Magnetic field dependence of electronic structures in a single and double quantum dots by 3D-MHFKS calculation

The 3-dimensional Mesh-Hartree-Fock-Kohn-Sham (3D-MHFKS) calculation is applied to study the magnetic (B-) field dependence of the electronic structures of circular-shaped vertical quantum dot (Q-dot) with electron number (N) in double barrier structure (DBS) and also coupled double Q-dots in triple barrier structure (TBS). One of the advantageous points of the 3D-MHFKS calculation is that the strength of coupling between two dots are explicitly evaluated by introducing the realistic barrier in TBS as a straightforward extension of 3D-MHFKS calculation of the single Q-dot in DBS. The calculated chemical potentials represented in B-N phase diagram are consistently and systematically discussed by showing the B-field dependence of the occupied single particle energy levels from the view point how the electronic states transfer sequentially from Fock-Darwin (FD) to lowest Landau (LL) and from LL to the spin flip (SF) and from SF to spin-polarized maximum density droplet (MDD) domains as increasing B-field in the Q-dots.     

Magnetic Thomas-Fermi-Weizsäcker model for quantum dots: A comparison with Kohn-Sham ground states

The magnetic extension of the Thomas-Fermi-Weizs?cker kinetic energy is used within density-functional-theory to numerically obtain the ground state densities and energies of two-dimensional quantum dots. The results are thoroughly compared with the microscopic Kohn-Sham ones in order to assess the validity of the semiclassical method. Circular as well as deformed systems are considered. Received 26 October 2000 and Received in final form 14 December 2000     

Magnetic reconstructions at the surface of the B2 FeV alloy

We present an ab initio study of the magnetic surface reconstructions of the B2 FeV alloy using a self-consistent tight-binding linearized muffin tin orbital method developed in the atomic spheres approximation. For (001) and (111), the surface reconstruction stabilizes configurations unstable in the bulk alloy. When Fe is at the (001) surface, a c(2×2) in-plane antiferromagnetic order is found to be the ground state with magnetic moments of -2.32 and 2.27. A p(1×1) ↓ ferromagnetic order is displayed in case of V toplayer with a magnetic moment of -1.83. At the (111) surface, we obtain for Fe toplayer two solutions p(1×1)↑ and p(2×1). The configuration p(1×1)↑ is found to be the ground state with a magnetic moment per atom of 2.34. For V toplayer, only the p(1×1) ↓ solution is obtained with a moment of -0.84. In all cases, the Fe-V coupling is always antiparallel like in the bulk. Our results are discussed and compared to experiments. Received 11 August 2000 and Received in final form 8 June 2001     

Synchronization of underdamped Josephson-junction arrays

Our recent experiments show that arrays of underdamped Josephson junctions radiate coherently only above a threshold number of junctions switched onto the radiating state. For each junction, the radiating state is a resonant step in the current-voltage characteristics due to the interaction between the junctions in the array and an electromagnetic cavity. Here we show that a model of a one-dimensional array of Josephson junctions coupled to a resonator can produce many features of the coherent be havior above threshold, including coherent radiation of power and the shape of the array current-voltage characteristic. The model also makes quantitative predictions about the degree of coherence of the junctions in the array. However, in this model there is no threshold; the experimental below-threshold region behavior could not be reproduced.Received: 11 April 2003, Published online: 23 July 2003PACS: 74.50.+r Tunneling phenomena; point contacts, weak links, Josephson effects - 85.25.-j Superconducting devices     

Confined Ge–Pt states in self-organized Pt nanowire arrays on Ge(001)

By means of band structure calculations within the density functional theory and the generalized gradient approximation, we investigate the electronic structure of self-organized Pt nanowires on the Ge(001) surface. In particular, we deal with a novel one-dimensional surface state confined in the nanowire array and clarify its origin. Due to large Pt contributions, the novel state is rather a mixed Ge–Pt hybrid state than a confined Ge surface state. Moreover, we compare our results to data from scanning tunneling microscopy.     

