Physica Status Solidi (C) Current Topics in Solid State Physics

The self-assembled GaInNAs quantum dot (QD) has proposed as a novel material system for long wavelength lasers on GaAs substrate. In this paper, we have investigated the thermal annealing effect on GaInNAs QDs. The increase of the PL intensity and blue shift of peak wavelength was observed by thermal annealing. For 600°C annealing, the PL intensity was increased with the increase of annealing time and maximum intensity was obtained at 2 hours. On the other hand, PL intensity was increased after 30s annealing, then decreased at longer time for 700°C annealing. A larger blue shift of peak wavelength compared to GaInNAs quantum well was observed. It is considered that the interdiffusion was enhanced in QD system due to its large strain and interface area between GaAs capping layer.

We report an experimental study of the coherent coupling between Surface Plasmon Polaritons (SPPs) and Quantum Well (QW) excitons in a hybrid metal- semiconductor nanostructure, consisting of a gallium arsenide quantum well placed in a close proximity of a metal nanoslit array. Exciton-SPP coupling is probed by low-temperature angle resolved spectroscopy. Our results give evidence of a distinct modification of the exciton dispersion relation due to interaction of the exciton with SPP fields at both interfaces of the metal film. An analysis of the experimental data within a coupled oscillator model indicates coupling strengths of several tens of meV.

We report a systematic study on 1.2 MeV Ar^8+ irradiated ZnO by x-ray diffraction (XRD), room temperature photoluminescence (PL) and ultraviolet-visible (UV-Vis) absorption measurements. ZnO retains its wurtzite crystal structure up to maximum fluence of 5 x 10^16 ions/cm^2. Even, the width of the XRD peaks changes little with irradiation. The UV-Vis absorption spectra of the samples, unirradiated and irradiated with lowest fluence (1 x 10^15 ions/cm^2), are nearly same. However, the PL emission is largely quenched for this irradiated sample. Red shift of the absorption edge has been noticed for higher fluence. It has been found that red shift is due to at least two defect centers. The PL emission is recovered for 5 x 10^15 ions/cm^2 fluence. The sample colour is changed to orange and then to dark brown with increasing irradiation fluence. Huge resistivity decrease is observed for the sample irradiated with 5 x 10^15 ions/cm^2 fluence. Results altogether indicate the evolution of stable oxygen vacancies and zinc interstitials as dominant defects for high fluence irradiation. Comment: Accepted in Physica Sattus Solidi (c)

We present calculations of the frequency-dependent spin susceptibility tensor of a two-dimensional electron gas with competing Rashba and Dresselhaus spin-orbit interaction. It is shown that the interplay between both types of spin-orbit coupling gives rise to an anisotropic spectral behavior of the spin density response function which is significantly different from that of vanishing Rashba or Dresselhaus case. Strong resonances are developed in the spin susceptibility as a consequence of the angular anisotropy of the energy spin-splitting. This characteristic optical modulable response may be useful to experimentally probe spin accumulation and spin density currents in such systems.

We study the microwave-induced photoconductivity of a two-dimensional electron system (2DES) in the presence of a magnetic field and a two-dimensional modulation. The microwave and Landau contributions are exactly taken into account, while the periodic potential is treated perturbatively. The longitudinal resistivity exhibits oscillations, periodic in $\omega / \omega_c$. Negative resistance states (NRS) develop for sufficiently high electron mobility and microwave power. This phenomenon appears in a narrow window region of values of the lattice parameter ($a$), around $a \sim l_B$, where $l_B$ is the magnetic length. It is proposed that these phenomena may be observed in artificially fabricated arrays of periodic scatterers at the interface of ultraclean heterostructures. {73.20.At,05.60.-k, 72.15.Rn}

We studied the optical absorption induced by 4.7eV pulsed laser radiation on Ge-doped a-SiO2 synthesized by a sol-gel technique. The absorption spectra in the ultraviolet spectral range were measured during and after the end of irradiation with an in situ technique, evidencing the growth of an absorption signal whose profile is characterized by two main peaks near 4.5eV and 5.7eV and whose shape depends on time. Electron spin resonance measurements performed ex situ a few hours after the end of exposure permit to complete the information acquired by optical absorption by detection of the paramagnetic Ge(1) and Ge-E' centers laser-induced in the samples.

We report on recent progress in the acousto-electrical control of self-assembled quantum dot and quantum post using radio frequency surface acoustic waves (SAWs). We show that the occupancy state of these optically active nanostructures can be controlled via the SAW-induced dissociation of photogenerated excitons and the resulting sequential bipolar carrier injection which strongly favors the formation of neutral excitons for quantum posts in contrast to conventional quantum dots. We demonstrate high fidelity preparation of the neutral biexciton which makes this approach suitable for deterministic entangled photon pair generation. The SAW driven acoustic charge conveyance is found to be highly efficient within the wide quantum well surrounding the quantum posts. Finally we present the direct observation of acoustically triggered carrier injection into remotely positioned, individual quantum posts which is required for a low-jitter SAW-triggered single photon source.

