Magnetoresistance, Micromagnetism, and Domain Wall Scattering in Epitaxial hcp Co Films

Large negative magnetoresistance (MR) observed in transport measurements of hcp Co films with stripe domains were recently reported and interpreted in terms of a novel domain wall (DW) scattering mechanism. Here detailed MR measurements, magnetic force microscopy, and micromagnetic calculations are combined to elucidate the origin of MR in this material. The large negative room temperature MR reported previously is shown to be due to ferromagnetic resistivity anisotropy. Measurements of the resistivity for currents parallel (CIW) and perpendicular to DWs (CPW) have been conducted as a function of temperature. Low temperature results show that any intrinsic effect of DWs scattering on MR of this material is very small compared to the anisotropic MR.Comment: 5 pages, 5 Figures, submitted to PR

Resistance of a domain wall in the quasiclassical approach

Starting from a simple microscopic model, we have derived a kinetic equation for the matrix distribution function. We employed this equation to calculate the conductance $G$ in a mesoscopic F'/F/F' structure with a domain wall (DW). In the limit of a small exchange energy $J$ and an abrupt DW, the conductance of the structure is equal to $G_{2d}=4\sigma_{\uparrow}\sigma_{\downarrow }/(\sigma_{\uparrow}+\sigma_{\downarrow})L$. Assuming that the scattering times for electrons with up and down spins are close to each other we show that the account for a finite width of the DW leads to an increase in this conductance. We have also calculated the spatial distribution of the electric field in the F wire. In the opposite limit of large $J$ (adiabatic variation of the magnetization in the DW) the conductance coincides in the main approximation with the conductance of a single domain structure $G_{1d}=(\sigma_{\uparrow}+\sigma_{\downarrow})/L$. The account for rotation of the magnetization in the DW leads to a negative correction to this conductance. Our results differ from the results in papers published earlier.Comment: 11 pages; replaced with revised versio

Der frühe Beginn der Zwangsstörung

Einleitung: Die vorliegende Untersuchung geht der Fragestellung nach, ob sich eine Zwangsstörung, die bereits im Kindes- bzw. Jugendalter beginnt, von einer Zwangsstörung, die erst im Erwachsenenalter beginnt, hinsichtlich Schweregrad und Symptomatik unterscheidet. Patienten und Methoden: Eine Stichprobe von 370 Patienten mit Zwangsstörung (ICD-10 F42), die sich zwischen 1998 und 2002 stationär in der Psychosomatischen Klinik Windach befanden, wurde in eine Early-Onset-Gruppe (Störungsbeginn ≤15 Jahre) und in eine Late-Onset-Gruppe (Störungsbeginn ≥16 Jahre) aufgeteilt. Die Gruppen wurden über ICD-10-Diagnosen und Y-BOCSWerte verglichen. Ergebnisse: Beim Schweregrad zeigte sich, dass 20,5% der Early-Onset-Gruppe, aber lediglich 8,7% der Late-Onset-Gruppe unter einer «massiven Zwangsstörung» leiden. Bei der Symptomatik zeigte sich, dass die Early-Onset-Gruppe häufiger die Diagnose «Zwangsgedanken und -handlungen gemischt» (76,9%)erhält als die Late-Onset-Gruppe (61,8%). Außerdem nennt die Early-Onset-Gruppe sowohl für die Gegenwart als auch für die Vergangenheit mehr Symptome als die Late-Onset-Gruppe (Gegenwart 8,2 vs. 7,0; Vergangenheit 5,5 vs. 3,9 Symptomgruppen). Weiter ergaben sich inhaltliche Unterschiede der Zwangsgedanken und Zwangshandlungen. Schlussfolgerungen: Early-Onset-Patienten scheinen häufiger von einer massiven Form der Zwangsstörung und einer größeren Symptomvielfalt betroffen zu sein als Late-Onset-Patienten. Ob es sich bei der Zwangsstörung mit Beginn im Kindes- und Jugendalter um einen abgrenzbaren Subtypus handelt, konnte jedoch in dieser Untersuchung nicht eindeutig geklärt werden und bedarf weiterer Forschungen.Introduction: This study investigates if obsessive compulsive disorder with early onset differs in severity and symptomatology from that with late onset. Patients and Methods: A sample of 370 patients with obsessive compulsive disorder (OCD; ICD 10 F42) who received in-patient treatment at the psychosomatic clinic of Windach between 1998 and 2002 were divided into an early-onset group (onset ≤15 years) and a late-onset group (onset ≥16 years). Groups were compared regarding ICD-10 diagnosis and Y-BOCS scores. Results: Considering severity of the disorder 20.5% of the early-onset group but merely 8.7% of the late-onset group suffered from an extreme form of OCD. With respect to symptomatology, the early-onset group was diagnosed with ‘obsessions and compulsions, mixed’ (76.9%) more often than the lateonset group (61.8%). Also, the early-onset group reported a wider variety of symptoms both for the present and for the past than the late-onset group (present 8,2 vs 7.0; past 5.5 vs 3.9 types of symptoms). There were also differences in the content of rumination and types of compulsive rituals. Conclusions: Patients with early-onset OCD seem to be more frequently affected by an extreme form of OCD and to experience a higher variety of symptoms than patients with late-onset OCD. If early-onset OCD can be considered a distinct subtype could not be answered unequivocally by the results of this study. This question needs additional research

