Super-Galilei Invariant Field Theories in 2+1 Dimensions

We extend the Galilei group of space-time transformations by gradation, construct interacting field-theoretic representations of this algebra, and show that non-relativistic Super-Chern-Simons theory is a special case. We also study the generalization to matrix valued fields, which are relevant to the formulation of superstring theory as a $1/N_c$ expansion of a field theory. We find that in the matrix case, the field theory is much more restricted by the supersymmetry.Comment: 25 pages + 2 postscript figures, phyzzx and psfig require

The Size of a Polymer of String-Bits: A Numerical Investigation

In string-bit models, string is described as a polymer of point-like constituents. We attempt to use string-bit ideas to investigate how the size of string is affected by string interactions in a non-perturbative context. Lacking adequate methods to deal with the full complications of bit rearrangement interactions, we study instead a simplified analog model with only ``direct'' potential interactions among the bits. We use the variational principle in an approximate calculation of the mean-square size of a polymer as a function of the number of constituents/bits for various interaction strengths g in three specific models.Comment: 14 pages, LaTeX, 9 postscript figure

Supersymmetric Quantum Mechanics for String-Bits

We develop possible versions of supersymmetric single particle quantum mechanics, with application to superstring-bit models in view. We focus principally on space dimensions $d=1,2,4,8$, the transverse dimensionalities of superstring in $3,4,6,10$ space-time dimensions. These are the cases for which ``classical'' superstring makes sense, and also the values of $d$ for which Hooke's force law is compatible with the simplest superparticle dynamics. The basic question we address is: When is it possible to replace such harmonic force laws with more general ones, including forces which vanish at large distances? This is an important question because forces between string-bits that do not fall off with distance will almost certainly destroy cluster decomposition. We show that the answer is affirmative for $d=1,2$, negative for $d=8$, and so far inconclusive for $d=4$.Comment: 17 pages, Late

Characterizing the chemical pathways for water formation -- A deep search for hydrogen peroxide

In 2011, hydrogen peroxide (HOOH) was observed for the first time outside the solar system (Bergman et al., A&A, 2011, 531, L8). This detection appeared a posteriori quite natural, as HOOH is an intermediate product in the formation of water on the surface of dust grains. Following up on this detection, we present a search for HOOH in a diverse sample of sources in different environments, including low-mass protostars and regions with very high column densities, such as Infrared Dark Clouds (IRDCs). We do not detect the molecule in any other source than Oph A, and derive 3$\sigma$ upper limits for the abundance of HOOH relative to H$_2$ lower than in Oph A for most sources. This result sheds a different light on our understanding of the detection of HOOH in Oph A, and shifts the puzzle to why this source seems to be special. Therefore we rediscuss the detection of HOOH in Oph A, as well as the implications of the low abundance of HOOH, and its similarity with the case of O$_2$. Our chemical models show that the production of HOOH is extremely sensitive to the temperature, and favored only in the range 20$-$30 K. The relatively high abundance of HOOH observed in Oph A suggests that the bulk of the material lies at a temperature in the range 20$-$30 K.Comment: 18 pages, 3 figures, invited refereed paper at the Faraday Discussion 16

Deconfinement Phase Transition in Hot and Dense QCD at Large N

We conjecture that the confinement- deconfinement phase transition in QCD at large number of colors $N$ and $N_f\ll N$ at $T\neq 0$ and $\mu\neq 0$ is triggered by the drastic change in $\theta$ behavior. The conjecture is motivated by the holographic model of QCD where confinement -deconfinement phase transition indeed happens precisely at $T=T_c$ where $\theta$ dependence experiences a sudden change in behavior. The conjecture is also supported by quantum field theory arguments when the instanton calculations (which trigger the $\theta$ dependence) are under complete theoretical control for $T>T_c$, suddenly break down immediately below $T<T_c$ with sharp changes in the $\theta$ dependence. Finally, the conjecture is supported by a number of numerical lattice results. We employ this conjecture to study confinement -deconfinement phase transition of hot and dense QCD in large $N$ limit by analyzing the $\theta$ dependence. We estimate the critical values for $T_c$ and $\mu_c$ where the phase transition happens by approaching the critical values from the hot and/or dense regions where the instanton calculations are under complete theoretical control. We also describe some defects of various codimensions within a holographic model of QCD by focusing on their role around the phase transition point.Comment: Talk at the Workshop honoring 60th anniversary of Misha Shifma

Universality and Clustering in 1+1 Dimensional Superstring-Bit Models

We construct a 1+1 dimensional superstring-bit model for D=3 Type IIB superstring. This low dimension model escapes the problems encountered in higher dimension models: (1) It possesses full Galilean supersymmetry; (2) For noninteracting polymers of bits, the exactly soluble linear superpotential describing bit interactions is in a large universality class of superpotentials which includes ones bounded at spatial infinity; (3) The latter are used to construct a superstring-bit model with the clustering properties needed to define an $S$-matrix for closed polymers of superstring-bits.Comment: 11 pages, Latex documen

