Nonlinear surface polaritons in 2D electron system in high magnetic field

The paper deals with the theoretical investigation of nonlinear surface polaritons (NSP) in isolated two-dimensional electron system (2DES) arranged at the interface between linear and nonlinear media and placed into the external quantizing magnetic field directed perpendicularly to 2DES. We consider that nonlinear medium dielectric permeability depends upon the tangential component of electric field only. It is shown that under the integer quantum Hall effect conditions all NSP characteristics are represented by the quantized values. It is found that the NSP spectrum contains two NSP modes - high-frequency and low-frequency ones. It is shown that the NSP can exist only in the case where the value of tangential component of electric field at the interface is less than a certain critical value. It is found that the resonant interaction between the NSP high-frequency mode and surface polariton mode occurs in the vicinity of the cyclotron resonance subharmonic. Received 23 September 2001 / Received in final form 31 January 2002 Published online 2 October 2002 RID="a" ID="a"e-mail: bludov@ire.kharkov.ua     

Plasmons in asymmetric double quantum well structures

We investigate the effects of spatial asymmetry, tunneling coupling, and exchange-correlation correction on the plasmon modes in asymmetric double quantum well (DQW) structures in a time-dependent local-density approximation. Special attention is paid to the properties of the ω _- mode which is always damped in symmetric DQW systems. In addition, the results on the spectral weight of the excitations are also presented. In general, all the modes carry finite spectral weights and should be observable in resonant inelastic light scattering experiments for the specified values of the parameters. Received 2 July 2002 Published online 19 December 2002 RID="a" ID="a"e-mail: c412-1@aphy.iphy.ac.cn     

Charge density wave transport in submicron antidot arrays in NbSe3

We demonstrate for the first time that a periodic array of submicrometer holes (antidots) can be patterned into thin single NbSe₃ crystals. We report on the study of Charge Density Wave (CDW) transport of the network of mesoscopic units between antidots. Size of the elementary unit can be as small as 0.5 μm along the chain axis and in cross-section. We observe size effects for Ohmic residual resistance and in CDW transport current-voltage characteristics in submicronic networks. Received: 25 November 1997 / Received in final form: 30 March 1998 / Accepted: 6 April 1998     

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     

Some new properties of deformed atomic clusters described in a projected spherical basis

Some properties of small sodium clusters, comprising up to 45 atoms, are described using a projected spherical single particle basis. The variation of the cluster shape and inner density with the number of atoms is studied. Seemingly chestnut, clusterization and halo like structures are identified for several metallic clusters. Static polarizabilities and plasmon frequencies are calculated and compared with experimental data and with results obtained in different approaches. Received 28 November 2000 and Received in final form 15 February 2001     

The role of layer-thickness deviation in dispersion and structure of plasmons in finite superlattices

We study the effects of layer thickness variations on the collective plasmon excitation modes of finite superlattices. Unlike other symmetry lowering mechanisms, thickness variation does not strongly localize the surface modes. We find that the reason for this insensitivity lies in the fact that the collective modes of a given finite structure must evolve continuously from the single-finite-superlattice at zero thickness deviation into modes of a pair of uncoupled finite structures at large thickness variation. We also show that this behavior is analogous to the evolution of molecular orbitals from atomic orbitals as the internuclear separation is reduced, in contrast to the analogy of the superlattice modes as a stack of coupled quantum wells. This emphasizes the difference between the electromagnetic symmetry of the finite superlattice and the structural symmetry. Received 16 April 2001 and Received in final form 6 July 2001     

Magnetic dipole and electric quadrupole responses of elliptic quantum dots in magnetic fields

The magnetic dipole (M1) and electric quadupole (E2) responses of two-dimensional quantum dots with an elliptic shape are theoretically investigated as a function of the dot deformation and applied static magnetic field. Neglecting the electron-electron interaction we obtain analytical results which indicate the existence of four characteristic modes, with different B-dispersion of their energies and associated strengths. Interaction effects are numerically studied within the time-dependent local-spin-density and Hartree approximations, assessing the validity of the non-interacting picture. Received 29 November 2001 Published online 6 June 2002     

Charge transport through ferromagnet/two-dimensional electron gas/d-wave superconductor junctions with Rashba spin-orbit coupling

Along the lines of Blonder, Tinkham and Klapwijk, we investigate the charge transport through ferromagnet/two-dimensional electronic gas/d-wave superconductor (F/2DEG/S) junctions in the presence of Rashba spin-orbit (SO) coupling and focus our attention on the interplay between spin polarization and spin precession. At zero spin polarization, the spin-mixing scattering resulted from Rashba SO coupling decreases the zero-bias conductance peak. Under spin polarization, spin precession introduces novel Andreev reflection, which competes with the effect of spin-mixing scattering. If the F layer is a half metal, the later effect is overwhelmed by that of novel Andreev reflection. As a result, the zero-bias conductance dip caused by spin polarization is enhanced, and at strong Rashba SO coupling, a split zero-bias peak is found in the gap. In an intermediate region where the two effects are comparable with each other, the zero-bias conductance shows a reentrant behavior as a function of Rashba SO coupling.     

