Noise-induced switching between vortex states with different polarization in classical two-dimensional easy-plane magnets

In the 2-dimensional anisotropic Heisenberg model with XY-symmetry there are non-planar vortices which exhibit a localized structure of the z-components of the spins around the vortex center. We study how thermal noise induces a transition of this structure from one polarization to the opposite one. We describe the vortex core by a discrete Hamiltonian and consider a stationary solution of the Fokker-Planck equation. We find a bimodal distribution function and calculate the transition rate using Langer's instanton theory (1969). The result is compared with Langevin dynamics simulations for the full many-spin model.Comment: 15 pages, 4 figures, Phys. Rev. B., in pres

Intrinsic hole mobility and trapping in a regio-regular poly(thiophene)

The transport properties of high-performance thin-film transistors (TFT) made with a regio-regular poly(thiophene) semiconductor (PQT-12) are reported. The room-temperature field-effect mobility of the devices varied between 0.004 cm2/V s and 0.1 cm2/V s and was controlled through thermal processing of the material, which modified the structural order. The transport properties of TFTs were studied as a function of temperature. The field-effect mobility is thermally activated in all films at T<200 K and the activation energy depends on the charge density in the channel. The experimental data is compared to theoretical models for transport, and we argue that a model based on the existence of a mobility edge and an exponential distribution of traps provides the best interpretation of the data. The differences in room-temperature mobility are attributed to different widths of the shallow localized state distribution at the edge of the valence band due to structural disorder in the film. The free carrier mobility of the mobile states in the ordered regions of the film is the same in all structural modifications and is estimated to be between 1 and 4 cm2/V s.Comment: 20 pages, 8 figure

Magnetic Vortex Core Reversal by Excitation of Spin Waves

Micron-sized magnetic platelets in the flux closed vortex state are characterized by an in-plane curling magnetization and a nanometer-sized perpendicularly magnetized vortex core. Having the simplest non-trivial configuration, these objects are of general interest to micromagnetics and may offer new routes for spintronics applications. Essential progress in the understanding of nonlinear vortex dynamics was achieved when low-field core toggling by excitation of the gyrotropic eigenmode at sub-GHz frequencies was established. At frequencies more than an order of magnitude higher vortex state structures possess spin wave eigenmodes arising from the magneto-static interaction. Here we demonstrate experimentally that the unidirectional vortex core reversal process also occurs when such azimuthal modes are excited. These results are confirmed by micromagnetic simulations which clearly show the selection rules for this novel reversal mechanism. Our analysis reveals that for spin wave excitation the concept of a critical velocity as the switching condition has to be modified.Comment: Minor corrections and polishing of previous versio

Critical dynamics in the 2d classical XY-model: a spin dynamics study

Using spin-dynamics techniques we have performed large-scale computer simulations of the dynamic behavior of the classical three component XY-model (i.e. the anisotropic limit of an easy-plane Heisenberg ferromagnet), on square lattices of size up to 192^2, for several temperatures below, at, and above T_KT. The temporal evolution of spin configurations was determined numerically from coupled equations of motion for individual spins by a fourth order predictor-corrector method, with initial spin configurations generated by a hybrid Monte Carlo algorithm. The neutron scattering function S(q,omega) was calculated from the resultant space-time displaced spin-spin correlation function. Pronounced spin-wave peaks were found both in the in-plane and the out-of-plane scattering function over a wide range of temperatures. The in-plane scattering function S^xx also has a large number of clear but weak additional peaks, which we interpret to come from two-spin-wave scattering. In addition, we observed a small central peak in S^xx, even at temperatures well below the phase transition. We used dynamic finite size scaling theory to extract the dynamic critical exponent z. We find z=1.00(4) for all T <= T_KT, in excellent agreement with theoretical predictions, although the shape of S(q,omega) is not well described by current theory.Comment: 31 pages, LaTex, 13 figures (38 subfigures) included as eps-files, needs psfig, 260 K

