M. Koskinen

Electron-gas clusters: the ultimate jellium model

The local spin-density approximation is used to calculate ground- and isomeric-state geometries of jellium clusters with 2 to 22 electrons. The positive background charge of the model is completely deformable, both in shape and in density. The model has no input parameters. The resulting shapes of the clusters exhibit breaking of axial and inversion symmetries; in general the shapes are far from ellipsoidal. Those clusters which lack inversion symmetry are extremely soft against odd-multipole deformations. Some clusters can be interpreted as molecules built from magic clusters. The deformation produces a gap at the Fermi level. This results in a regular odd-even staggering of the total energy per electron and of the HOMO level. The strongly deformed 14-electron cluster is semimagic. Stable isomers are predicted. The splitting of the plasmon resonance due to deformation is estimated on a classical argument.

Formation of Wigner molecules in small quantum dots

It was recently argued that in small quantum dots the electrons could crystallize at much higher densities than in the infinite two-dimensional electron gas. We compare predictions that the onset of spin polarization and the formation of Wigner molecules occurs at a density parameter

Quantum Dots in Magnetic Fields: Phase Diagram and Broken Symmetry at the Maximum-Density-Droplet Edge

r_s\approx 4 a_B^*

Rotational and vibrational spectra of quantum rings

to the results of a straight-forward diagonalization of the Hamiltonian matrix.

Many-body spectrum and particle localization in quantum dots and finite rotating bose condensates

Quantum dots in magnetic fields are studied within the current spin-density-functional formalism avoiding any spatial symmetry restrictions of the solutions. We find that the maximum-density droplet reconstructs into states with broken internal symmetry: The Chamon-Wen edge coexists with a modulation of the charge density along the edge. The phase boundaries between the polarization transition, the maximum-density droplet, and its reconstruction are in agreement with recent experimental results.

Current-spin-density-functional study of persistent currents in quantum rings

Many-body spectra of quantum rings confining a few electrons are determined by exact diagonalization techniques. Surprisingly, a model of localized electrons accurately describes such systems at rather high electron densities. Group-theoretical methods provide the necessary tools to uncover rotational and vibrational structures in the spectra.

Unrestricted Shapes of Jellium Clusters

The yrast spectra (i.e. the lowest states for a given total angular momentum) of quantum dots in strong magnetic fields, are studied in terms of exact numerical diagonalization and analytic trial wave functions. We argue that certain features (cusps) in the many-body spectrum can be understood in terms of particle localization due to the strong field. A new class of trial wavefunctions supports the picture of the electrons being localized in Wigner molecule-like states consisting of consecutive rings of electrons, with low-lying excitations corresponding to rigid rotation of the outer ring of electrons. The geometry of the Wigner molecule is independent of interparticle interactions and the statistics of the particles.

Universal vortex formation in rotating traps with bosons and fermions

We present a numerical study of persistent currents in quantum rings using current spin density functional theory (CSDFT). This formalism allows for a systematic study of the joint effects of both spin, interactions and impurities for realistic systems. It is illustrated that CSDFT is suitable for describing the physical effects related to Aharonov-Bohm phases by comparing energy spectra of impurity-free rings to existing exact diagonalization and experimental results. Further, we examine the effects of a symmetry-breaking impurity potential on the density and current characteristics of the system and propose that narrowing the confining potential at fixed impurity potential will suppress the persistent current in a characteristic way. PACS numbers: 73.20.Dx, 73.23.Ra, 71.15.Mb

Electron-Hole Duality and Vortex Rings in Quantum Dots

A jellium model with a completely relaxable background charge density is used to study metal clusters containing 2 to 22 electrons. The resulting shapes of the clusters exhibit breaking of axial and inversion symmetries, as well as molecular formation. The clusters without inversion symmetry are soft against deformation. The strongly deformed 14-electron cluster is found to be semi-magic. Stable-shape isomers are predicted.

Vortex localization in rotating clouds of bosons and fermions

We show that the rotation of trapped quantum mechanical particles with a repulsive interaction can lead to vortex formation, irrespective of whether the particles are bosons or (unpaired) fermions. The exact many-particle wave function constitutes similar structures in both cases, implying that this vortex formation is indeed universal.