Volume 101, Issue 8, 15 April 2007
- special topic: plenary and invited papers from the 28th international conference on the physics of semiconductors, vienna, austria, 2006
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
Index of content:
- SPECIAL TOPIC: PLENARY AND INVITED PAPERS FROM THE 28TH INTERNATIONAL CONFERENCE ON THE PHYSICS OF SEMICONDUCTORS, VIENNA, AUSTRIA, 2006
Preface to Special Topic: Plenary and Invited Papers from the 28th International Conference on the Physics of Semiconductors, Vienna, Austria, 2006101(2007); http://dx.doi.org/10.1063/1.2732073View Description Hide Description
101(2007); http://dx.doi.org/10.1063/1.2722722View Description Hide Description
Electron-electron interactions have strong effects on the low-energy excitations of a one-dimensional metal. Luttinger liquidtheory, which is supposed to describe this situation, predicts, among other things, that an injected electron will split into separate charge and spin excitations, which propagate at different velocities. We shall review some experiments and theoretical analyses where spin-charge separation can have manifest consequences, including discussion of the spin-incoherent regime, which can occur in low-density electron systems, when the temperature is low compared to the energy scale for charge excitations, but larger than the spin-exchange energy for neighboring electrons.
101(2007); http://dx.doi.org/10.1063/1.2722724View Description Hide Description
We present a method that allows for deterministic coupling of charge-tunable quantum dots to high-photonic crystal nanocavity modes. The realization of cavity-mediated coherent coupling of two distant spins is hindered by large fluctuations in quantum dot optical (trion) transition energy and interdot separation. We show that flexible cavity design and gate-voltage-tunable trion transitions in quantum dot molecules can be used to overcome these limitations and to achieve conditional quantum dynamics of two confined spins.
101(2007); http://dx.doi.org/10.1063/1.2722725View Description Hide Description
For low biases the linear conductance of quantum dots is based on elastic transport processes. At finite bias in the Coulomb blockade regime, inelastic cotunneling sets in once the applied bias exceeds the energy between ground and excited state in the dot. Here we report on transport experiments through an Aharonov-Bohm ring containing a quantum dot in each arm of the ring. The tunnel coupling between the two dots can be tuned by electrostatic gates. For strong tunnel coupling and low bias we observe pronounced Aharonov-Bohm oscillations in the ring with visibilities exceeding 80%. For quantum dots which are purely capacitively coupled, the Aharonov-Bohm amplitude is reduced to a more standard 10%. For finite bias, where transport through excited states becomes possible and a conductance onset is observed, the visibility of the Aharonov-Bohm oscillations remains basically unchanged, while the phase typically undergoes a change of . We discuss these observations in view of the possible elastic and inelastic transport processes and their contributions to coherent transport.
Self-assembly of periodic nanoclusters of Si and Ge along atomically straight steps of a vicinal Si(111)a)101(2007); http://dx.doi.org/10.1063/1.2722726View Description Hide Description
The very initial stage of the molecular beam epitaxy of Si and Ge on substrates with atomically straight steps has been studied by scanning tunneling microscopy and spectroscopy. The atomically straight steps have been prepared on a miscut Si(111) substrate by annealing at with kink-up direct current. The length of the steps can be maximized by selecting a proper annealing time. The steps have a well-defined step-edge structure. The growth of both Si and Ge at temperatures between 250 and starts with formation of a single-adatom-row nanowire (0.67 nm in width) along the lower edge of each step. Subsequent growth of Si and Ge at temperatures between 250 and results in formation of one-dimensional arrays of nanoclusters (less than 2.0 nm in width) in the unfaulted halves of the structure along the upper step edges. Scanning tunneling spectroscopy reveals localized electronic states of the nanoclusters. Differences between the growth of Si and Genanoclusters are discussed.
Excited excitonic states observed in semiconductor quantum dots using polarization resolved optical spectroscopya)101(2007); http://dx.doi.org/10.1063/1.2722729View Description Hide Description
We present results on the polarization-resolved photoluminescence emitted from InGaAs/AlGaAs single quantum dots(QDs)grown in inverted tetrahedral pyramids. The emitted light was detected for two mutually perpendicular linear polarization directions in the less conventional cleaved-edge geometry, in addition to the standard top-emission geometry. Whereas the in-plane linear polarization was isotropic, as a consequence of the high symmetry of the system, we found a strong polarizationanisotropy of the edge-emitted light revealing QD states of predominantly heavy- or light-hole character. By temperature control of the charge state, several neutral and charged light-hole like exciton complexes were identified. In particular, a biexciton showing a twofold radiative recombination path, leading to two nearly perpendicularly polarized emission multiplets, was identified. These results are also of technological relevance for any design of optoelectronic QD-integrated devices.