Self-consistent local-equilibrium model for density profile and distribution of dissipative currents in a Hall bar under strong magnetic fields

Recent spatially resolved measurements of the electrostatic-potential variation across a Hall bar in strong magnetic fields, which revealed a clear correlation between current-carrying strips and incompressible strips expected near the edges of the Hall bar, cannot be understood on the basis of existing equilibrium theories. To explain these experiments, we generalize the Thomas-Fermi--Poisson approach for the self-consistent calculation of electrostatic potential and electron density in {\em total} thermal equilibrium to a {\em local equilibrium} theory that allows to treat finite gradients of the electrochemical potential as driving forces of currents in the presence of dissipation. A conventional conductivity model with small values of the longitudinal conductivity for integer values of the (local) Landau-level filling factor shows that, in apparent agreement with experiment, the current density is localized near incompressible strips, whose location and width in turn depend on the applied current.Comment: 9 pages, 7 figure

New Mechanism for Electronic Energy Relaxation in Nanocrystals

The low-frequency vibrational spectrum of an isolated nanometer-scale solid differs dramatically from that of a bulk crystal, causing the decay of a localized electronic state by phonon emission to be inhibited. We show, however, that an electron can also interact with the rigid translational motion of a nanocrystal. The form of the coupling is dictated by the equivalence principle and is independent of the ordinary electron-phonon interaction. We calculate the rate of nonradiative energy relaxation provided by this mechanism and establish its experimental observability.Comment: 4 pages, Submitted to Physical Review

Spin Degeneracy and Conductance Fluctuations in Open Quantum Dots

The dependence of mesoscopic conductance fluctuations on parallel magnetic field is used as a probe of spin degeneracy in open GaAs quantum dots. The variance of fluctuations at high parallel field is reduced from the low-field variance (with broken time-reversal symmetry) by factors ranging from roughly two in a 1 square-micron dot at low temperature, to four or greater in 8 square-micron dots. The factor of two is expected for simple Zeeman splitting of spin degenerate channels. A possible explanation for the unexpected larger factors in terms of field-dependent spin orbit scattering is proposed.Comment: Includes new reference to related theoretical work, cond-mat/0010064. Other minor changes. Related papers at http://marcuslab.harvard.ed

Reduced visibility of quantum oscillations in the spin-boson model

The loss of coherence of quantum oscillations is of fundamental interest as well as of practical importance in quantum computing. In solid-state experiments the oscillations show, next to the familiar exponential decay on time scales $T_{1/2}$, an overall loss of amplitude. We solve the spin-Boson for a large class of initial conditions without the Markov approximation at the pure dephasing point. It is shown that a loss of visibility occurs in the form of a fast initial drop for factorized initial conditions and an overall reduction for entangled initial conditions. This loss of amplitude is distict from $T_2$-decoherence with the difference being most drastic for environments with real or pseudo-gaps. This result is explained by bandwith effects in quantum noise as well as in terms of higher-order phase-breaking processes. For several experiments, such gapped environments are identified. We confirm that this physics is valid beyond the pure dephasing point.Comment: 13 pages, 2 figures; significant updat

Plankton ecology: The past two decades of progress

This is a selected account of recent developments in plankton ecology. The examples have been chosen for their degree of innovation during the past two decades and for their general ecological importance. They range from plankton autecology over interactions between populations to community ecology. The autecology of plankton is represented by the hydromechanics of plankton (the problem of life in a viscous environment) and by the nutritional ecology of phyto- and zooplankton. Population level studies are represented by competition, herbivory (grazing), and zooplankton responses to predation. Community ecology is represented by the debate about bottom- up vs. top-down control of community organization, by the PEG model of seasonal plankton succession, and by the recent discovery of the microbial food web

An Analysis of Private School Closings

We add to the small literature on private school supply by exploring exits of K-12 private schools. We find that the closure of private schools is not an infrequent event, and use national survey data from the National Center for Education Statistics to study closures of private schools. We assume that the probability of an exit is a function of excess supply of private schools over the demand, as well as the school's characteristics such as age, size, and religious affiliation. Our empirical results generally support the implications of the model. Working Paper 07-0

A RAVE investigation on Galactic open clusters I. Radial velocities and metallicities

Context. Galactic open clusters (OCs) mainly belong to the young stellar population in the Milky Way disk, but are there groups and complexes of OCs that possibly define an additional level in hierarchical star formation? Current compilations are too incomplete to address this question, especially regarding radial velocities (RVs) and metallicities ([M/H]). Aims. Here we provide and discuss newly obtained RV and [M/H] data, which will enable us to reinvestigate potential groupings of open clusters and associations. Methods. We extracted additional RVs and [M/H] from the RAdial Velocity Experiment (RAVE) via a cross-match with the Catalogue of Stars in Open Cluster Areas (CSOCA). For the identified OCs in RAVE we derived (RV) over bar and ([M/H]) over bar from a cleaned working sample and compared the results with previous findings. Results. Although our RAVE sample does not show the same accuracy as the entire survey, we were able to derive reliable (RV) over bar for 110 Galactic open clusters. For 37 OCs we publish (RV) over bar for the first time. Moreover, we determined ([M/H]) over bar for 81 open clusters, extending the number of OCs with ([M/H]) over bar by 69