Photorefractive ac-enhanced nonlinear response of sillenites: Low- and high-contrast effects

Analytically and numerically we investigate the dependence of the first Fourier harmonics of the space-charge field, induced in an AC-biased sillenite crystal by a light-interference pattern, on the light contrast m. It is shown that within the whole contrast range, 0 < m < 1, these dependences are controlled by the only scalar parameter - the space-charge wave quality factor Q. In the low-contrast limit, m Q ^-2, this factor defines the degree of enhancement of the nonlinear response while for larger contrasts it characterizes strong saturation effects. The data obtained are compared with the results of the previous studies of the AC-response. The possibilities of experimental detection of predicted dependences and their possible implications are discussed. Received 13 September 2002 / Received in final form 27 December 2002 Published online 11 February 2003     

Wigner lattice of ripplopolarons in a multielectron bubble in helium

The properties of ripplonic polarons in a multielectron bubble in liquid helium are investigated on the basis of a path-integral variational method. We find that the two-dimensional electron gas can form deep dimples in the helium surface, or ripplopolarons, to solidify as a Wigner crystal. We derive the experimental conditions of temperature, pressure and number of electrons in the bubble for this phase to be realized. This predicted state is distinct from the usual Wigner lattice of electrons: it melts by dissociation of the ripplopolarons when the electrons shed their localizing dimple as the pressure on the multielectron bubble drops below a critical value. Received 20 February 2003 Published online 11 April 2003 RID="a" ID="a"Also at: TU Eindhoven, Eindhoven, The Netherlands e-mail: devreese@uia.ua.ac.be     

Effective magnetic properties of a composite material with circular conductive elements

Effective magnetic properties of a composite meta-material consisting of periodically arranged circular conductive elements are studied theoretically. A general expression for the effective bulk permeability is obtained with mutual effects and lattice ordering being taken into account. The resonance frequency of the permeability is found to be strongly dependent on the size and shape of the unit cell. Frequency dispersion of the permeability is studied with special attention paid to the frequency range, where negative values of the permeability are possible. Corresponding recommendations for optimisation of the meta-materials with negative permeability are made. The results are confirmed by numerical simulations of the finite structure behaviour in an external magnetic field. Received 19 April 2002 Published online 31 July 2002     

Collective electronic excitations in a semiconductor superlattice in the Landau and Wannier-Stark ladder regime

Using a mean-field approximation, we have developed a systematic treatment of collective electronic modes in a semiconductor superlattice (SL) in the presence of strong electric and magnetic fields parallel to the SL axis. The spectrum of collective modes with zero wavevector along the SL axis is shown to consist of a principle magnetoplasmon mode and an infinite set of Bernstein-like modes. For non-zero wavevector along the SL axis, in addition to the cyclotron modes, extra collective modes are found at the frequencies |Nω _c±Mω _s|, which we call cyclotron-Stark modes (ω _c and ω _s are respectively the cyclotron and Stark frequencies, N and M are integer numbers). The frequencies of the modes propagating in “oblique” direction with respect to the SL axis show oscillatory behavior as a function of electric field strength. All the modes considered have very weak spatial dispersion and they are not Landau damped. The specific predictions made for the dispersion relations of the collective excitations should be observable in resonant Raman scattering experiments. Received 29 August 2002 / Received in final form 25 February 2003 Published online 4 June 2003 RID="a" ID="a"e-mail: 612033@inbox.ru     

Collective oscillations in quantum rings: A broken symmetry case

We present calculations within density functional theory of the ground state and collective electronic oscillations in small two-dimensional quantum rings. No spatial symmetries are imposed to the solutions and, as in a recent contribution, a transition to a broken symmetry solution in the intrinsic reference frame for an increasingly narrow ring is found. The oscillations are addressed by using real-time simulation. Conspicuous effects of the broken symmetry solution on the spectra are pointed out. Received 6 April 2000 and Received in final form 9 June 2000     

Surface state on first-order ferroelectrics

In the presence of a surface the Landau-Devonshire equations of ferroelectricity must be extended to include a boundary condition. For a ferroelectric with a second-order transition in the case when the polarization p(z) increases at the surface, it is well known that a surface state occurs in a range of temperature above the bulk critical temperature t_CB . Here we explore the corresponding effect for a first-order ferroelectric. We show that a surface state can occur, but only if the surface effect is sufficiently strong. Analytic expressions are derived and illustrated for p(z), the surface value p_S =p(0) and the free energy. The transition from the paraelectric state (p=0) to the surface state is first order, and for completeness we establish the dependence of the three critical temperatures (supercooling, thermodynamic and superheating) on a boundary-condition parameter y. In a final section, we derive and illustrate expressions for p(z)in the temperature range t<t_CB .     