101(2007); http://dx.doi.org/10.1063/1.2722731View Description Hide Description
We report a systematic and comprehensive computational study of the electronic structure of GaN and InN surfaces in various orientations, including the polar plane, as well as the nonpolar and planes. Surfaceband structures and density-of-states plots show the energetic position of surface states, and by correlating the electronic structure with atomistic information we are able to identify the microscopic origins of each of these states. Fermi-level pinning positions are identified, depending on surface stoichiometry and surface polarity. For polar InN we find that all the surface states are located above the conduction-band minimum, and explain the source of the intrinsic electron accumulation that has been universally observed on InN surfaces.
Observation of subband standing waves in superlattices by low-temperature scanning tunneling spectroscopya)101(2007); http://dx.doi.org/10.1063/1.2722732View Description Hide Description
The local density of states (LDOS) of InAs/GaSb long period superlattices was investigated on the cleaved (110) surface at atomic resolution by low-temperature scanning tunneling microscopy and low-temperature scanning tunneling spectroscopy (LT-STS). We present definitive features showing the broken-gap energy band profiles (the overlap of the conduction band of the InAs and the valence band of the GaSb in energy) in the LT-STS spectra complemented by distinct, atomically resolved topographic images. Furthermore, vivid standing waves in LDOS corresponding to the single quantum well-like subband confined in the InAs layer were observed.
101(2007); http://dx.doi.org/10.1063/1.2722734View Description Hide Description
Spin-dependent transport measurements through a double quantum dot are a valuable tool for detecting both the coherent evolution of the spin state of a single electron, as well as the hybridization of two-electron spin states. In this article, we discuss a model that describes the transport cycle in this regime, including the effects of an oscillating magnetic field (causing electron spin resonance) and the effective nuclear fields on the spin states in the two dots. We numerically calculate the current flow due to the induced spin flips via electron spin resonance, and we study the detector efficiency for a range of parameters. The experimental data are compared with the model and we find a reasonable agreement.
Atomic scale study of the impact of the strain and composition of the capping layer on the formation of InAs quantum dotsa)101(2007); http://dx.doi.org/10.1063/1.2722738View Description Hide Description
The impact of the capping material on the structural properties of self-assembledInAsquantum dots(QDs) was studied at the atomic scale by cross-sectional scanning tunneling microscopy. Capping with lattice matched layers and with strained layers was analyzed. When the different capping materials are lattice matched to the substrate, the differences in the QDproperties can be dominated by chemical effects: InAs/InP QDs capped with InP have a 2 ML smaller height than those capped with InGaAs or InGaAsP due to As/P exchange induced decomposition. The height of the dots is found to be much more strongly affected when strained capping layers are used. InAs/GaAs, QDs capped with InGaAs are considerably taller than typical GaAs-capped dots. When GaAsSb is used as the capping layer, the dots are almost full pyramids with a height of 9.5 nm, indicating that dot decomposition is almost completely suppressed. This indicates that the dot/capping layer strain plays a major role in inducing dot decomposition during capping.
101(2007); http://dx.doi.org/10.1063/1.2722742View Description Hide Description
On the ultrastrong vacuum Rabi coupling of an intersubband transition in a semiconductor microcavitya)101(2007); http://dx.doi.org/10.1063/1.2722757View Description Hide Description
In this invited paper of the 28th International Conference of the Physics of Semiconductors (ICPS-28), we discuss the peculiar quantum electrodynamical properties of a semiconductormicrocavity system, in which a cavity photon mode is strongly coupled to an intersubband transition of a dopedquantum well system. In this kind of semiconductor system, it is possible to achieve an unprecedented ultrastrong coupling regime, in which the vacuum Rabi frequency is comparable to the electronic transition frequency. We discuss the anomalous quantum properties of the quantum ground state (a squeezed vacuum) and of the intersubband cavity polaritonexcitations. We address the role of dissipation and point out some future perspectives concerning the investigation of quantum vacuum radiation effects induced by an ultrafast time modulation of the quantum vacuum.