Arbitrary waves incident on a solid embedded nanoparticle are studied. The acoustic vibrational frequencies are shown to correspond to the poles of the scattering cross section in the complex frequency plane. The location of the poles is unchanged even if the incident wave is nonplanar. A second approach approximating the infinite matrix as a very large shell surrounding the nanoparticle provides an alternate way of predicting the mode frequencies. The wave function of the vibration is also provided.

We study the evolution of a quantum dot controlled by a frequency-swept (chirped), linearly polarized laser pulse in the presence of carrier-phonon coupling. The final occupation of the exciton state is limited both due to phonon-induced transitions between the adiabatic spectral branches and because of phonon-assisted transitions to the biexciton state. When the biexciton shift is large enough, the quantum dot can be modeled as a two-level system, which corresponds to excitation with circularly polarized light. For this case, we compare different methods of simulations: (i) a time convolutionless method, (ii) correlation expansion and (iii) path integrals. We show that results obtained from these methods agree perfectly at low temperatures.

It is shown that in many cases an adequate description of optical spectra of semiconductor quantum dots requires a treatment beyond the commonly used adiabatic approximation. We have developed a theory of phonon-assisted optical transitions in semiconductor quantum dots, which takes into account non-adiabaticity of the exciton-phonon system. Effects of non-adiabaticity lead to a mixing of different exciton and phonon states that provides a key to the understanding of surprisingly high intensities of phonon satellites observed in photoluminescence spectra of quantum dots. A breakdown of the adiabatic approximation gives an explanation also for discrepancies between the serial law, observed in multi-phonon optical spectra of some quantum dots, and the Franck-Condon progression, prescribed by the adiabatic approach.

We study the quantum jumps of physical quantities in a strongly correlated many electron systems based on a new p - adic functional integral approach. It is shown that a description in terms of the p - adic numbers leads to the fractal behavior and can describe the quantum jumps in the conductivity and magnetization as a function of voltage and magnetic field in nanotechnology devices and low-dimensional strongly correlated organic metals and other materials. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

A new version of a laser-heated aerodynamic levitation apparatus that enables the study of high temperature liquids by neutron spectroscopy is presented. This levitator was developed to be used on the IN6 neutron TimeOf-Flight (TOF) spectrometer at the European High Flux Neutron Source of the Institute Laue Langevin (ILL) in Grenoble, France. This device overcomes earlier restrictions to small sample sizes as routinely used for neutron diffraction studies on levitated liquids at high temperature, and makes other improvements in signal-to-noise ratio. A factor of more than three increase in sample volume in combination with the high flux and low background instrument IN6 makes quasielastic neutron scattering (QENS) studies on levitated molten oxides feasible. Here, we present results of a first QENS experiment towards the study of the relaxation dynamics of a molten CaO-Al2O3 sample. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have derived a universal relaxation function for heterogeneous materials using the maximum entropy principle for nonextensive systems. The power law exponents of the relaxation function are simply related to a global fractal parameter and for large time to the entropy nonextensivity parameter q. For intermediate times the relaxation follows a stretched exponential behavior. The asymptotic power law behaviors both in the time and the frequency domains coincide with those of the Weron generalized dielectric function derived in the stochastic theory from an extension of the Levy central limit theorem. These results are in full agreement with the Jonscher universality principle and find application in the characterization of the dielectric properties of aerogels catalytic supports as well as in the problem of the relation between morphology and dielectric properties of polymer composites.

We show that optical properties of linear molecular aggregates undergo drastic changes when aggregates are deposited on a metal surface. The dipole-dipole interactions of monomers with their images can result in strong {re-structuring of both the exciton band and the absorption spectrum, depending on the arrangement of the monomer transition dipoles with respect to the surface.

Systematic ensemble photoluminescence studies have been performed on type-I InP-quantum dots in Al0.20Ga0.80InP barriers, emitting at approximately 1.85 eV at 5 K. The influence of different barrier configurations as well as the incorporation of additional tunnel barriers on the optical properties has been investigated. The confinement energy between the dot barrier and the surrounding barrier layers, which is the sum of the band discontinuities for the valence and the conduction bands, was chosen to be approximately 190 meV by using Al0.50Ga0.50InP. In combination with 2 nm thick AlInP tunnel barriers, the internal quantum efficiency of these barrier configurations can be increased by up to a factor of 20 at elevated temperatures with respect to quantum dots without such layers.

