J. H. Jefferson

Dynamics of Paramagnetic Metallofullerenes in Carbon Nanotube Peapods

We filled SWNTs with the paramagnetic fullerene Sc@C82 to form peapods. The interfullerene 1D packing distance measured using TEM is d = 1.1 +/- 0.02 nm. The Sc@C82 in SWNT peapods continuously rotated during the 2 s TEM exposure time, and we did not see the Sc atoms. However, Sc@C82 metallofullerenes in MWNT peapods have periods of fixed orientation, indicated by the brief observation of Sc atoms. La@C82 peapods were also prepared and their rotational behavior examined. The interfullerene 1D packing of both La@C82 and Sc@C82 peapods is identical and thus independent of the charge transfer state for these paramagnetic fullerenes. The La@C82 metallofullerenes in the peapods have fixed orientations for extended periods of time, up to 50 s in some cases. The La@C82 spontaneously rotates rapidly between fixed orientations.

INTERACTING ELECTRONS IN POLYGONAL QUANTUM DOTS

The low-lying eigenstates of a system of two electrons confined within a two-dimensional quantum dot with a hard polygonal boundary are obtained by means of exact diagonalization. The transition from a weakly correlated charge distribution for small dots to a strongly correlated ‘‘Wigner molecule’’ for large dots is studied, and the behavior at the crossover is determined. In sufficiently large dots, a recently proposed mapping to an effective charge-spin model is investigated, and is found to produce the correct ordering of the energy levels and to give a good first approximation to the size of the level spacings. We conclude that this approach is a valuable method to obtain the low-energy spectrum of few-electron quantum dots.

Spin-dependent thermoelectric transport coefficients in near perfect quantum wires

Thermoelectric transport coefficients are determined for semiconductor quantum wires with weak thickness fluctuations. Such systems exhibit anomalies in conductance near 1/4 and 3/4 of 2e 2 /h on the rising edge to the first conductance plateau, explained hy singlet and triplet resonances of conducting electrons with a single weakly bound electron in the wire [T. Rejec, A. Ramsak. and J.H. Jefferson, Phys. Rev. B 62, 12 985 (2000)]. We extend this work to study the Seebeck thermopower coefficient and linear thermal conductance within the framework of the Landauer-Buttiker formalism, which also exhibit anomalous structures. These features are generic and robust, surviving to temperatures of a few degrees. It is shown quantitatively how at elevated temperatures thermal conductance progressively deviates from the Wiedemann-Franz law.

Bandgap modulation of narrow-gap carbon nanotubes in a transverse electric field

We propose a method to modulate the bandgaps in narrow-gap carbon nanotubes using a transverse electric field. Unlike previous investigations, we include curvature effects of the nanotubes by incorporating both π- and σ-orbitals in our tight-binding calculations. The calculations show that the narrow curvature-induced bandgaps decrease quadratically with electric field amplitude to zero. As the electric field amplitude continues to increase, the bandgap then expands in a similar manner to that presented in earlier studies on metallic nanotubes. The bandgap dependence is verified by analytical calculations, which also agree with preceding analyses for the limit of no curvature.

Influence of composition on the piezoelectric effect and on the conduction band energy levels of InxGa1-xAs/GaAs quantum dots

We address fundamental issues relating to the symmetry of the shape and the nonuniform composition of InGaAs quantum dot islands. Using atomistic simulations in the framework of the Tersoff empirical potential, we study the effect of compositional gradients in the In distribution on the piezoelectric effect in quantum dots. We demonstrate that the internal piezoelectric fields contribute strongly to the experimentally observed optical anisotropies. This is confirmed by accurate high-resolution transmission electron microscopy analysis over hundreds of islands grown in different conditions that reveals the absence of structural anisotropy under our growth conditions.

Zero-field spin splitting and spin lifetime in n-InSb/In1−xAlxSb asymmetric quantum well heterostructures

The spin-orbit (SO) coupling parameters for the lowest conduction subband due to structural inversion asymmetry (SIA) and bulk inversion asymmetry (BIA) are calculated for a range of carrier densities in [001]-grown δ-doped n-type InSb∕In1−xAlxSb quantum wells using the established eight-band k⋅p formalism [ J. Deng et al. Phys. Rev. B 59 R5312 (1999)]. We present calculations for conditions of zero bias at 10 K. It is shown that both the SIA and BIA parameters scale approximately linearly with carrier density, and exhibit a marked dependence on well width when alloy composition is adjusted to allow maximum upper barrier height for a given well width. In contrast to other material systems, the BIA contribution to spin splitting is found to be of significant and comparable value to the SIA mechanism in these structures. We calculate the spin lifetime τs[11 0] for spins oriented along [11 0] based on D’yakonov–Perel’ mechanism using both the theory of Averkiev et al. J. Phys.: Condens. Matter 14 R271 (2002) and also directly the rate of precession of spins about the effective magnetic field, taking into account all three SO couplings, which show good agreement. τs[11 0] is largest in the narrowest wells over the range of moderate carrier densities considered, which is attributed to the reduced magnitude of the k-cubic BIA parameter in narrow wells. The inherently large BIA induced SO coupling in these systems is shown to have considerable effect on τs[11 0], which exhibits significant reduction in the maximum spin lifetime compared to previous studies that consider systems with relatively weak BIA induced SO coupling. The relaxation rate of spins oriented in the [001] direction is found to be dominated by the k-linear SIA and BIA coupling parameters and at least an order of magnitude greater than in the [11 0] direction.

Entanglement between static and flying qubits in quantum wires

A weakly bound electron in a semiconductor quantum wire is shown to become entangled with an itinerant electron via the Coulomb interaction. The degree of entanglement and its variation with energy of the injected electron, may be tuned by choice of spin and initial momentum. Full entanglement is achieved close to energies where there are spin-dependent resonances. Possible realisations of related device structures are discussed.

Effects of Doping on Electronic Structure and Correlations in Carbon Peapods

Sc@C(82) peapods, which form encapsulated spin-1/2 antiferromagnetic chains under doping with electrons or holes are investigated using hybrid density functional theory. The narrow fullerene bands become shifted relative to nanotube bands resulting in charge transfer and conducting channels along both the fullerene chain and the nanotube. This is accompanied by a reduction in the magnetic moments on the fullerenes, consistent with a 1D Hubbard-Anderson model description.

Atomistic simulation of InxGa1-xAs/GaAs quantum dots with nonuniform composition

An atomistic model of an InxGa1-xAs/GaAs quantum dot with nonuniform composition is investigated. An empirical inter-atomic potential, the Tersoff potential, is used to obtain dynamic relaxation through energy minimisation. Bond deformations are analysed in order to predict the components of the strain tensor with resolution on the atomic scale, revealing the nature of the lattice distortion in both the dot pyramid and the capping layer. The piezoelectric charges are then computed directly from the off diagonal components of the strain, revealing a wider distribution of the dipoles compared to those previously reported by other groups

Entanglement of two delocalized electrons

Several convenient formulas for the entanglement of two indistinguishable delocalized spin-1/2 particles are introduced. These generalize the standard formula for concurrence, valid only in the limit of localized or distinguishable particles. Several illustrative examples are given.