880,266 research outputs found
Test fixture insures high degree of accuracy in flexure tests
Modified die set improves accuracy in load application, minimizes problems of parallelism, and eliminates testing errors normally encountered during flexure tests. Test results are given for a comparison test of the old and new fixtures
Heterostructure unipolar spin transistors
We extend the analogy between charge-based bipolar semiconductor electronics
and spin-based unipolar electronics by considering unipolar spin transistors
with different equilibrium spin splittings in the emitter, base, and collector.
The current of base majority spin electrons to the collector limits the
performance of ``homojunction'' unipolar spin transistors, in which the
emitter, base, and collector all are made from the same magnetic material. This
current is very similar in origin to the current of base majority carriers to
the emitter in homojunction bipolar junction transistors. The current in
bipolar junction transistors can be reduced or nearly eliminated through the
use of a wide band gap emitter. We find that the choice of a collector material
with a larger equilibrium spin splitting than the base will similarly improve
the device performance of a unipolar spin transistor. We also find that a
graded variation in the base spin splitting introduces an effective drift field
that accelerates minority carriers through the base towards the collector.Comment: 9 pages, 2 figure
Centralizers of maximal regular subgroups in simple Lie groups and relative congruence classes of representations
In the paper we present a new, uniform and comprehensive description of
centralizers of the maximal regular subgroups in compact simple Lie groups of
all types and ranks. The centralizer is either a direct product of finite
cyclic groups, a continuous group of rank 1, or a product, not necessarily
direct, of a continuous group of rank 1 with a finite cyclic group. Explicit
formulas for the action of such centralizers on irreducible representations of
the simple Lie algebras are given.Comment: 27 page
Exactly-solvable problems for two-dimensional excitons
Several problems in mathematical physics relating to excitons in two
dimensions are considered. First, a fascinating numerical result from a
theoretical treatment of screened excitons stimulates a re-evaluation of the
familiar two-dimensional hydrogen atom. Formulating the latter problem in
momentum space leads to a new integral relation in terms of special functions,
and fresh insights into the dynamical symmetry of the system are also obtained.
A discussion of an alternative potential to model screened excitons is given,
and the variable phase method is used to compare bound-state energies and
scattering phase shifts for this potential with those obtained using the
two-dimensional analogue of the Yukawa potential. The second problem relates to
excitons in a quantising magnetic field in the fractional quantum Hall regime.
An exciton against the background of an incompressible quantum liquid is
modelled as a few-particle neutral composite consisting of a positively-charged
hole and several quasielectrons with fractional negative charge. A complete set
of exciton basis functions is derived, and these functions are classified using
a result from the theory of partitions. Some exact results are obtained for
this complex few-particle problem.Comment: 66 pages, 9 figure
Effective spacetime and Hawking radiation from moving domain wall in thin film of 3He-A
An event horizon for "relativistic" fermionic quasiparticles can be
constructed in a thin film of superfluid 3He-A. The quasiparticles see an
effective "gravitational" field which is induced by a topological soliton of
the order parameter. Within the soliton the "speed of light" crosses zero and
changes sign. When the soliton moves, two planar event horizons (black hole and
white hole) appear, with a curvature singularity between them. Aside from the
singularity, the effective spacetime is incomplete at future and past
boundaries, but the quasiparticles cannot escape there because the
nonrelativistic corrections become important as the blueshift grows, yielding
"superluminal" trajectories. The question of Hawking radiation from the moving
soliton is discussed but not resolved.Comment: revtex file, 4 pages, 2 figures, submitted to JETP Let
Nondegenerate three-dimensional complex Euclidean superintegrable systems and algebraic varieties
A classical (or quantum) second order superintegrable system is an integrable n-dimensional Hamiltonian system with potential that admits 2n−1 functionally independent second order constants of the motion polynomial in the momenta, the maximum possible. Such systems have remarkable properties: multi-integrability and multiseparability, an algebra of higher order symmetries whose representation theory yields spectral information about the Schrödinger operator, deep connections with special functions, and with quasiexactly solvable systems. Here, we announce a complete classification of nondegenerate (i.e., four-parameter) potentials for complex Euclidean 3-space. We characterize the possible superintegrable systems as points on an algebraic variety in ten variables subject to six quadratic polynomial constraints. The Euclidean group acts on the variety such that two points determine the same superintegrable system if and only if they lie on the same leaf of the foliation. There are exactly ten nondegenerate potentials. ©2007 American Institute of Physic
Directed Explicit Model Checking with HSF-SPIN
We present the explicit state model checker HSF-SPIN which is based on the model checker SPIN and its Promela modeling language. HSF-SPIN incorporates directed search algorithms for checking safety and a large class of LTL-specified liveness properties. We start off from the A* algorithm and define heuristics to accelerate the search into the direction of a specified failure situation. Next we propose an improved nested depth-first search algorithm that exploits the structure of Promela Never-Claims. As a result of both improvements, counterexamples will be shorter and the explored part of the state space will be smaller than with classical approaches, allowing to analyze larger state spaces. We evaluate the impact of the new heuristics and algorithms on a set of protocol models, some of which are real-world industrial protocols
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