J. T. Devreese

Atomic-scale structure of self-assembled In(Ga)As quantum rings in GaAs

We present an atomic-scale analysis of the indium distribution of self-assembled In(Ga)As quantum rings (QRs) which are formed from InAs quantum dots by capping with a thin layer of GaAs and subsequent annealing. We find that the size and shape of QRs as observed by cross-sectional scanning tunneling microscopy (X-STM) deviate substantially from the ring-shaped islands as observed by atomic force microscopy on the surface of uncapped QR structures. We show unambiguously that X-STM images the remaining quantum dot material whereas the AFM images the erupted quantum dot material. The remaining dot material shows an asymmetric indium-rich crater-like shape with a depression rather than an opening at the center and is responsible for the observed electronic properties of QR structures. These quantum craters have an indium concentration of about 55% and a diameter of about 20nm which is consistent with the observed electronic radius of QR structures.

Photoluminescence of spherical quantum dots

In order to interpret the phonon-assisted optical transitions in semiconductor quantum dots, a theory is developed comprising the exciton interaction with both adiabatic and Jahn-Teller phonons and also the external nonadiabaticity (pseudo-Jahn-Teller effect). The effects of nonadiabaticity of the exciton-phonon system are shown to lead to a significant enhancement of phonon-assisted transition probabilities and to multiphonon optical spectra that are considerably different from the Franck-Condon progression. The calculated relative intensity of the phonon satellites and its temperature dependence compare well with the experimental data on the photoluminescence of CdSe quantum dots, both colloidal and embedded in glass.

High pressure structural phase transformation in gallium nitride

Abstract Under normal conditions GaN crystallizes in the wurtzite structure. At high pressure (30–50 GPa) GaN undergoes a structural phase transformation to the rocksalt structure. The total energy of both structures as well as of the zincblende structure is calculated, for different unit cell volumes, using first-principles non-local pseudopotentials. For the wurtzite structure we obtain a = 3.126 A , c = 5.119 A and an internal parameter u = 0.3767. In the rocksalt structure we get a = 4.098 A and the zinchlende lattice constant is found to be a = 4.419 A . At low pressure the wurtzite structure has the lowest energy. At 55.1 GPa there is a phase transformation to the rocksalt structure. No transition to the zincblende structure is observed.

Effect of the confining potential on the magneto‐optical spectrum of a quantum dot

The energy levels of electrons confined to a circular quantum well with hard walls are calculated in the presence of a perpendicular magnetic field. The results are compared with the case of soft‐wall confinement (parabolic potential). There are important differences in the transition energies of the magneto‐optical spectrum: (i) in contrast to the parabolic case where only two transition energies are found, in the hard‐wall case there are many transitions possible which have different energies. Only a small number of them however have sufficient oscillator strength to be observable; (ii) with increasing magnetic field the energies approach the two‐dimensional results much faster than for the soft‐wall case. In quantum dots with many electrons we calculate the Fermi energy as a function of the magnetic field.

Feynman path-integral treatment of the BEC-impurity polaron

The description of an impurity atom in a Bose-Einstein condensate can be cast in the form of Frohlichs polaron Hamiltonian, where the Bogoliubov excitations play the role of the phonons. An expression for the corresponding polaronic coupling strength is derived, relating the coupling strength to the scattering lengths, the trap size and the number of Bose condensed atoms. This allows to identify several approaches to reach the strong-coupling limit for the quantum gas po- larons, whereas this limit was hitherto experimentally inaccessible in solids. We apply Feynmans path-integral method to calculate for all coupling strengths the polaronic shift in the free energy and the increase in the effective mass. The effect of temperature on these quantities is included in the description. We find similarities to the acoustic polaron results and indications of a transition between free polarons and self-trapped polarons. The prospects, based on the current theory, of investigating the polaron physics with ultracold gases are discussed for lithium atoms in a sodium condensate.

A resonance of the electronic polaron appearing in the optical absorption of alkali halides

Abstract Excited states (scattering states) of free and bound electronic polarons in non metals are introduced and investigated in the continuum approximation. It is suggested that transitions to these states might lead to prominent resonances in the optical absorption at energies approximately twice the bandgap energy. A shift towards higher energies of the corresponding resonances in the energy loss function is calculated. Such resonances and the predicted shift are found in the experimental data for alkali halides; previously they have generally been attributed to plasma excitations. Limitations of the present model, due to the continuum approximation, (and related to the oscillator strength of the transitions) are discussed. The electronic polaron coupling constant a is calculated and tabulated for a number of alkali halides.

Photoluminescence of Tetrahedral Quantum-Dot Quantum Wells

Taking into account the tetrahedral shape of a quantum-dot quantum well (QDQW) when describing excitonic states, phonon modes, and the exciton-phonon interaction in the structure, we obtain within a nonadiabatic approach a quantitative interpretation of the photoluminescence spectrum of a single CdS/HgS/CdS QDQW. We find that the exciton ground state in a tetrahedral QDQW is bright, in contrast to the dark ground state for a spherical QDQW. The position of the phonon peaks in the photoluminescence spectrum is attributed to interface optical phonons. We also show that the experimental value of the Huang-Rhys parameter can be obtained only within the nonadiabatic theory of phonon-assisted transitions.

Multiphonon Raman scattering in semiconductor nanocrystals: importance of nonadiabatic transitions

Multi-phonon Raman scattering in semiconductor nanocrystals is treated taking into account both adiabatic and non-adiabatic phonon-assisted optical transitions. Because phonons of various symmetries are involved in scattering processes, there is a considerable enhancement of intensities of multi-phonon peaks in nanocrystal Raman spectra. Cases of strong and weak band mix- ing are considered in detail. In the first case, fundamental scattering takes place via internal electron-hole states and is participated by s- and d-phonons, while in the second case, when the intensity of the one-phonon Raman peak is strongly influenced by the interaction of an electron and of a hole with in- terface imperfections (e. g., with trapped charge), p-phonons are most active. Calculations of Raman scattering spectra for CdSe and PbS nanocrystals give a good quantitative agreement with recent experimental results.

ELECTRONIC STRUCTURE AND PHONON-ASSISTED LUMINESCENCE IN SELF-ASSEMBLED QUANTUM DOTS

We present photoluminescence (PL) measurements on an ensemble of InAs/GaAs self-assembled quantum dots embedded in GaAs. We observe a transition from an inhomogeneously broadened photoluminescence band under non-resonant excitation into up to five phonon-assisted bands under selective excitation. We interpret the phonon-assisted PL as being indicative of an enhanced electron–phonon interaction. We also perform theoretical calculations of the single-particle energy spectrum of self-assembled quantum dots, in the framework of the single-band effective-mass approximation for electrons and using the Luttinger Hamiltonian for holes. Finally, by taking advantage of the computed wave functions we evaluate the Huang-Rhys parameter: We find an enhancement of the electron–phonon interaction that partially accounts for the experimental results.

On the superconducting phase boundary for a mesoscopic square loop

Abstract A self-consistent solution of the Ginzburg-Landau equations for a mesoscopic superconducting square loop has been obtained. It has been shown that the inhomogeneous distribution of the amplitude of the order parameter inside the loop leads to the appearance of certain areas where it is much more difficult to rotate the superconducting condensate and which therefore sustain much higher applied magnetic fields. The interplay between the square symmetry of the loop and the cylindrical symmetry of the magnetic field results in different oscillatory superconducting phase boundaries which correspond to various phase boundary definitions. The most “realistic” criterion to define the phase boundary magnetic field( H )-temperature( T ) is formulated; it allows to obtain a good agreement between the calculated H ( T ) curve and the experimentally observed one.