Spectroscopy of few-electron collective excitations in charge-tunable artificial atoms

We report the investigation of electronic excitations in InGaAs self-assembled quantum dots using resonant inelastic light scattering. The dots can be charged via a gate by N=1, em leader,6 electrons. We observe excitations, which are identified as transitions of electrons, predominantly from the s to the p shell (s-p transitions) of the quasiatoms. We find that the s-p transition energy decreases and the observed band broadens, when the p shell is filled with 1 to 4 electrons. By a theoretical model, which takes into account the full Coulomb interaction in the few-electron artificial atom, we can confirm the experimental results to be an effect of the Coulomb interaction in the quantum dot.     

Inelastic light scattering by electrons and plasmons in metals

The cross section of the inelastic light scattering by electron-hole plasma in metals is studied. The Coulomb interaction of electron excitations is taken into account self-consistently. The system of the Boltzmann equation for electronic fluctuations and Maxwell’s equations for the interaction field is solved. The Raman spectra consist of the electron-hole background, diffuson and plasmon resonances. The widths of this background and resonance are determined by the electron collision rate as well as by the decay of the incident and scattered radiation in the metal. The line shape depends on the screening of the electron-light interaction, i.e., on the incident radiation frequency. Zh. éksp. Teor. Fiz. 112, 679–689 (August 1997) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Spin excitations in few-electrons AlGaAs/GaAs quantum dots probed by inelastic light scattering

We present recent studies of electronic excitations in nanofabricated AlGaAs/GaAs semiconductor quantum dots (QDs) by resonant inelastic light scattering. The resonant light scattering spectra are dominated by excitations from parity-allowed inter-shell transitions between Fock–Darwin levels. In QDs with very few electrons the resonant spectra are characterized by distinct charge and spin excitations that reveal the strong impact of both exchange and correlation effects. A sharp inter-shell spin excitation of the triplet spin QD state with four electrons is identified.     

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.     

Critical opalescence in the pure Coulomb system

Based on the dielectric formalism and quantum field theory methods, the phenomenon of critical opalescence is explained for light scattering in pure matter as a two-component electron-nuclear system with Coulomb interaction. A similar phenomenon is shown to occur in the case of neutron scattering in pure substances as well. The obtained results are valid for quantum case and arbitrary strong Coulomb interaction. Thus, the relations between structure factors derived for the electron-nuclear system are the exact result of the quantum statistical mechanics.     

Fano-type features in magnetoabsorption in semiconductors

It is shown that bulk semiconductors exhibit strongly asymmetric Fano-type resonance profiles in magnetoabsorption processes involving the formation of hot electron-hole pairs (EHPs) and accompanied by the scattering of the EHPs by defects. This result is valid for transitions to electronic states with large Landau quantum numbers, when the Coulomb interaction plays a small role. The physical reason for such a sharp change in the magnetoabsorption coefficient as compared with the expected result for the ordinary density of states in a quantizing magnetic field is that the electronic excitations are quasi-one-dimensional. The form of the resonance absorption is in good qualitative agreement with the experimental data. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 619–622 (25 April 1996)     

Quantum Monte Carlo studies of bound and unbound states

Over the last fifteen years, there has been tremendous progress in understanding how nuclear structure arises from the nucleon-nucleon interaction. I describe the contribution to this progress made by quantum Monte Carlo computational methods, as well as directions to be taken in future work. Most effort in the past has concentrated on energy spectra, and we have had good success in computing the spectra of systems with A≤12. We are now shifting our attention to off-diagonal and scattering/reaction properties of the light nuclei. I also discuss briefly the relation of quantum Monte Carlo methods to other ab initio methods and their particular relevance for weakly-bound nuclei produced at radioactive-beam facilities.     