101(2007); http://dx.doi.org/10.1063/1.2722764View Description Hide Description
The spin-orbit interaction offers an avenue for the electrical generation and manipulation of electron spin polarization in semiconductors without magnetic materials or magnetic fields. In semiconductor heterostructures, the spin-orbit coupling modifies the electron factor and introduces momentum-dependent spin splittings. In addition, spin-orbit coupling enables the electrical generation of spin polarization through these spin splittings and the spin Hall effect. Here we present an overview of recent measurements of spin dynamics, spin splittings, and electrically generated spin polarization. We demonstrate manipulation of the spin-orbit coupling using electric and magnetic fields to change the orbital motion of the electrons and using strain and quantum confinement to tune the spin splittings in semiconductor heterostructures.
101(2007); http://dx.doi.org/10.1063/1.2722765View Description Hide Description
Single InAs quantum dots embedded in a planar cavity, formed by mismatched sets of GaAs/AlAs distributed Bragg reflectors, can be a useful source of triggered polarization-entangled photon pairs. We demonstrate this with a fidelity exceeding 70% for the expected entangled state. Quantum dot based devices may therefore be of great use in quantum communications and quantum information processing.
101(2007); http://dx.doi.org/10.1063/1.2722769View Description Hide Description
Entangled photon pairs are emitted from a biexciton decay cascade of single quantum dots when spectral filtering is applied. We show this by experimentally measuring the density matrix of the polarization state of the photon pair emitted from a continuously pumped quantum dot. The matrix clearly satisfies the Peres criterion for entanglement. By applying in addition a temporal window, the quantum dot becomes an entangled light source.
101(2007); http://dx.doi.org/10.1063/1.2722774View Description Hide Description
The optical properties of individual quantum dotsdoped with a single Mn atom and charged with a single carrier are analyzed. The emission of the neutral, negatively and positively charged excitons coupled with a single magnetic atom (Mn) are observed in the same individual quantum dot. The spectrum of the charged excitons in interaction with the Mn atom shows a rich pattern attributed to a strong anisotropy of the hole-Mn exchange interaction slightly perturbed by a small valence-band mixing. The anisotropy in the exchange interaction between a single magnetic atom and a single hole is revealed by comparing the emission of a charged Mn-doped quantum dot in longitudinal and transverse magnetic field.
Imaging correlated wave functions of few-electron quantum dots: Theory and scanning tunneling spectroscopy experimentsa)101(2007); http://dx.doi.org/10.1063/1.2722782View Description Hide Description
We show both theoretically and experimentally that scanning tunnelingspectroscopy (STS) images of semiconductor quantum dots may display clear signatures of electron-electron correlation. We apply many-body tunnelingtheory to a realistic model, which fully takes into account correlation effects and dot anisotropy. Comparing measured STS images of freestanding InAs quantum dots with those calculated by the full configuration interaction method, we explain the wave-function sequence in terms of images of one- and two-electron states. The STS map corresponding to double charging is significantly distorted by electron correlation with respect to the noninteracting case.
101(2007); http://dx.doi.org/10.1063/1.2722783View Description Hide Description
In this article we analyze spin dynamics for electrons confined to semiconductor quantum dots due to the contact hyperfine interaction. We compare mean-field (classical) evolution of an electron spin in the presence of a nuclear field with the exact quantum evolution for the special case of uniform hyperfine coupling constants. We find that (in this special case) the zero-magnetic-field dynamics due to the mean-field approximation and quantum evolution are similar. However, in a finite magnetic field, the quantum and classical solutions agree only up to a certain time scale , after which they differ markedly.
101(2007); http://dx.doi.org/10.1063/1.2722785View Description Hide Description
We discuss methods for imaging the nonequilibrium spin polarization of electrons in Fe/GaAs spin transport devices. Both optically and electrically injected spin distributions are studied by scanning magneto-opticalKerr rotation microscopy. Related methods are used to demonstrate electrical spin detection of optically injected spin-polarized currents. Dynamical properties of spin transport are inferred from studies based on the Hanle effect, and the influence of strain on spin transport data in these devices is discussed.
101(2007); http://dx.doi.org/10.1063/1.2722786View Description Hide Description
We present a detailed experimental study of the effects of the optically induced transition from the excitonic, insulating regime, to the plasma, metallic regime, on the spectra and on the photoluminescencedynamics of GaAs. The transition is rather abrupt and presents a Mott-like behavior. The critical temperature, of 49 K, corresponds to the exciton binding energy. Through the study of the characteristics of the photoluminescencedynamics, the critical density for the transition has been obtained with unprecedented resolution.