Gravitationally dresed RG flows and zig-zag invariant strings

We propose a world-sheet realization of the zigzag-invariant bosonic and fermionic strings as a perturbed Wess-Zumino-Novikov-Witten model at large negative level $k$ on a group manifold $G$ coupled to 2D gravity. In the large $k$ limit the zigzag symmetry can be obtained as a result of a self-consistent solution of the gravitationally dressed RG equation. The only solution found for simple group is $G=SL(2)$. More general target-space geometries can be obtained via tensoring of various cosets based on SL(2). In the supersymmetric case the zigzag symmetry fixes the maximal target-space dimension of the confining fermionic string to be seven.Comment: 16 pages, corrected typos, version to be published in Phys.Lett.

Towards c=0 Flows

We discuss some implications of the gravitational dressing of the renormalization group for conformal field theories perturbed by relevant operators. The renormalization group flows are defined with respect to the dilatation operator associated with the $J_0^{(0)}$ mode of the $SL(2,R)$ affine algebra. We discuss the possibility of passing under the $c=25$ barrier along renormalization group flows in some models.Comment: LaTex file, 11 pages, QMW Preprint, QMW 94-2

Higher Spin Fields in Siegel Space, Currents and Theta Functions

Dynamics of four-dimensional massless fields of all spins is formulated in the Siegel space of complex $4\times 4$ symmetric matrices. It is shown that the unfolded equations of free massless fields, that have a form of multidimensional Schrodinger equations, naturally distinguish between positive- and negative-frequency solutions of relativistic field equations, i.e. particles and antiparticles. Multidimensional Riemann theta functions are shown to solve massless field equations in the Siegel space. We establish the correspondence between conserved higher-spin currents in four-dimensional Minkowski space and those in the ten-dimensional matrix space. It is shown that global symmetry parameters of the current in the matrix space should be singular to reproduce a nonzero current in Minkowski space. The \D-function integral evolution formulae for 4d massless fields in the Fock-Siegel space are obtained. The generalization of the proposed scheme to higher dimensions and systems of higher ranks is considered.Comment: LaTeX, 38 pages, v.3: clarifications, acknowledgements and references added, typos corrected, v.4: more comments and references added, typos corrected, the version to appear in JHE

Effect of pulse thermal treatments on the Ni(Ti)–n-21R(6H)-SiC contact parameters

We present experimental investigations of the effect of rapid thermal treatment with incoherent IR radiation, as well as electric-spark and electron-beam treatments, on the electric parameters of Ni(Ti) n-21R(6H)-SiC contacts. The results obtained show that pulse thermal treatment is an efficient technique for local change of parameters of heterogeneous metal/silicon carbide structures

SiC Schottky-barrier diodes formed with TiBx and ZrBx amorphous layers

Electrical and structural properties of Schottky-barrier diodes formed with TiBx and ZrBx amorphous layers on n-6H-, 15R- and 4H-SiC (with epi-layer) were studied. High thermal stability of ideality factors and barrier heights in the formed contacts was explained by the thermal stability of an interface TiBx(ZrBx)-SiC after rapid thermal annealing at 800°N for 60 s

Methods and sensors for accurate wavefront measurements

The wavefront measurement is an important part both in adaptive optics and in optical shop testing. A number of wavefront sensors based on interferometric or on Hartmann principle is known; this thesis investigates particular technologies that can help to increase the accuracy and/or speed of existing wavefront sensors either by optimising the wavefront reconstruction algorithm or by optimising the hardware. The topics discussed are interferogram analysis, optimising of the Hartmann mask geometry, and design of a 2D heterodyne phase detector.Electrical Engineering, Mathematics and Computer Scienc

Simulation of incoherent radiation absorption in 3C-, 6H- and 4H-SiC at rapid thermal processing

For 3C-, 6H- and 4H-SiC polytypes of different conduction types and with various impurity concentrations we investigated absorption of incoherent radiation from the near IR spectral region at rapid thermal annealing. General regularities of both radiation absorption and sample heating are considered. We evaluated various processing modes; one should take this into account when developing technological procedures based on rapid thermal processing for SiC

Phase retrieval of large-scale time-varying aberrations using a non-linear Kalman filtering framework

This paper presents a computationally efficient framework in which a single focal-plane image is used to obtain a high-resolution reconstruction of dynamic aberrations. Assuming small-phase aberrations, a non-linear Kalman filter implementation is developed whose computational complexity scales close to linearly with the number of pixels of the focal-plane camera. The performance of themethod is tested in a simulation of an adaptive optics system, where the small-phase assumption is enforced by considering a closed-loop system that uses a low-resolution wavefront sensor to control a deformable mirror. The results confirmthe computational efficiency of the algorithm and showa large robustness against noise and model uncertainties. </p

Challenges in simulating advanced control methods for AO

This paper discusses various practical problems arising in the design and simulation of predictive control methods for adaptive optics. Although there has been increased attention towards optimal prediction and control methods for AO systems, they are often tested in simplified simulation environments. The use of advanced AO simulators however, is a valuable alternative to the use of real data or laboratory experiments, as they provide both a flexible environment which is ideal for testing a new algorithm and are more accessible to non-experts. Topics that are often not explicitly discussed, such as the identification of a turbulence dynamics model from data, the use of matrix structures in AO systems to decrease the computational complexity and the implementation of Kalman filters to optimally deal with realistic noise conditions are examined. All topics discussed are illustrated by an accompanying Matlab code, which is based on the existing Matlab AO toolbox OOMAO.Team Raf Van de Pla