Design of a Magnetic Bearing

A popular approach to nano-positioning requirements in precision engineering in general and micro-lithography in particular is to subdivide the stage positioning architecture into a coarse positioning module with micrometer accuracy (Long Stroke), onto which a fine positioning module (Short Stroke) is cascaded. The latter is responsible for correcting the residual error of the coarse positioning module to the last nanometers. High accuracy positioning in 6 Degrees Of Freedom put severe constraints on the actuators and/or bearing systems. Actuators are used for generating a varying force being part of a control loop. Bearing systems should generate a force as constant as possible in the bearing direction, but the force perpendicular to that direction should be as low as possible. Actuators could serve as a bearing system, but on the one hand this would require the actuators to be large and thus heavy and on the other hand a substantial amount of heat is continuously dissipated in order to generate the static forces. Such heat generation does not contribute to the positioning performance of the actuators, but significantly affects the thermal stability of the application. The latter implication will be overcome if the bearing system is established by a system with permanent magnets

QCD effective action with dressing functions - consistency checks in the perturbative regime

In a previous paper, we presented solution to the Slavnov--Taylor identity for the QCD effective action, and argued that the action terms containing (anti)ghost fields are unique. These terms have the same form as those in the classical action, but the gluon and (anti)ghost effective fields are convoluted with gluon and ghost dressing functions G_A and G_c, the latter containing perturbative and nonperturbative effects (but not including the soliton-like vacuum effects). In the present work we show how the perturbative QCD (pQCD) can be incorporated into the framework of this action, and we present explicit one-loop pQCD expressions for G_A and G_c. We then go on to check the consistency of the obtained results by considering an antighost Dyson--Schwinger equation (DSE). By solving the relations that result from the Legendre transformation leading to the effective action, we obtain the effective fields as power expansions of sources. We check explicitly that the aforementioned one-loop functions G_A and G_c fulfil the antighost DSE at the linear source level. We further explicitly check that these one-loop G_A and G_c have the regularization-scale and momentum dependence consistent with the antighost DSE at the quadratic source level. These checks suggest that the the effective action with dressing functions represents a consistent framework for treating QCD, at least at the one-loop level.Comment: 17 pages, revtex4; dimensional regularization used instead of Pauli-Villars, the check of identity in the linear-in-sources Dyson-Schwinger equation now includes the finite part; conclusions unchanged; to appear in Phys.Rev.

Treibhausgasflüsse beim Anbau von Winterweizen und Kleegras

At the experimental station Viehhausen, 30 km north of Munich in southern Germany, N2O, CO2 and CH4 fluxes between soil and atmosphere were measured to investigate the influence of site-related factors and cultivation-technique on the emissions of these greenhouse gases. Clover-grass as well as several wheat cropping systems (with and without biogas slurry) were analyzed under the conditions of organic farming. This paper shows the results of the vegetation period of 2009. The N2O emissions from the wheat fields were higher than those from clover-grass fields. Ploughing-in of the legume-grass biomass resulted in releasing distinctive N2O emissions. For CH4 fluxes the arable soils were a net sink, especially in cropping systems with winter wheat

Next-to-leading Corrections to the Higgs Boson Transverse Momentum Spectrum in Gluon Fusion

We present a fully analytic calculation of the Higgs boson transverse momentum and rapidity distributions, for nonzero Higgs $p_\perp$, at next-to-leading order in the infinite-top-mass approximation. We separate the cross section into a part that contains the dominant soft, virtual, collinear, and small-$p_\perp$-enhanced contributions, and the remainder, which is organized by the contributions due to different parton helicities. We use this cross section to investigate analytically the small-$p_\perp$ limit and compare with the expectation from the resummation of large logarithms of the type $\ln{m_H/p_\perp}$. We also compute numerically the cross section at moderate $p_\perp$ where a fixed-order calculation is reliable. We find a $K$-factor that varies from $\approx1.6-1.8$, and a reduction in the scale dependence, as compared to leading order. Our analysis suggests that the contribution of current parton distributions to the total uncertainty on this cross section at the LHC is probably less than that due to uncalculated higher orders.Comment: 40 pages, 10 figures, JHEP style (minor changes, added reference

An approach to solve Slavnov-Taylor identities in nonsupersymmetric non-Abelian gauge theories

We present a way to solve Slavnov--Taylor identities in a general nonsupersymmetric theory. The solution can be parametrized by a limited number of functions of spacetime coordinates, so that all the effective fields are dressed by these functions via integral convolution. The solution restricts the ghost part of the effective action and gives predictions for the physical part of the effective action.Comment: revised version, section 3 is enlarged, 24 pages, Latex2e, no figures, version accepted by Phys. Rev.

Permutable subnormal subgroups of finite groups

The aim of this paper is to prove certain characterization theorems for groups in which permutability is a transitive relation, the so called PT -groups. In particular, it is shown that the finite solvable PT -groups, the finite solvable groups in which every subnormal subgroup of defect two is permutable, the finite solvable groups in which every normal subgroup is permutable sensitive, and the finite solvable groups in which conjugate-permutability and permutability coincide are all one and the same class. This follows from our main result which says that the finite modular p-groups, p a prime, are those p-groups in which every subnormal subgroup of defect two is permutable or, equivalently, in which every normal subgroup is permutable sensitive. However, there exist finite insolvable groups which are not PT -groups but all subnormal subgroups of defect two are permutable

Two-point microrheology and the electrostatic analogy

The recent experiments of Crocker et al. suggest that microrheological measurements obtained from the correlated fluctuations of widely-separatedprobe particles determine the rheological properties of soft, complex materials more accurately than do the more traditional particle autocorrelations. This presents an interesting problem in viscoelastic dynamics. We develop an important, simplifing analogy between the present viscoelastic problem and classical electrostatics. Using this analogy and direct calculation we analyze both the one and two particle correlations in a viscoelastic medium in order to explain this observation

Ultrafast carrier relaxation in GaN, In_(0.05)Ga_(0.95)N and an In_(0.05)Ga_(0.95)/In_(0.15)Ga_(0.85)N Multiple Quantum Well

Room temperature, wavelength non-degenerate ultrafast pump/probe measurements were performed on GaN and InGaN epilayers and an InGaN multiple quantum well structure. Carrier relaxation dynamics were investigated as a function of excitation wavelength and intensity. Spectrally-resolved sub-picosecond relaxation due to carrier redistribution and QW capture was found to depend sensitively on the wavelength of pump excitation. Moreover, for pump intensities above a threshold of 100 microJ/cm2, all samples demonstrated an additional emission feature arising from stimulated emission (SE). SE is evidenced as accelerated relaxation (< 10 ps) in the pump-probe data, fundamentally altering the re-distribution of carriers. Once SE and carrier redistribution is completed, a slower relaxation of up to 1 ns for GaN and InGaN epilayers, and 660 ps for the MQW sample, indicates carrier recombination through spontaneous emission.Comment: submitted to Phys. Rev.

Spin transport of electrons through quantum wires with spatially-modulated strength of the Rashba spin-orbit interaction

We study ballistic transport of spin-polarized electrons through quantum wires in which the strength of the Rashba spin-orbit interaction (SOI) is spatially modulated. Subband mixing, due to SOI, between the two lowest subbands is taken into account. Simplified approximate expressions for the transmission are obtained for electron energies close to the bottom of the first subband and near the value for which anticrossing of the two lowest subbands occurs. In structures with periodically varied SOI strength, {\it square-wave} modulation on the spin transmission is found when only one subband is occupied and its possible application to the spin transistor is discussed. When two subbands are occupied the transmission is strongly affected by the existence of SOI interfaces as well as by the subband mixing