Modelling of magnetic effects in near-field optics

Green's dyadic technique represents a powerful tool for calculations in electrodynamics, especially in modelling optical properties of nanoscopic objects. The method does not only provide field distributions, but also maps of susceptibilities and densities of states. Whereas the formalism is well established for dielectrics and electric fields, I present here a straight forward extension to tensors of both electric and magnetic type as well as mixed ones and furthermore to the situation where objects with dielectric and magnetic permeabilities are present together. As examples, characteristic field patterns are compared for elementary dielectric and magnetic perturbations. Green's tensors calculated for a coral structure reveal that mixed susceptibilities can exhibit other symmetries than pure electric or magnetic ones. Maps of all tensor components can thus give essential clues to the interpretation of near-field images. Received 15 December 2002 Published online 20 June 2003 RID="a" ID="a"Files “maths.ps” and “tensors.ps” are only available in electronic form at http://www.edpsciences.org RID="b" ID="b"e-mail: Ursula.Schroeter@uni-konstanz.de     

Polarizability of truncated spheroidal particles supported by a substrate: model and applications

The light scattering by three-dimensional clusters supported by a substrate is modelled by representing clusters by truncated spheroids whose polarizability is calculated via a multipolar development of the potential in the quasi-static limit. The determination of the mean island radius, density and aspect ratio from the optical response is examined. The strong influence of both the particle-substrate interaction and the particle shape on the optical behaviour is demonstrated, showing the limits of effective medium and dipolar theories. The Surface Differential Reflectance spectra of silver on MgO(100) and titanium or aluminium on α-Al₂O₃(0001) surfaces have then been modelled by using the above model, illustrating the capability of optical means to deal with various metals, including those belonging to transition series. In all cases, it is highlighted that the aspect ratio is central in modelling the optical response of supported particles. Received 5 June 2000 and Received in final form 31 July 2001     

Evidence for the short-period oscillations in spin-resolved photoemission of thin Cr(110) films

The spin-resolved electronic structure of thin Cr overlayers on top of the Fe(110) surface was investigated by means of spin- and angle-resolved photoelectron spectroscopy. The initial fast drop of photoelectron spin-polarization at the Fermi level, followed by weak oscillatory behavior with the period of about 2 ML, can give an evidence for the first time spectroscopic observation of the short period oscillations in (110)-oriented thin Cr films.     

Transmission resonance via quantum bound states in confined arrays of antidots

We have studied the transmission resonances for a confined array of antidots, using the lattice Green's function method. Two kinds of resonant peaks via quasibound states are found. One kind of resonant peak corresponds to the split quasibound states. The split states originate from the superposition of quasibound states respectively localized in different (T or crossed) junctions, while the number of quasibound states in each junction is associated with the arm-width of the junction. Electrons in these split states are mainly localized in the junctions. The other kind of resonant peaks correspond to the high quasibound states which exist in (transverse and longitude) multi-period confined arrays of antidots. It is interesting to note that electrons in some of the high quasibound states are mainly localized in the intersection of the junctions rather than in the junctions themselves.     

Optical extinction of an assembly of spherical particles in an absorbing medium: Application to silver clusters in absorbing organic materials

The theory presented by Gerardy and Ausloos for the calculation of the linear optical response of aggregates of spherical particles is analytically continued for absorbing embedding media. The method is based on the calculation of the extinction rate by a single particle embedded in an absorbing matrix. Explicit expressions for the extinction and scattering cross-sections are given. The method is applied to calculate the energy losses in several organic matrices with embedded silver clusters. Comparison with experimental data shows a very good agreement. Received: 21 December 1998     

Interplay of electronic,magnetic and structural properties of surface-supported clusters

We present first theoretical evidence revealing the influence of structural changes on the spin-polarized surface states of large Co nanoislands grown on Cu(111). The minority density of electronic states possesses a pronounced peak whose energetic position depends sensitively on the Co layers stacking order. Our results suggest a way to deduce the stacking order of large Co nanoislands using scanning tunnelling microscopy/spectroscopy.     

Formation of sharp metal-organic semiconductor interfaces: Ag and Sn on CuPc

A detailed investigation of the chemistry and electronic structure during the formation of the interfaces between thin films of the archetypal organic molecular semiconductor copper phthalocyanine (CuPc) and Ag or Sn deposited on it was performed using photoemission and near-edge X-ray absorption spectroscopies with synchrotron light. Our study demonstrates the formation of sharp, abrupt interfaces, a behavior which is of particular importance for applications in organic devices. Moreover, for Ag on CuPc we demonstrate that this interface is free from any reaction, whereas there is slight interface reaction for Sn/CuPc.     

Local triboelectricity on oxide surfaces

In triboelectric phenomena, electric charges are transferred when two materials are touched or rubbed together. We present in this paper a study of this effect performed on metallic oxides at the nanometric scale by an Atomic Force Microscope in the resonant mode. We show that following the electrification processes, positive or negative charges can be deposited. From our experimental data, we conclude that the charge transfer results in an equilibrium final state, the occupied states in the gap being “surface states” with large density and spread under the surface along a characteristic distance of about 100 nm. Received 18 March 1998 and Received in final form 8 July 1998