Linear and nonlinear optical properties of hydrated and dehydrated silica micro-spheres under an electric bias

The linear refractive indices and nonlinear second-order susceptibility of hydrated and dehydrated silica micro-spheres are studied using attenuated total reflectance (ATR) and the second harmonic generation (SHG) method in direct transmission, respectively. A dramatic change of the effective dielectric constant of silica suspension under an electric bias was observed, which is attributed to particle redistribution in the fluid. Dielectric constants of dehydrated silica spheres change slightly under an electric field due to Pockels effect, for which we measure a linear electro-optical coefficient of r₃₃ ∼3.4±0.7 pm/V. The transmission second harmonic generation comes from the third-order susceptibility χ⁽³⁾, which is a coupling of two photons and the electrostatic field induced by the surface –OH charges as characterized by the Gouy-Chapman model. The SH signal from the dehydrated silica vanishes because of the loss of –OH groups on the particle surfaces. Dehydration of silica beads is irreversible. The optical properties of dried silica spheres do not recover their original hydrated state when distilled water is added.     

Coherent quantum transport in two-dimensional electron gas/superconductor double junctions with Rashba spin-orbit coupling

Based on the extended Blonder-Tinkham-Klapwijk (BTK) approach, we have investigated the coherent quantum transport in two-dimensional electron gas/superconductor (2DEG/SC) double tunneling junctions in the presence of the Rashba spin-orbit coupling (RSOC). It is found that all the reflection coefficients in BTK theory as well as conductance spectra oscillate with the external voltage and energy. The oscillation feature of conductance can be tuned largely by the RSOC for low insulating barriers, while for high insulating barriers it is almost independent of the RSOC. These phenomena are essentially different from those found in ferromagnet/superconductor double tunneling junctions.     

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     

Conductance properties in spin field-effect transistors

Conductance properties in spin field-effect transistors (SFET) are studied by taking into account the Rashba spin-orbit coupling strength, the presence of external magnetic field, the angle between the direction of magnetization and the conductance band mismatch between the ferromagnetic contacts and the channel. It is shown that the conductance of the SFET has high peaks while the value of external magnetic field varies. These peaks become more and more pronounced with the potential barriers strength increasing. The conductance peaks also appear by increasing the strength of the spin-orbit coupling. It is found that the conductance exhibits quantum oscillating behavior when varying the angle between the direction of magnetization in the two contacts. The influence of conductance band mismatch between the contacts and channel is also discussed.     

Conserving quasiparticle calculations for small metal clusters

A novel approach for GW-based calculations of quasiparticle properties for finite systems is presented, in which the screened interaction is obtained directly from a linear response calculation of the density-density correlation function. The conserving nature of our results is shown by explicit evaluation of the f-sum rule. As an application, energy renormalizations and level broadenings are calculated for the closed-shell Na₉ ⁺ and Na₂₁ ⁺ clusters, as well as for Na₄. Pronounced improvements of conserving approximations to RPA-level results are obtained.     

Weak and strong coupling regimes, vacuum Rabi splitting and two types of resonances

For two discrete-level quantum systems in interaction, we follow the displacement in the complex plane of the eigen-energies of the compound system when the excited level of one of the two systems is enlarged. These new points are usually called resonances and describe mixed unstable states. This allows us to define and to calculate a critical value of the coupling constant which separates two well-known coupling regimes. These two regimes are thus described in a unified way. In the study, resonances which are usually not taken into account occur. They are studied in the large continuum case provided by the coupling of the hydrogen atom to the states of the transverse electromagnetic field in the vacuum. We justify that some of these resonances be neglected in this case.     

Creation of entanglement between two electron spins induced by many spin ensemble excitations

We theoretically explore the possibility of creating spin entanglement by simultaneously coupling two electronic spins to a nuclear ensemble. By microscopically modeling the spin ensemble as a single mode boson field, we use the time-dependent Fr?hlich transformation (TDFT) method developed recently [Y. Li, C. Bruder, C.P. Sun, Phys. Rev. A 75, 032302 (2007)] to calculate the effective coupling between the two spins. Our investigation shows that the total system realizes a solid state based architecture for cavity QED. Exchanging such kind of effective boson in a virtual process can result in an effective interaction between two spins. It is discovered that a maximum entangled state can be obtained when the velocity of the electrons matches the initial distance between them in a suitable way. Moreover, we also study how the number of collective excitations influences the entanglement. It is shown that the larger the number of excitation is, the less the two spins entangle each other.     

