanographene is always zero regardless of the value of the field strength. The variations of the state energies will be directly reflected in the density of states. The numbers and frequencies of the density of states’ divergent peaks are strongly dependent on the field strength and the type of the nanographene. Finally, the electron wave functions are found to be localized at certain zigzag lines at zero electric field.