Magnetization dynamics in racetrack memory

Various devices have been proposed which use magnetic domain walls (DWs) in nanosized magnetic structures to perform logic operations or store information. In particular in ‘Racetrack memory’ bits of information represented by DWs are shifted in a magnetic wire to be stored. For these memory and logic devices to be successful, great control of DW motion is of vital importance. In cooperation with IBM’s Almaden research laboratory a pump-probe Kerr magnetooptical scanning microscope has been developed. In order to control DW injection, motion and reset, magnetic fields have to be applied locally on the nanowire. For this a special Damascene CMOS chip has been fabricated at the 200 mm wafer facility at IBM Microelectronics Research Laboratory (MRL). Probing of the local magnetization is done with a focused pulsed laser spot of 400 nm diameter where the polarization rotation caused by the Kerr effect is measured after reflection. In order to achieve optimal focusing a perpendicular incident laser beam is focused with a high numerical aperture objective. Synchronized ‘pumping’ in this scheme is achieved by successively: 1 injecting a DW; 2 propagate the DW down the nanowire with either current through or an applied field pulse over the nanowire; 3 and finally resetting the whole nanowire to its original magnetization by applying a large field together with the injection of an opposite magnetic domain. With this setup field and current induced DW motion is studied in permalloy nanowires ranging in width from 200 to 700 nm and thickness of 20 nm. For control of DWs in Racetrack memory it is important to understand the different mechanism for driving a DW already in motion (dynamic) and driving a DW that is currently at rest (static). The propagation field, the minimum field below which no DW motion takes place, is measured for both dynamic DWs and static DWs. It is found that Static DWs require a much higher field than DWs already in motion. A model is build where this effect is related to the wire roughness, successfully describing the existence of a propagation field, the difference between both propagation fields and a specific effect related to the method of injection. For Racetrack memory to be successful the critical current needs to be small (the current needed to move a DW solely by current) and the DW velocity high. Much of the influence of intrinsic magnetic properties of materials on DW dynamics is unknown. One important property affecting DW velocity and possibly also the critical current is Gilbert damping. Gilbert damping in permalloy can be tuned by doping the nanowires with osmium. This is used to prepare a sample series with increasing Gilbert damping. Measurement of the field induced DW velocity revealed a profile well known that includes the Walker breakdown (a maximum field where further increasing field strength does not further increase the DW velocity). From this profile the dependence of the Walker breakdown, DW mobility and maximum DW velocity on Gilbert damping has been determined. With the same sample series also current induced field assisted DW motion has been measured. Current induced DW motion is known to be driven by two effects: adiabatic and ballistic- spin momentum transfer (SMT) which relative contribution is parameterized by beta in the Landau Lifshitz Gilbert equation (LLG). Measurement of DW velocity depending on current density revealed the relative contribution of the two SMT schemes. Also the influence of Gilbert damping on the relative contribution of both schemes has been explored. A pronounced dependence of the measured spin torque efficiency on osmium concentration was found. This result may be interpreted as a sign that the intensively debated ratio ¿ / ¿ is far from constant over the range of ¿ studied

Magnetoresistance of Three-Constituent Composites: Percolation Near a Critical Line

Scaling theory, duality symmetry, and numerical simulations of a random network model are used to study the magnetoresistance of a metal/insulator/perfect conductor composite with a disordered columnar microstructure. The phase diagram is found to have a critical line which separates regions of saturating and non-saturating magnetoresistance. The percolation problem which describes this line is a generalization of anisotropic percolation. We locate the percolation threshold and determine the t = s = 1.30 +- 0.02, nu = 4/3 +- 0.02, which are the same as in two-constituent 2D isotropic percolation. We also determine the exponents which characterize the critical dependence on magnetic field, and confirm numerically that nu is independent of anisotropy. We propose and test a complete scaling description of the magnetoresistance in the vicinity of the critical line.Comment: Substantially revised version; description of behavior in finite magnetic fields added. 7 pages, 7 figures, submitted to PR

Space station integrated propulsion and fluid systems study

This Databook addresses the integration of fluid systems of the Space Station program. It includes a catalog of components required for the Space Station elements fluid systems and information on potential hardware commonality. The components catalog is in four parts. The first part lists the components defined for all the fluid systems identified in EP 2.1, Space Station Program Fluid Systems Configuration Databook. The components are cross-referenced in three sections. Section 2.1 lists the components by the fluid system in which they are used. Section 2.2 lists the components by type. Section 2.3 lists by the type of fluid media handled by the component. The next part of the catalog provides a description of the individual component. This section (2.4) is made up of data retrieved from Martin Marietta Denver Aerospace component data base. The third part is an assessment of propulsion hardware technology requirements. Section 2.5 lists components identified during the study as requiring development prior to flight qualification. Finally, Section 2.6 presents the results of the evaluation of commonality between components. The specific requirements of each component have been reviewed and duplication eliminated