Collective quantum tunneling of strongly correlated electrons in commensurate mesoscopic rings

We present a new effect that is possible for strongly correlated electrons in commensurate mesoscopic rings: the collective tunneling of electrons between classically equivalent configurations, corresponding to ordered states possessing charge and spin density waves (CDW, SDW) and charge separation (CS). Within an extended Hubbard model at half filling studied by exact numerical diagonalization, we demonstrate that the ground state phase diagram comprises, besides conventional critical lines separating states characterized by different orderings (e.g. CDW, SDW, CS), critical lines separating phases with the same ordering (e.g. CDW-CDW) but with different symmetries. While the former also exist in infinite systems, the latter are specific for mesoscopic systems and directly related to a collective tunnel effect. We emphasize that, in order to construct correctly a phase diagram for mesoscopic rings, the examination of CDW, SDW and CS correlation functions alone is not sufficient, and one should also consider the symmetry of the wave function that cannot be broken. We present examples demonstrating that the jumps in relevant physical properties at the conventional and new critical lines are of comparable magnitude. These transitions could be studied experimentally e.g. by optical absorption in mesoscopic systems. Possible candidates are cyclic molecules and ring-like nanostructures of quantum dots. Received 27 November 2000     

Structure of quantum disordered wave functions: weak localization, far tails, and mesoscopic transport

We report on the comprehensive numerical study of the fluctuation and correlation properties of wave functions in three-dimensional mesoscopic diffusive conductors. Several large sets of nanoscale samples with finite metallic conductance, modeled by an Anderson model with different strengths of diagonal box disorder, have been generated in order to investigate both small and large deviations (as well as the connection between them) of the distribution function of eigenstate amplitudes from the universal prediction of random matrix theory. We find that small, weak localization-type, deviations contain both diffusive contributions (determined by the bulk and boundary conditions dependent terms) and ballistic ones which are generated by electron dynamics below the length scale set by the mean free path ℓ. By relating the extracted parameters of the functional form of nonperturbative deviations (“far tails”) to the exactly calculated transport properties of mesoscopic conductors, we compare our findings based on the full solution of the Schr?dinger equation to different approximative analytical treatments. We find that statistics in the far tail can be explained by the exp-log-cube asymptotics (convincingly refuting the log-normal alternative), but with parameters whose dependence on ℓ is linear and, therefore, expected to be dominated by ballistic effects. It is demonstrated that both small deviations and far tails depend explicitly on the sample size--the remaining puzzle then is the evolution of the far tail parameters with the size of the conductor since short-scale physics is supposedly insensitive to the sample boundaries. Received 19 August 2002 Published online 19 November 2002     

Electronic properties of model quantum-dot structures in zero and finite magnetic fields

We have computed electronic structures and total energies of circularly confined two-dimensional quantum dots and their lateral dimers in zero and finite uniform external magnetic fields using different theoretical schemes: the spin-density-functional theory (SDFT), the current-and-spin-density-functional theory (CSDFT), and the variational quantum Monte Carlo (VMC) method. The SDFT and CSDFT calculations employ a recently-developed, symmetry-unrestricted real-space algorithm allowing solutions which break the spin symmetry. Results obtained for a six-electron dot in the weak confinement limit and in zero magnetic field as well as in a moderate confinement and in finite magnetic fields enable us to draw conclusions about the reliability of the more approximative SDFT and CSDFT schemes in comparison with the VMC method. The same is true for results obtained for the two-electron quantum dot dimer as a function of inter-dot distance. The structure and role of the symmetry-breaking solutions appearing in the SDFT and CSDFT calculations for the above systems are discussed. Received 16 October 2001 and Received in final form 17 January 2002     

Polaronic effects in one-dimensional quantum antidot arrays

We study the effect of polaronic corrections arising from theelectron-longitudinal optical phonon interaction on the energyspectrum of a two-dimensional electron system with a one-dimensionalperiodic antidot array geometry created by a weak electrostaticmodulation potential, and subjected to a weak magnetic fieldmodulation as well as a uniform strong perpendicular staticmagnetic field. To incorporate the effects of electron-phononinteractions within the framework of Fröhlich polaron theory, wefirst apply a displaced-oscillator type unitary transformation todiagonalise the relevant Fröhlich Hamiltonian, and we thendetermine the parameters of this transformation together with theparameter included in the electronic trial wave function . On thebasis of this technique, it has been shown that the polaroniccorrections have non-negligible effects on the electronic spectrumof a two-dimensional electron system with a quantum antidot array,since switching such an interaction results in shifting thedegeneracy restoring points of Landau levels wherein the flatbandcondition is fulfilled, thus suppressing the Weiss oscillations.     

Triggered single photons and entangled photons from a quantum dot microcavity

Current quantum cryptography systems are limited by the attenuated coherent pulses they use as light sources: a security loophole is opened up by the possibility of multiple-photon pulses. By replacing the source with a single-photon emitter, transmission rates of secure information can be improved. We have investigated the use of single self-assembled InAs/GaAs quantum dots as such single-photon sources, and have seen a tenfold reduction in the multi-photon probability as compared to Poissonian pulses. An extension of our experiment should also allow for the generation of triggered, polarization-entangled photon pairs. The utility of these light sources is currently limited by the low efficiency with which photons are collected. However, by fabricating an optical microcavity containing a single quantum dot, the spontaneous emission rate into a single mode can be enhanced. Using this method, we have seen 78% coupling of single-dot radiation into a single cavity resonance. The enhanced spontaneous decay should also allow for higher photon pulse rates, up to about 3 GHz. Received 8 July 2001 and Received in final form 25 August 2001