85,034 research outputs found
The Poisson geometry of SU(1,1)
We study the natural Poisson structure on the Lie group SU(1,1) and related
questions. In particular, we give an explicit description of the
Ginzburg-Weinstein isomorphism for the sets of admissible elements. We also
establish an analogue of Thompson's conjecture for this group.Comment: 11 pages, minor correction
Quantising Higher-spin String Theories
In this paper, we examine the conditions under which a higher-spin string
theory can be quantised. The quantisability is crucially dependent on the way
in which the matter currents are realised at the classical level. In
particular, we construct classical realisations for the algebra,
which is generated by a primary spin- current in addition to the
energy-momentum tensor, and discuss the quantisation for . From these
examples we see that quantum BRST operators can exist even when there is no
quantum generalisation of the classical algebra. Moreover, we find
that there can be several inequivalent ways of quantising a given classical
theory, leading to different BRST operators with inequivalent cohomologies. We
discuss their relation to certain minimal models. We also consider the
hierarchical embeddings of string theories proposed recently by Berkovits and
Vafa, and show how the already-known strings provide examples of this
phenomenon. Attempts to find higher-spin fermionic generalisations lead us to
examine the whether classical BRST operators for ( odd)
algebras can exist. We find that even though such fermionic algebras close up
to null fields, one cannot build nilpotent BRST operators, at least of the
standard form.Comment: CTP TAMU-24/94, KUL-TF-94/11, SISSA-135/94/E
CTMC calculations of electron capture and ionization in collisions of multiply charged ions with elliptical Rydberg atoms
We have performed classical trajectory Monte Carlo (CTMC) studies of electron
capture and ionization in multiply charged (Q=8) ion-Rydberg atom collisions at
intermediate impact velocities. Impact parallel to the minor and to the major
axis, respectively, of the initial Kepler electron ellipse has been
investigated. The important role of the initial electron momentum distribution
found for singly charged ion impact is strongly disminished for higher
projectile charge, while the initial spatial distribution remains important for
all values of Q studied.Comment: 3 pages, 5 figure
Stress-Induced Delamination Of Through Silicon Via Structures
Continuous scaling of on-chip wiring structures has brought significant challenges for materials and processes beyond the 32 nm technology node in microelectronics. Recently three-dimensional (3-D) integration with through-silicon-vias (TSVs) has emerged as an effective solution to meet the future interconnect requirement. Thermo-mechanical reliability is a key concern for the development of TSV structures used in die stacking as 3-D interconnects. This paper examines the effect of thermal stresses on interfacial reliability of TSV structures. First, the three-dimensional distribution of the thermal stress near the TSV and the wafer surface is analyzed. Using a linear superposition method, a semi-analytic solution is developed for a simplified structure consisting of a single TSV embedded in a silicon (Si) wafer. The solution is verified for relatively thick wafers by comparing to numerical results obtained by finite element analysis (FEA). Results from the stress analysis suggest interfacial delamination as a potential failure mechanism for the TSV structure. Analytical solutions for various TSV designs are then obtained for the steady-state energy release rate as an upper bound for the interfacial fracture driving force, while the effect of crack length is evaluated numerically by FEA. Based on these results, the effects of TSV designs and via material properties on the interfacial reliability are elucidated. Finally, potential failure mechanisms for TSV pop-up due to interfacial fracture are discussed.Aerospace Engineerin
Isothermal Shock Formation in Non-Equatorial Accretion Flows around Kerr Black Holes
We explore isothermal shock formation in non-equatorial, adiabatic accretion
flows onto a rotating black hole, with possible application to some active
galactic nuclei (AGNs). The isothermal shock jump conditions as well as the
regularity condition, previously developed for one-dimensional (1D) flows in
the equatorial plane, are extended to two-dimensional (2D), non-equatorial
flows, to explore possible geometrical effects. The basic hydrodynamic
equations with these conditions are self-consistently solved in the context of
general relativity to explore the formation of stable isothermal shocks. We
find that strong shocks are formed in various locations above the equatorial
plane, especially around a rapidly-rotating black hole with the prograde flows
(rather than a Schwarzschild black hole). The retrograde flows are generally
found to develop weaker shocks. The energy dissipation across the shock in the
hot non-equatorial flows above the cooler accretion disk may offer an
attractive illuminating source for the reprocessed features, such as the iron
fluorescence lines, which are often observed in some AGNs.Comment: 22 pages with 11 figures, presented at 5th international conference
on high energy density laboratory astrophysics in Tucson, Arizona. accepted
to Ap
Higher-spin strings and W minimal models
We study the spectrum of physical states for higher-spin generalisations of
string theory, based on two-dimensional theories with local spin-2 and spin-
symmetries. We explore the relation of the resulting effective Virasoro string
theories to certain minimal models. In particular, we show how the
highest-weight states of the minimal models decompose into Virasoro
primaries.Comment: 13 pages, CTP TAMU-43/93, KUL-TF-93/9
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