[en] We study the D-brane solutions to a type IIB superstring in ten dimensions and find an interpretation in terms of the compactification of a twelve-dimensional three-brane of (a specific) F-theory on torus T². In this framework, there also exists a two-brane which may be argued to be equivalent to the three-brane by utilizing the electric-magnetic duality in eleven dimensions. In this context, we propose the existence of an isometry in one of the transverse directions to the three-brane in F-theory. As a consequence the two-brane may be identified with the three-brane in twelve dimensions. The twelve-dimensional picture of D-branes in type IIB theory suggests the reformulation of the type IIB superstring in terms of the three-brane of F-theory. (orig.)

[en] We show that the symmetric as well as asymmetric chiral gauging of the SL(2,R) Wess-Zumino-Witten (WZW) models give rise to consistent three dimensional string backgrounds with curvature singularities. Close connection of our solution with the three dimensional black string is presented. The symmetries of the background fields are also analysed. (author). 19 refs

[en] Purpose: As three-dimensional diode arrays increase in popularity for patient-specific quality assurance for intensity-modulated radiation therapy (IMRT), it is important to evaluate an array’s susceptibility to setup errors. The ArcCHECK phantom is set up by manually aligning its outside marks with the linear accelerator’s lasers and light-field. If done correctly, this aligns the ArcCHECK cylinder’s central axis (CAX) with the linear accelerator’s axis of rotation. However, this process is prone to error. This project has developed an analytical expression including a perturbation factor to quantify the effect of shifts. Methods: The ArcCHECK is set up by aligning its machine marks with either the sagittal room lasers or the light-field of the linear accelerator at gantry zero (IEC). ArcCHECK has sixty-six evenly-spaced SunPoint diodes aligned radially in a ring 14.4 cm from CAX. The detector response function (DRF) was measured and combined with inverse-square correction to develop an analytical expression for output. The output was calculated using shifts of 0 (perfect alignment), +/−1, +/−2 and +/−5 mm. The effect on a series of simple inputs was determined: unity, 1-D ramp, steps, and hat-function to represent uniform field, wedge, evenly-spaced modulation, and single sharp modulation, respectively. Results: Geometric expressions were developed with perturbation factor included to represent shifts. DRF was modeled using sixth-degree polynomials with correlation coefficient 0.9997. The output was calculated using simple inputs such as unity, 1-D ramp, steps, and hat-function, with perturbation factors of: 0, +/−1, +/−2 and +/−5 mm. Discrepancies have been observed, but large fluctuations have been somewhat mitigated by aliasing arising from discrete diode placement. Conclusion: An analytical expression with perturbation factors was developed to estimate the impact of setup errors on an ArcCHECK phantom. Presently, this has been applied to simple functions, but is being developed to handle more realistic clinical cases

[en] Schild close-quote s null (tensionless) strings are discussed in certain flat and curved backgrounds. We find closed, stationary, null strings as natural configurations existing on the horizons of spacetimes which possess such null hypersurfaces. Examples of these are obtained in Schwarzschild and Rindler spacetimes. A dynamic null string is also identified in Rindler spacetime. Furthermore, a general prescription (with explicit examples) is outlined by means of which null string configurations can be obtained in a large class of cosmological backgrounds. copyright 1996 The American Physical Society

No abstract available

[en] The generalized Raychaudhuri equations derived by Capovilla and Guven are exclusively for extremal, timelike Nambu-Goto membranes. In this article, we construct the corresponding equations for string world sheets in the presence of a background Kalb-Ramond field. We analyze the full set of equations by concentrating on special cases in which the generalized shear or the generalized rotation or both are set to zero. If only the generalized shear is set to zero then it is possible to identify the components of the generalized rotation with the projections of the field strength of the Kalb-Ramond potential. copyright 1996 The American Physical Society

[en] Transparent gate structures were fabricated by electron beam evaporation of Sn-doped In/sub 2/O/sub 3/ on oxidized p-Si substrates. The samples were oxidized at 1100⁰C in dry oxygen. Admittance-voltage-frequency measurements were made under optical illumination. Interface state density distributions and hole and electron capture cross-sections were obtained using the recently developed optical metal-oxide-semiconductor (MOS) admittance technique. The experimental interface state density profile contained two peaked distributions, one near the valence band-edge E/sub v/, and the other near the conduction band-edge E/sub c/, overlying a concave background. The peak near E/sub c/ was sharper and the peak density was higher than is the case of the peak near E/sub v/. The capture cross-section vs band-gap energy profile also displayed a peaked distribution for interface states under each of the peaks. With increasing illumination, the state density at the peak increased, the peak energy location moved closer to the respective band-edge, and the capture cross-section decreased. The experimental results show the presence of defects at unpassivated Si-SiO/sub 2/ interfaces, which exchange electrons/holes with silicon bands under illumination

[en] The dynamics of two interacting Thirring fields has been investigated within the dimensional regularization framework. The coupling constants are renormalized in the same way as observed in the non-perturbative approach of Ansel'm et al (Sov. Phys. - JETP 36: 608 (1959)). Functionsβsub(i)(g₁, g₂, g₃) and γsub(i)(g₁, g₂, g₃), pertaining to the stability and anomalous behaviour of the problem, are computed up to a third order in the coupling parameters. With the help of these, subsidiary non-linear differential equations of the renormalization group are studied in 2-epsilon dimension. The results show up some peculiar features of the theory: a zero of βsub(i)(g₁, g₂, g₃) corresponding to g₂ approximately α√epsilon, a characteristic of phi theory. The scale invariant limit is reached when g₂ → 0 (i.e. the two Thirring fields are decoupled) and also when g₁ = xg₂ = g₃, where x is a root of 2x³ + 2x² - 1 = 0. The branch-point zero makes the transition to the epsilon tends to 0 limit non-unique. The anomalous dimensions are obtained and seen to match that of the Dashen-Frishman model (Phys. Lett.; 46B 439 (1973)). The existence of a non-trivial scale invariant limit distinguishes the model from many simple field theories. (author)

[en] In many plasma processing applications, like plasma immersion ion implantation (PIII), the substrate is immersed in low pressure plasma and is biased with negative voltage pulses. In typical PIII, the pulse duration is much larger than the ion response time, and hence the ion matrix sheath expands and ion implantation happens on the biased substrate. It is assumed that for pulse duration shorter than ion response times, the ions remain stationary and electrons are repelled by the negative bias. In the present investigation, the negative pulse duration is varied between ion and electron plasma response times; so as to study the electron behavior assuming ions are stationary. The results indicate that the electrons that are lost to the walls come from the ion matrix sheath and probably from the bulk plasma as well. The pulse duration, when it is less than the ion response time, plays a crucial role in determining the number of electrons lost to the walls

[en] Plasma response to high positive and negative voltage pulses is studied using pulsed capacitive excitation in a uniform and unmagnetized plasma. The positive or negative voltage pulse is applied to a metallic electrode, covered by a dielectric (Kapton) film, immersed in a low pressure argon plasma. The pulse magnitude is much greater than the electron temperature (U₀ >> kT_e/e). Experiments are carried out for different plasma parameters, to find how the plasma perturbations propagate for various applied pulse widths in comparison to ion plasma period ( f_i^-1). Plasma perturbations are studied by varying the thickness of the dielectric. For positive pulse bias, depending on the dielectric thickness, excitation of solitary electron holes, or solitary ion holes are observed. For negative pulse bias, varying the dielectric thicknesses, only ion rarefaction waves are excited.