Probing collective modes of correlated states of few electrons in semiconductor quantum dots

Low-lying collective excitations above highly correlated ground states of few interacting electrons confined in GaAs semiconductor quantum dots are probed by resonant inelastic light scattering. We highlight that separate studies of the changes in the spin and charge degrees of freedom offer unique access to the fundamental interactions. The case of quantum dots with four electrons is found to be determined by a competition between triplet and singlet ground states that is uncovered in the rich light scattering spectra of spin excitations. These light scattering results are described within a configuration-interaction framework that captures the role of electron correlation with quantitative accuracy. Recent light scattering results that reveal the impact of anisotropic confining potentials in laterally coupled quantum dots are also reviewed. In these studies, inelastic light scattering methods emerge as powerful probes of collective phenomena and spin configurations in quantum dots with few electrons.     

Crossover and coexistence of quasiparticle excitations in the fractional quantum hall regime at nu < or =1/3

New low-lying excitations are observed by inelastic light scattering at filling factors nu=p/(phip+/-1) of the fractional quantum Hall regime with phi=4. Coexisting with these modes throughout the range nu < or =1/3 are phi=2 excitations seen at 1/3. Both phi=2 and phi=4 excitations have distinct behaviors with temperature and filling factor. The abrupt first appearance of the new modes in the low-energy excitation spectrum at nu > or near 1/3 suggests a marked change in the quantum ground state on crossing the phi=2-->phi=4 boundary at nu=1/3.     

Formation of zero-dimensional hole states during molecular-beam epitaxy of Ge on Si (100)

The characteristic features of electronic spectra in Ge/Si (100) heterostructures obtained by molecular-beam epitaxy are investigated by capacitance spectroscopy. It is observed that the self-organization of a Ge film into an island film when the effective germanium thickness exceeds six monolayers is accompanied by the appearance of hole bound states, which can be attributed to size quantization and the Coulomb interaction of carriers in the array of Ge quantum dots. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 2, 125–130 (25 July 1998)     

Q-dependent light scattering by electrons in LaB6

The inelastic light scattering by intraband electronic excitations in metallic lanthanum hexaboride has been studied in the temperature range of 10–300 K. General agreement has been obtained between the measured spectra and the spectra calculated within the band theory taking into account the renormalization of electron energies owing to electron-phonon scattering. The electron-phonon coupling constant λ and electron relaxation frequency Γ have been estimated. The dependence of the electron self-energies on the direction and magnitude of the wave vector has been revealed, implying the anisotropic electron-phonon interaction or the contribution from other electron scattering mechanisms.     

Barrier D − complexes in a high-mobility two-dimensional electron system

The cyclotron excitation spectrum of selectively doped AlGaAs/GaAs quantum wells with a high (up to 2 × 10⁷ cm²/(V s)) mobility of electrons has been studied by means of the Raman scattering. The lines of the Raman scattering by the excitations of D ^? complexes, the objects in which two electrons localized in a quantum well are coupled to a charged impurity in a barrier, have been detected and identified. Spin-singlet D ^? complexes have been shown to exist in the entire range of the electron filling factor, from v → 0 to v = 2, owing to the specificity of the Coulomb interaction in two-dimensional systems. The excitation energies of the singlet D ^? complexes have been studied as functions of the electron density, quantum well width, and magnetic field.     

Evidence of correlation in spin excitations of few-electron quantum dots

We report inelastic light scattering measurements of spin and charge excitations in nanofabricated AlGaAs/GaAs quantum dots with few electrons. A narrow spin excitation peak is observed and assigned to the intershell triplet-to-singlet monopole mode of dots with four electrons. Configuration-interaction theory provides precise quantitative interpretations that uncover large correlation effects that are comparable to exchange Coulomb interactions.     

Brillouin scattering studies of rare gas solids

Abstract

The scattering of light by elementar excitations in the matter is results in two phenomena, discriminated by the zero wavevector frequency of the excitation: if this frequency is zero, one deals with Brillouin scattering, and with Raman scattering in the other case. Brillouin scattering results from the interaction of light with thermal excitations (acoustic phonons in a crystal) of a material, or, from a classical point of view, with density waves. Contrary to Raman scattering, the selection rules allow always the observation of at least one mode. It is a powerful technic in the study of rare gases under pressure: at ambient temperature, rare gases crystallize in the face centered cubic structure (except helium which structure was recently found to be hexagonal) and are therefore Raman and infrared inactive.