Ternary and quaternary cubic c-AlxIn1-xN/GaN and c-AlxGayIn1-x-y/GaN heterostructures lattice-matched to c-GaN on freestanding 3C-SiC substrates were grown by plasma-assisted molecular beam epitaxy. The c-AlxGayIn1-x-y alloy permits the independent control of band gap and lattice parameter. The ternary and quaternary films were grown at 620 C. Different alloy compositions were obtained by varying the Al and Ga fluxes. The alloy composition was measured by Energy Dispersive X-ray Spectroscopy (EDX) and Rutherford Backscattering Spectrometry (RBS). X-ray reciprocal space map of asymmetric (-1-13) reflex were used to measure the lattice parameters and to verify the lattice match between the alloy and the c-GaN buffer.

Force-constant and positional disorder have been introduced into diamond lattice models in an attempt to mimic the vibrational properties of a realistic amorphous silicon model. Neither type of disorder is sufficient on its own to mimic the realistic model. By comparing the spectral densities of these models, it is shown that a combination of both disorders is a better representation, but still not completely satisfactory. Topological disorder in these models was investigated by renumbering the atoms and examining the dynamical matrix graphically. The dynamical matrix of the realistic model is similar to that of a positionally-disordered lattice model, implying that the short-range order in both systems is similar.

We study chaotic properties of eigenstates for periodic quasi-1D waveguides with “regular” and “random” surfaces. Main attention is paid to the role of the so-called “gradient scattering” which is due to large gradients in the scattering walls. We demonstrate numerically and explain theoretically that the gradient scattering can be quite strong even if the amplitude of scattering profiles is very small in comparison with the width of waveguides. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have measured the Anisotropic Magnetoresistance (AMR) of ultra thin (5nm) Ga(0.95)Mn(0.05)As films. We find that the sign of the AMR can be positive or negative, which may depend on the direction of the current with respect to the crystal. At low temperatures, transport measurements and SQUID magnetometry suggest that the magnetisation has a component pointing out of the plane of the film.

We study the effect of magnetic anisotropy in a single electron transistor with ferromagnetic electrodes and a non-magnetic island. We identify the variation $\delta \mu$ of the chemical potential of the electrodes as a function of the magnetization orientation as a key quantity that permits to tune the electrical properties of the device. Different effects occur depending on the relative size of $\delta \mu$ and the charging energy. We provide preliminary quantitative estimates of $\delta \mu$ using a very simple toy model for the electrodes.

The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced Hc2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).

The influence of surface anisotropy on the magnetization processes of maghemite nanoparticles with ellipsoidal shape is studied by means of Monte Carlo simulations. Radial surface anisotropy is found to favor the formation of hedgehog-like spin structures that become more stable as the surface anisotropy constant at the surface $k_S$ is increased form the value at the core. We have studied the change in the low temperature hysteresis loops with the particle aspect ratio and with $k_S$, finding a change in the magnetization reversal mode as $k_S$ or the particle elongation is increased.

Electron-electron interactions seem to play a surprisingly small role in the description of the integer quantum Hall effect, considering that for just slightly different filling factors the interactions are of utmost importance causing the interaction-mediated fractional quantum Hall effect. However, recent imaging experiments by Cobden et al. and Ilani et al. constitute strong evidence for the importance of electron-electron interactions even in the integer effect. The experiments report on measurements of the conductance and electronic compressibility of mesoscopic MOSFET devices that show disagreement with predictions from the single particle model. By diagonalising a random distribution of Gaussian scatterers and treating the interactions in Hartree-Fock approximation we investigate the role of electron-electron interactions for the integer quantum Hall effect and find good agreement with the experimental results.

The shot noise suppression in a sample containing a layer of self-assembled InAs quantum dots has been investigated experimentally and theoretically. The observation of a non-monotonic dependence of the Fano factor on the bias voltage in a regime where only few quantum dot ground states contribute to the tunneling current is analyzed by a master equation model. Under the assumption of tunneling through states without Coulomb interaction this behaviour can be qualitatively reproduced by an analytical expression.

The notion of artifical atom relies on the capability to change the number of carriers one by one in semiconductor quantum dots, and the resulting changes in their electronic structure. Organic molecules with transition metal atoms that have a net magnetic moment and display hysteretic behaviour are known as single molecule magnets (SMM). The fabrication of CdTe quantum dots chemically doped with a controlled number of Mn atoms and with a number of carriers controlled either electrically or optically paves the way towards a new concept in nanomagnetism: the artificial single molecule magnet. Here we study the magnetic properties of a Mn-doped CdTe quantum dot for different charge states and show to what extent they behave like a single molecule magnet.