Single-particle excitations in quasi-zero- and quasi-one-dimensional electron systems

High mobility one- and two-dimensional electron systems in nanowire-based quantum heterostructures

Free-standing semiconductor nanowires in combination with advanced gate-architectures hold an exceptional promise as miniaturized building blocks in future integrated circuits. However, semiconductor nanowires are often corrupted by an increased number of close-by surface states, which are detrimental with respect to their optical and electronic properties. This conceptual challenge hampers their potentials in high-speed electronics and therefore new concepts are needed in order to enhance carrier mobilities. We have introduced a novel type of core-shell nanowire heterostructures that incorporate modulation or remote doping and hence may lead to high-mobility electrons. We demonstrate the validity of such concepts using inelastic light scattering to study single modulation-doped GaAs/Al0.16Ga0.84As core-multishell nanowires grown on silicon. We conclude from a detailed experimental study and theoretical analysis of the observed spin and charge density fluctuations that one- and two-dimensional electron channels are formed in a GaAs coaxial quantum well spatially separated from the donor ions. A total carrier density of about 3 × 10(7) cm(-1) and an electron mobility in the order of 50,000 cm(2)/(V s) are estimated. Spatial mappings of individual GaAs/Al0.16Ga0.84As core-multishell nanowires show inhomogeneous properties along the wires probably related to structural defects. The first demonstration of such unambiguous 1D- and 2D-electron channels and the respective charge carrier properties in these advanced nanowire-based quantum heterostructures is the basis for various novel nanoelectronic and photonic devices.

Correlated electrons in optically tunable quantum dots: building an electron dimer molecule

We observe the low-lying excitations of a molecular dimer formed by two electrons in a GaAs semiconductor quantum dot in which the number of confined electrons is tuned by optical illumination. By employing inelastic light scattering we identify the intershell excitations in the one-electron regime and the distinct spin and charge modes in the interacting few-body configuration. In the case of two electrons, a comparison with configuration-interaction calculations allows us to link the observed excitations with the breathing mode of the molecular dimer and to determine the singlet-triplet energy splitting.

Resonant Raman transitions into singlet and triplet states in InGaAs quantum dots containing two electrons

Semiconductor quantum dots containing two electrons, also called artificial quantum-dot helium atoms, are model structures to investigate the most fundamental many-particle states induced by Coulomb interaction and the Pauli exclusion principle. Here, electronic excitations in quantum-dot helium are investigated by resonant Raman spectroscopy in magnetic fields. We observe transitions from the ground state into the excited singlet state and, in the depolarized Raman configuration which allows spin-flip processes, into the triplet state.

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.

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.

Where is the luttinger liquid in one-dimensional semiconductor quantum wire structures?

We present the theoretical basis for analyzing resonant Raman scattering experiments in one-dimensional systems described by the Luttinger-liquid fixed point. We make experimentally testable predictions for distinguishing Luttinger liquids from the Fermi liquid and argue that presently available quantum wire systems are not in the regime where Luttinger-liquid effects are important.

Dynamic conductance of carbon nanotubes

The dynamic conductance of carbon nanotubes was investigated using the nonequilibrium Green's function formalism within the context of a tight-binding model. Specifically, we have studied the ac response of tubes of different helicities, both with and without defects, and an electronic heterojunction. Because of the induced displacement currents, the dynamic conductance of the nanotubes differs significantly from the dc conductance displaying both capacitive and inductive responses. The important role of photon-assisted transport through nanotubes is revealed and its implications for experiments discussed.

LOCALIZED OPTICAL PHENOMENA AND THE CHARACTERIZATION OF MATERIALS INTERFACES

▪ Abstract  This review focuses on the characterization of interfaces, specifically on the optical methods of characterization that utilize some form of spatial localization to circumvent the special problems accruing to interfaces. Experiments utilizing radiation in only the infrared through the ultraviolet regions of the electromagnetic spectrum are considered. We specifically exclude the vast and important array of experimental techniques that utilize the vacuum ultraviolet and X-ray spectral regions. In addition, the interfaces considered are those composed of solids with liquids, thin films, and other solids, thus largely ignoring the literature of ultrahigh vacuum surface science.