Artifical compound eyes - Different concepts and their application to ultra flat image acquisition sensors

Two different approaches for ultra flat image acquisition sensors on the basis of artificial compound eyes are examined. In apposition optics the image reconstruction is based on moiré- or static sampling while the superposition eye approach produces an overall image. Both types of sensors are compared with respect to theoretical limitations of resolution, sensitivity and system thickness. Explicit design rules are given. A paraxial 3×3 matrix formalism is used to describe the arrangement of three microlens arrays with different pitches to find first order parameters of artificial superposition eyes. The model is validated by analysis of the system with raytracing software. Measurements of focal length of anamorphic reflow lenses, which are key components of the superposition approach, under oblique incidence are performed. For the second approach, the artificial apposition eye, a first demonstrator system is presented. The monolithic device consists of a UV-replicated reflow microlens array on a thin silica-substrate with a pinhole array in a metal layer on the backside. The pitch of the pinholes differs from the lens array pitch to enable an individual viewing angle for each channel. Imaged test patterns are presented and measurements of the angular sensitivity function are compared to calculations using commercial raytracing software

Tumorbiologie des Oropharynxkarzinoms

Hintergrund Oropharynxkarzinome (OPSCC) unterscheiden sich abhängig von noxenbasierter oder durch humane Papillomaviren (HPV) getriebener Ätiologie in klinischen Faktoren und der Prognose. Zugrunde liegend sind molekulare Unterschiede der Tumorbiologie. Ziel der Arbeit Ziel war die Darstellung wichtiger molekularbiologischer Charakteristika der Genetik, Epigenetik und Immunologie von OPSCC. Material und Methoden Es handelt sich um eine Übersichtsarbeit zu einer Auswahl molekularbiologischer Faktoren der Tumorbiologie von OPSCC aus Genetik, Epigenetik und Immunologie. Ergebnisse Genetische Veränderungen und deren Auswirkungen auf Kanzerogenese und Tumorbiologie werden in zunehmender Tiefe verstanden. Epigenetische Phänomene ergänzen funktionelle Zusammenhänge. Die epigenetischen Regulationsmechanismen der Gene sind komplex. Daher besteht in diesem Feld weiterhin großer Forschungsbedarf. Immunologische Aspekte der Molekularbiologie gewinnen im Kontext der aktuellen Entwicklungen in der Immunonkologie an Bedeutung. Schlussfolgerung Die Tumorbiologie von Oropharynxkarzinomen unterscheidet sich v. a. bezüglich des HPV-Status. Zusätzlich werden HPV-unabhängige Subgruppen genetisch, epigenetisch und immunologisch zunehmend charakterisiert. Aus diesen Erkenntnissen können logische Grundprinzipien neuer Therapiekonzepte abgeleitet werden

Far-infrared study of the Jahn-Teller distorted C60 monoanion in C60 tetraphenylphosphoniumiodide

We report high-resolution far-infrared transmission measurements on C(60)-tetraphenylphosphoniumiodide as a function of temperature. In the spectral region investigated (20-650 cm(-1)), we assign intramolecular modes of the C(60) monoanion and identify low-frequency combination modes. The well-known F(1u)(1) and F(1u)(2) modes are split into doublers at room temperature, indicating a D(5d) or D(3d) distorted ball. This result is consistent with a dynamic Jahn-Teller effect in the strong-coupling limit or with a static distortion stabilized by low-symmetry perturbations. The appearance of silent odd modes is in keeping with symmetry reduction of the hall, while activation of even modes is attributed to interband electron-phonon coupling and orientational disorder in the fulleride salt. Temperature dependences reveal a weak transition in the region 125-150 K in both C(60)(-) and counterion modes, indicating a bulk, rather than solely molecular, effect. Anomalous softening (with decreasing temperature) in several modes may correlate with the radial character of those vibrations. [S0163-1829(98)03245-7]