Experimental results will be reviewed on rare gases and rare gas mixtures in the fluid phase, like He-Ne and He-H₂. These results will be discussed in relation with recent measurements of the frequency of global oscillations of Jupiter.     

Raman Scattering of Light by Quantum Rings in a Magnetic Field

The effect of the Coulomb interaction in the intermediate state on the inelastic resonant process of light scattering by electrons in quantum rings in a magnetic field normal to the ring plane is investigated theoretically. By way of examples, one- and two-electron quantum rings are considered.     

Kinematical and Dynamical Inelastic Light Scattering in Conducting Crystals

Fundamentals of the kinematical quantum theory of inelastic light scattering from electronic excitations in conducting crystals are reviewed. Keeping both the →Δ and →p·→A terms in the photon-electron interaction the general expression for the differential scattering cross-section is obtained. Emphasizing opacity effects and anisotropies arising from the Doppler shift the scattering kinematics is analyzed. The scattering from free-carrier density fluctuations is considered briefly. The basic concepts of a recently established phenomenological theory of dynamical light diffraction in opaque crystals are discussed, and some aspects of a predicted phonon induced anomalous transmission of light below the plasma edge in semiconductors are studied.     

Theory of Raman light scattering by nanocrystal acoustic vibrations

A theory of Raman scattering of light by acoustic phonons in spherical nanocrystals of zinc-blende and wurtzite semiconductors has been developed with the inclusion of the complex structure of the valence band. The deformation-potential approximation was used to describe the exciton-phonon interaction. It is shown that this approximation allows only Raman scattering processes involving spheroidal acoustic phonons with a total angular momentum F=0 or 2. The effect of phonon quantum confinement on linewidth in Raman scattering spectra and scattered polarization is analyzed. An expression for the shape of the spectral line corresponding to nonresonant scattering from F=0 phonons was obtained. Fiz. Tverd. Tela (St. Petersburg) 41, 1473–1483 (August 1999)     

Excitations in the quantum condensated fermion gas

The excitations (both collective and quasi-pair states) connected with quantum condensation of a fermion gas are examined. These gapless excitations do not only turn up for finite-range interaction but also for Coulomb interaction, i.e., as electronic excitations in a superconductor.     

Light scattering observations of spin reversal excitations in the fractional quantum Hall regime

Resonant inelastic light scattering experiments access the low lying excitations of electron liquids in the fractional quantum Hall regime in the range 2/5≥ν≥1/3. Modes associated with changes in the charge and spin degrees of freedom are measured. Spectra of spin reversed excitations at filling factor ν?1/3 and at ν?2/5 identify a structure of lowest spin-split Landau levels of composite fermions (CFs) that is similar to that of electrons. Observations of spin wave excitations enable determinations of energies required to reverse spin. The spin reversal energies obtained from the spectra illustrate the significant residual interactions of composite fermions. At ν=1/3 energies of spin reversal modes are larger but relatively close to spin conserving excitations that are linked to activated transport. Predictions of composite fermion theory are in good quantitative agreement with experimental results.     

Angular dependence of Raman scattering selection rules for long-wavelength optical phonons in short-period GaAs/AlAs superlattices

The angular dependence of Raman scattering selection rules for optical phonons in short-period (001) GaAs/AlAs superlattices is calculated and experimentally studied. Experiments are performed using a micro-Raman setup, in the scattering geometry with the wavevectors of the incident and scattered light lying in the plane of superlattices (so-called in-plane geometry). Phonon frequencies are calculated using the Born model taking the Coulomb interaction into account in the rigid-ion approximation. Raman scattering spectra are calculated in the framework of the deformation potential and electro-optical mechanisms. Calculations show an angular dependence of the selection rules for optical phonons with different directions of the wavevectors. Drastic differences in the selection rules are found for experimental and calculated spectra. Presumably, these differences are due to the Fröhlich mechanism in Raman scattering for short-period superlattices.