The evolution of Bernstein modes in quantum wires with increasing deviation from parabolic confinement

We investigate the evolution of the interaction of the magnetoplasmon resonance with the harmonics of the cyclotron resonance as the confinement of an electron gas in a quantum wire increasingly deviates from the parabolic case. The occurrence of the Bernstein modes is observed in a time-dependent Hartree model of a two-dimensional electron gas in a single quantum wire.Comment: 9 pages, 4 figures, uses IOP macros, to appear in the Journal of Physics: Condensed Matte

Non-adiabatic Current Excitation in Quantum Rings

We investigate the difference in the response of a one-dimensional semiconductor quantum ring and a finite-width ring to a strong and short-lived time-dependent perturbation in the THz regime. In both cases the persistent current is modified through a nonadiabatic change of the many-electron states of the system, but by different mechanisms in each case.Comment: LaTeX, 5 pages with 6 embedded postscript figures, submitted to 20th Nordic Semiconductor Meeting, Tampere (2003

Influence of shape of quantum dots on their far-infrared absorption

We investigate the effects of the shape of quantum dots on their far-infrared absorption in an external magnetic field by a model calculation. We focus our attention on dots with a parabolic confinement potential deviating from the common circular symmetry, and dots having circular doughnut shape. For a confinement where the generalized Kohn theorem does not hold we are able to interprete the results in terms of a mixture of a center-of-mass mode and collective modes reflecting an excitation of relative motion of the electrons. The calculations are performed within the time-dependent Hartree approximation and the results are compared to available experimental results.Comment: RevTeX, 16 pages with 10 postscript figures included. Submitted to Phys. Rev.

Far-Infrared Excitations below the Kohn Mode: Internal Motion in a Quantum Dot

We have investigated the far-infrared response of quantum dots in modulation doped GaAs heterostructures. We observe novel modes at frequencies below the center-of-mass Kohn mode. Comparison with Hartree-RPA calculations show that these modes arise from the flattened potential in our field-effect confined quantum dots. They reflect pronounced relative motion of the charge density with respect to the center-of-mass.Comment: 8 pages, LaTeX with integrated 6 PostScript figure

The Fano regime of one-dot Aharonov-Bohm interferometers

We use the Landauer-B\"{u}ttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi {\it et al.} [Phys. Rev. Lett. {\bf 88}, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.Comment: To appear in Phys. Rev.

Rapid changes in the size of different functional organ and muscle groups during refueling in a long-distance migrating shorebird

The adaptive value of size changes in different organ and muscle groups was studied in red knots (Calidris canutus islandica) in relation to their migration. Birds were sampled on five occasions: at arrival in Iceland in May 1994, two times during subsequent refueling, at departure toward, and on return from, the high arctic breeding grounds. During their 24-d stopover in May, body mass increased from 144.3 to 214.5 g. Mass gains were lowest over the first week (0.85 g/d, only fat-free tissue deposited). Over the subsequent 10 d, average mass increased by 5.0 g/d (fat contributing 78%), and over the last week before takeoff, it increased by 2.0 g/d (fat contributing over 100% because of loss of lean components). There were no sex differences in body and fat mass gains. Over the first interval, lean masses of heart, stomach, and liver increased. During the middle 10 d, sizes of leg muscle, intestine, liver, and kidneys increased. Stomach mass decreased over the same interval. In the last interval before takeoff, the stomach atrophied further and the intestine, leg muscles, and liver became smaller too, but pectoral muscles and heart increased in size. Sizes of "exercise organs" such as pectoral muscle and heart were best correlated with body mass, whereas sizes of organs used during foraging (leg muscles) and nutrient extraction (intestine, liver) were best correlated with rate of mass gain. Kidneys changed little before takeoff, which suggests that they are needed as much during flight as during refueling

Magnetization in short-period mesoscopic electron systems

We calculate the magnetization of the two-dimensional electron gas in a short-period lateral superlattice, with the Coulomb interaction included in Hartree and Hartree-Fock approximations. We compare the results for a finite, mesoscopic system modulated by a periodic potential, with the results for the infinite periodic system. In addition to the expected strong exchange effects, the size of the system, the type and the strength of the lateral modulation leave their fingerprints on the magnetization.Comment: RevTeX4, 10 pages with 14 included postscript figures To be published in PRB. Replaced to repair figure

Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption

We investigate several spin-related phenomena in a confined two-dimensional electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb interaction of the electrons. The exchange term of the interaction causes a large splitting of the spin levels whenever the chemical potential lies within a Landau band (LB). This splitting can be reinterpreted as an enhancement of an effective g-factor, g*. The increase of g* when a LB is half filled can be accompanied by a spontaneous formation of a static spin-inversion state (SIS) whose details depend on the system sision state (SIS) whose details depend on the system size. The coupling of the states of higher LB's into the lowest band by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The far-infrared absorption of the system, relatively insensitive to the spin splitting, develops clear signs of the SIS.Comment: 7 figure

Far-infrared spectra of lateral quantum dot molecules

We study effects of electron-electron interactions and confinement potential on the magneto-optical absorption spectrum in the far-infrared range of lateral quantum dot molecules. We calculate far-infrared (FIR) spectra for three different quantum dot molecule confinement potentials. We use accurate exact diagonalization technique for two interacting electrons and calculate dipole-transitions between two-body levels with perturbation theory. We conclude that the two-electron FIR spectra directly reflect the symmetry of the confinement potential and interactions cause only small shifts in the spectra. These predictions could be tested in experiments with nonparabolic quantum dots by changing the number of confined electrons. We also calculate FIR spectra for up to six noninteracting electrons and observe some additional features in the spectrum.Comment: For better quality Figs download manuscript from http://www.fyslab.hut.fi/~mma/FIR/Helle_qdmfir.ps.g