Quantum states in a magnetic anti-dot

We study a new system in which electrons in two dimensions are confined by a non homogeneous magnetic field. The system consists of a heterostructure with on top of it a superconducting disk. We show that in this system electrons can be confined into a dot region. This magnetic anti-dot has the interesting property that the filling of the dot is a discrete function of the magnetic field. The circulating electron current inside and outside the anti-dot can be in opposite direction for certain bound states. And those states exhibit a diamagnetic to paramagnetic transition with increasing magnetic field. The absorption spectrum consists of many peaks, some of which violate Kohn's theorem, and which is due to the coupling of the center of mass motion with the other degrees of freedom.Comment: 6 pages, 12 ps figure

Energy-Momentum dispersion relation of plasmarons in bilayer graphene

The relation between the energy and momentum of plasmarons in bilayer graphene is investigated within the Overhauser approach, where the electron-plasmon interaction is described as a field theoretical problem. We find that the Dirac-like spectrum is shifted by $\Delta E(\mathbf{k})\sim 100\div150\,{\rm meV}$ depending on the electron concentration $n_{e}$ and electron momentum. The shift increases with electron concentration as the energy of plasmons becomes larger. The dispersion of plasmarons is more pronounced than in the case of single layer graphene, which is explained by the fact that the energy dispersion of electrons is quadratic and not linear. We expect that these predictions can be verified using angle-resolved photoemission spectroscopy (ARPES).Comment: 4 pages, 3 figure

The interaction between a superconducting vortex and an out-of-plane magnetized ferromagnetic disk: influence of the magnet geometry

The interaction between a superconducting vortex in a type II superconducting film (SC) and a ferromagnet (FM) with out-of-plane magnetization is investigated theoretically within the London approximation. The dependence of the interaction energy on the FM-vortex distance, film thickness and different geometries of the magnetic structures: disk, annulus(ring), square and triangle are calculated. Analytic expressions and vectorplots of the current induced in the SC due to the presence of the FM are presented. For a FM disk with a cavity, we show that different local minima for the vortex position are possible, enabling the system to be suitable to act as a qubit. For FMs with sharp edges, like e.g. for squares and triangles, the vortex prefers to enter its equilibrium position along the corners of the magnet.Comment: Preprint, 10 pages, 10 figures, submitted to Phys. Rev.

Optical properties of (In,Ga)As capped InAs quantum dots grown on [11k] substrates

Using three-dimensional k.p calculation including strain and piezoelectricity, we showed that the size of the quantum dot (QD) in the growth direction determines the influence of the (In,Ga)As capping layer on the optical properties of [11k] grown InAs QDs, where k=1,2,3. For flat dots, increase of In concentration in the capping layer leads to a decrease of the transition energy, as is the case of [001] grown QDs, whereas for large dots an increase of the In concentration in the capping layer is followed by an increase of the transition energy up to a critical concentration of In, after which the optical transition energy starts to decrease

Field-enhanced critical parameters in magnetically nanostructured superconductors

Within the phenomenological Ginzburg-Landau theory, we demonstrate the enhancement of superconductivity in a superconducting film, when nanostructured by a lattice of magnetic particles. Arrays of out-of-plane magnetized dots (MDs) extend the critical magnetic field and critical current the sample can sustain, due to the interaction of the vortex-antivortex pairs and surrounding supercurrents induced by the dots and the external flux lines. Depending on the stability of the vortex-antivortex lattice, a peak in the Hext-T boundary is found for applied integer and rational matching fields, which agrees with recent experiments [Lange et al., Phys. Rev. Lett. 90, 197006 (2003)]. Due to compensation of MDs- and Hext-induced currents, we predict the field-shifted jc-Hext characteristics, as was actually realized in previous experiment but not commented on [Morgan and Ketterson, Phys. Rev. Lett. 80, 3614 (1998)].Comment: 8 pages, 5 figures, to appear in Europhysics Letter

Quantum dot size dependent influence of the substrate orientation on the electronic and optical properties of InAs/GaAs quantum dots

Using 3D k.p calculation including strain and piezoelectricity we predict variation of electronic and optical properties of InAs/GaAs quantum dots (QDs) with the substrate orientation. The QD transition energies are obtained for high index substrates [11k], where k = 1,2,3 and are compared with [001]. We find that the QD size in the growth direction determines the degree of influence of the substrate orientation: the flatter the dots, the larger the difference from the reference [001] case.Comment: Submitted to Appl. Phys. Let