Floating Wigner molecules and possible phase transitions in quantum dots

A floating Wigner crystal differs from the standard one by a spatial averaging over positions of the Wigner-crystal lattice. It has the same internal structure as the fixed crystal, but contrary to it, takes into account rotational and/or translational symmetry of the underlying jellium background. We study properties of a floating Wigner molecule in few-electron spin-polarized quantum dots, and show that the floating solid has the lower energy than the standard Wigner crystal with fixed lattice points. We also argue that internal rotational symmetry of individual dots can be broken in arrays of quantum dots, due to degenerate ground states and inter-dot Coulomb coupling.Comment: 6 pages incl 3 figure

Dimer states in atomic mixtures

A mixture of heavy atoms in a Mott state and light spin-1/2 fermionic atoms is studied in an optical lattice. Inelastic scattering processes between both atomic species excite the heavy atoms and renormalize the tunneling rate as well as the interaction of the light atoms. An effective Hamiltonian for the latter is derived that describes tunneling of single fermions, tunneling of fermionic pairs and an exchange of fermionic spins. Low energy states of this Hamiltonian are a N\'eel state for strong effective repulsion, dimer states for moderate interaction, and a density wave of paired fermions for strong effective attraction.Comment: 10 pages, 3 figure, extended versio

System approach to disparity estimation

A system approach to disparity estimation using dynamic programming is presented. The four step system can calculate a dense correspondence map between a stereo pair with parallel or nonparallel camera geometry. Results are presented with CCIR 601 format stereo images

Evolution of field spiral galaxies up to redshifts z=1

We have gained VLT/FORS spectra and HST/ACS images of a sample of 220 distant field spiral galaxies. Spatially resolved rotation curves were extracted and fitted with synthetic velocity fields that take into account all geometric and observational effects, like blurring due to the slit width and seeing influence. The maximum rotation velocity Vmax could be determined for 124 galaxies that cover the redshift range 0.1<z<1.0. The luminosity-rotation velocity distribution of this sample is offset from the Tully-Fisher relation (TFR) of local low-mass spirals, whereas the distant high-mass spirals are compatible with the local TFR. We show that the slope of the local and the intermediate-z TFR would be in compliance if its scatter decreased by more than a factor of 3 between z~0.5 and z~0. On the other hand, the distant low-luminosity disks have much lower stellar M/L ratios than their local counterparts, while high-luminosity disks barely evolved in M/L over the covered redshift range. This could be the manifestation of the "downsizing" effect, i.e. the succesive shift of the peak of star formation from high-mass to low-mass galaxies towards lower redshifts. This trend might be canceled out in the TF diagram due to the simultaneous evolution of multiple parameters. We also estimate the ratios between stellar and total masses, finding that these remained constant since z=1, as would be expected in the context of hierarchically growing structure. (Abridged)Comment: 20 pages, 5 figures, ApJ, accepte

A Study of Sediment Transport in Norwegian Glacial Rivers, 1969

From original report: The Norwegian Water Resources and Electricity Board, Institute of Water Resources, Department of Hydrology, Oslo. September 1970. Report No. 6/70.Permission to translate this Norwegian report was kindly given by G. Østrem, and the translation by Helga Carstens, while she was in Alaska, is greatly appreciated. Unfortunately, Mrs. Carstens returned to her homeland, Norway, before final editing of the manuscript could be completed. Consequently, any errors in translation are due to the editor, and for these errors, the editor apologizes to the authors. Not included in this translation is an English summary contained in the original report. To keep printing costs down, the original figures and tables, which fortunately had English titles, are used in this translation. This report is the first of a series of reports being prepared for the Norwegian Water Resources and Electricity Board. The second report for 1970 has been published with an English summary and contains an extension of the data contained in the 1969 report. Because this work deals with problems very similar to those in Alaska, it was decided to translate the first report and circulate a limited number of copies to workers in the U. S. and Canada. Research very similar to the Norwegian work was initiated in Alaska under the editor's direction in cooperation with the U. S. Geological Survey. -- G. L. Guymon.This work and the translation of this report were supported by funds provided by the United States Department of the Interior, Office of Water Resources Research (Proj. A-042-ALAS), as authorized under the amended Water Resources Act of 1964

Ising instability of a Holstein phonon mode in graphene

We study the thermal distribution of phonons in a graphene sheet. Due to the two electronic bands there are two out-of-plane phonon modes with respect to the two sublattices. One of these modes undergoes an Ising transition by spontaneously breaking the sublattice symmetry. We calculate the critical point, the renormalization of the phonon frequency and the average lattice distortion. This transition might be observable in Raman scattering and in transport properties.Comment: 5 pages, 2 figure

Lower Bound for the Fermi Level Density of States of a Disordered D-Wave Superconductor in Two Dimensions

We consider a disordered d--wave superconductor in two dimensions. Recently, we have shown in an exact calculation that for a lattice model with a Lorentzian distributed random chemical potential the quasiparticle density of states at the Fermi level is nonzero. As the exact result holds only for the special choice of the Lorentzian, we employ different methods to show that for a large class of distributions, including the Gaussian distribution, one can establish a nonzero lower bound for the Fermi level density of states. The fact that the tails of the distributions are unimportant in deriving the lower bound shows that the exact result obtained before is generic.Comment: 15 preprint pages, no figures, submitted to PR

On the mechanism of irradiation enhanced exchange bias

By means of layer resolved ion irradiation the mechanisms involved in the irradiation driven modifications of the exchange bias effect in NiFe/FeMn bilayers have been investigated. It is shown that not only the locations of the defects but also the magnetic coupling between both layers during the irradiation process is of crucial importance. This requires an extension of current models accounting for defects in exchange bias systems.Comment: 3 pages, 3 figures, revised version, added results from further structural characterization by TEM, submitted to Europhysics Letter

Inelastic scattering of atoms in a double well

We study a mixture of two light spin-1/2 fermionic atoms and two heavy atoms %in a Mott state in a double well potential. Inelastic scattering processes between both atomic species excite the heavy atoms and renormalize the tunneling rate and the interaction of the light atoms (polaron effect). The effective interaction of the light atoms changes its sign and becomes attractive for strong inelastic scattering. This is accompanied by a crossing of the energy levels from singly occupied sites at weak inelastic scattering to a doubly occupied and an empty site for stronger inelastic scattering. We are able to identify the polaron effect and the level crossing in the quantum dynamics.Comment: 12 pages, 9 figure