Electronic structure of quantum dots

Spherical solid model for muon and hydrogen in metals

The spherical solid model and the spin density functional formalism have been applied to calculate the screening of a positive point charge at different lattice sites in Al, Na and Cu. Results are obtained for the Knight shift, the electric field gradient, the heat of solution and the diffusion barrier. It is found essential to use the spin-polarised form to evaluate the Knight shift, especially at low metallic densities and for impurities with a high nuclear charge. Both the Knight shift and the electric field gradient are found to be markedly different for substitutional and interstitial positions. The calculated heat of solution of hydrogen is lowest for the octahedral position in FCC Al…

Bright beaches of nanoscale potassium islands on graphite in STM imaging,

We demonstrate, via scanning tunneling microscopy (STM) measurements performed at 48 K, the existence of “bright beaches” at the edges of K islands (diameter ∼ 5 – 500     nm ) on the graphite surface. The enhanced tunneling current is only observed in monolayer-high islands on graphite, and not in islands of similar geometry on top of a K monolayer film. First-principles density functional calculations and STM simulations suggest that this is an STM field effect, which appears as the positive tip attracts donated electrons back to the metallic K islands. The restored charge accumulates preferentially at the island edges. peerReviewed

Magnetic properties of quantum dots and rings

Exact many-body methods as well as current-spin-density functional theory are used to study the magnetism and electron localization in two-dimensional quantum dots and quasi-one-dimensional quantum rings. Predictions of broken-symmetry solutions within the density functional model are confirmed by exact configuration interaction (CI) calculations: In a quantum ring the electrons localize to form an antiferromagnetic chain which can be described with a simple model Hamiltonian. In a quantum dot the magnetic field localizes the electrons as predicted with the density functional approach.

The Kadanoff-Baym approach to double excitations in finite systems

We benchmark many-body perturbation theory by studying neutral, as well as non-neutral, excitations of finite lattice systems. The neutral excitation spectra are obtained by time-propagating the Kadanoff–Baym equations in the Hartree–Fock and the second Born approximations. Our method is equivalent to solving the Bethe–Salpeter equation with a high-level kernel while respecting self-consistency, which guarantees the fulfillment of a frequency sum rule. As a result, we find that a time-local method, such as Hartree–Fock, can give incomplete spectra, while already the second Born approximation, which is the simplest time-non-local approximation, reproduces well most of the additional excitati…

Muon states in metals: Recent progress

We report on our results in two interesting questions related to muon spin rotation studies in condensed matter: (i) energetics of muons in metals, including lattice relaxation and zero point motion in self-trapping phenomena, and (ii) systematics of Knight shifts and hyperfine fields.

Vortex localization in rotating clouds of bosons and fermions

Finite quantal systems at high angular momenta may exhibit vortex formation and localization. These phenomena occur independent of the statistics of the repulsively interacting particles, which may be of bosonic or fermionic nature. We analyze the relation between vortex localization and formation of stable Wigner molecules at high angular momenta in the view of particle-hole duality.Trial wave functions for the vortex states and the corresponding fermion-boson relations are discussed.

Positron detrapping from defects: A thermodynamic approach

The rate of positron detrapping in thermal equilibrium from lattice defects has been calculated by relating it to the specific trapping rate. The results for vacancies, dislocations and surfaces each show a different temperature dependence for the escape rate. For vacancies a measure of the importance of the detrapping can be obtained from the ratio of the vacancy formation energy to the positron binding energy in the defect. The positronium desorption rate from a surface is also calculated and agreement with experimental results is found.