Electronic Structure of Multiple Dots

We calculate, via spin density functional theory (SDFT) and exact diagonalization, the eigenstates for electrons in a variety of external potentials, including double and triple dots. The SDFT calculations employ realistic wafer profiles and gate geometries and also serve as the basis for the exact diagonalization calculations. The exchange interaction J between electrons is the difference between singlet and triplet ground state energies and reflects competition between tunneling and the exchange matrix element, both of which result from overlap in the barrier. For double dots, a characteristic transition from singlet ground state to triplet ground state (positive to negative J) is calculated. For the triple dot geometry with 2 electrons we also find the electronic structure with exact diagonalization. For larger electron number (18 and 20) we use only SDFT. In contrast to the double dot case, the triple dot case shows a quasi-periodic fluctuation of J with magnetic field which we attribute to periodic variations of the basis states in response to changing flux quanta threading the triple dot structure.Comment: 3 pages, 4 figure

Does anticipation help or hinder performance in a subsequent speech?

This study examined the effects of anticipatory processing on a subsequent speech in high and low socially anxious participants. Forty participants (n = 20 in each group) gave two speeches, one after no anticipatory processing and one after 10-minutes of anticipatory processing. In anticipatory processing, high socially anxious participants were more anxious, and experienced more negative and unhelpful self-images than low socially anxious participants did. However, both groups rated memories of past speeches as having a somewhat helpful effect on their speech preparation. High socially anxious participants tended to use the observer perspective more in the anticipated speech, while, in the unanticipated speech, they might have been switching between observer and field perspectives. Low socially anxious participants tended to use the field perspective in both speeches. High and low socially anxious participants reported better speech performances after the anticipated, compared to after the unanticipated speech. Results suggest that anticipatory processing may have both positive and negative effects on socially anxious individuals' cognitive processing and performance before and during a speech

Strategic cognition in paranoia: the use of thought control strategies in a non-clinical population

Background: Recent work in the area of cognition and emotion has focused on the process as well as the content of thought. Metacognitive approaches have included studies of people's relationship with internal experience (cf. Teasdale and Barnard, 1993), and the overarching beliefs that guide allocation of internal resources to manage distress (cf. Wells, 2000). At the same time, cognitive models of psychosis have emphasized the clinical value of a multidimensional understanding of paranoia (Chadwick, 2006; Freeman and Garety, 2004b). Method: This study examined paranoia in a non-clinical group, specifically (i) the relationship between a single measure of trait paranoia and dimensions of paranoid thought frequency, belief conviction and distress, and (ii) the metacognitive strategies that people use. It was predicted that trait paranoia would be associated with (i) dimensions of thought frequency, belief conviction and distress, and (ii) the internal strategies of “punishment” and “worry.” Results: Regression analyses showed that trait paranoia uniquely predicted frequency, conviction and distress associated with paranoid thoughts. Trait paranoia accounted for the use of “reappraisal”, whereas “punishment” and “worry” were accounted for by anxiety. Conclusions: The implications for clinical work and further research are discussed

Inhomogeneous Nuclear Spin Flips

We discuss a feedback mechanism between electronic states in a double quantum dot and the underlying nuclear spin bath. We analyze two pumping cycles for which this feedback provides a force for the Overhauser fields of the two dots to either equilibrate or diverge. Which of these effects is favored depends on the g-factor and Overhauser coupling constant A of the material. The strength of the effect increases with A/V_x, where V_x is the exchange matrix element, and also increases as the external magnetic field B_{ext} decreases.Comment: 5 pages, 4 figures (jpg

Semiconductor quantum dots for electron spin qubits

We report on our recent progress in applying semiconductor quantum dots for spin-based quantum computation, as proposed by Loss and DiVincenzo (1998 Phys. Rev. A 57 120). For the purpose of single-electron spin resonance, we study different types of single quantum dot devices that are designed for the generation of a local ac magnetic field in the vicinity of the dot. We observe photon-assisted tunnelling as well as pumping due to the ac voltage induced by the ac current driven through a wire in the vicinity of the dot, but no evidence for ESR so far. Analogue concepts for a double quantum dot and the hydrogen molecule are discussed in detail. Our experimental results in laterally coupled vertical double quantum dot device show that the Heitler–London model forms a good approximation of the two-electron wavefunction. The exchange coupling constant J is estimated. The relevance of this system for two-qubit gates, in particular the SWAP operation, is discussed. Density functional calculations reveal the importance of the gate electrode geometry in lateral quantum dots for the tunability of J in realistic two-qubit gates

The classical phase space for quantum dots

この論文は国立情報学研究所の電子図書館事業により電子化されました。We numerically integrate Hamilton's equations for the classical phase space trajectories of electrons confined in a closed quantum dot. The dot potential contour is obtained numerically via full, 3D spin density functional calculations of a realistic, GaAs-AlGaAs heterostructure based device. We show that the phase space of the dot is mixed and, as a function of increasing energy, becomes increasingly chaotic. We show that this is related to the well-screened, symmetric shape of the potential at low energies and the un-screened, irregular perimeter shape at the Fermi surface. We employ an iterative method to search for periodic orbits in the dot, employing the computed matrizant