109,865 research outputs found
Measurement of tan beta in associated t H^\pm Production in gamma gamma Collisions
The ratio of neutral Higgs field vacuum expectation values, tan beta, is one
of the most important parameters to determine in type-II Two-Higgs Doublet
Models (2HDM), specifically the Minimal Supersymmetric Standard Model (MSSM).
Assuming the energies and integrated luminosity of a future high energy e^+e^-
linear collider of sqrt{s}=500, 800, 1000, and 1500 GeV and L=1 ab^{-1} we show
that associated t H^+/- production in gamma gamma collisions can be used to
make an accurate determination of tan beta for low and high tan beta by
precision measurements of the gamma gamma -> H^+/- t + X cross section.Comment: 7 pages, 11 figures, uses REVTEX
Quantum computation in semiconductor quantum dots of electron-spin asymmetric anisotropic exchange
The universal quantum computation is obtained when there exists asymmetric
anisotropic exchange between electron spins in coupled semiconductor quantum
dots. The asymmetric Heisenberg model can be transformed into the isotropic
model through the control of two local unitary rotations for the realization of
essential quantum gates. The rotations on each qubit are symmetrical and depend
on the strength and orientation of asymmetric exchange. The implementation of
the axially symmetric local magnetic fields can assist the construction of
quantum logic gates in anisotropic coupled quantum dots. This proposal can
efficiently use each physical electron spin as a logical qubit in the universal
quantum computation.Comment: 4 pages, 1 figur
Generation of tunable Terahertz out-of-plane radiation using Josephson vortices in modulated layered superconductors
We show that a moving Josephson vortex in spatially modulated layered
superconductors generates out-of-plane THz radiation. Remarkably, the magnetic
and in-plane electric fields radiated are of the same order, which is very
unusual for any good-conducting medium. Therefore, the out-of-plane radiation
can be emitted to the vacuum without the standard impedance mismatch problem.
Thus, the proposed design can be more efficient for tunable THz emitters than
previous proposals, for radiation only propagating along the ab-plane.Comment: 7 pages, 1 figure. Phys. Rev. B (2005), in pres
Assessment of crystallographic influence on material properties of calcite brachiopods
Calcium carbonate biominerals are frequently analysed in materials science due to their abundance, diversity and unique material properties. Aragonite nacre is intensively studied, but less information is available about the material properties of biogenic calcite, despite its occurrence in a wide range of structures in different organisms. In particular, there is insufficient knowledge about how preferential crystallographic orientations influence these material properties. Here, we study the influence of crystallography on material properties in calcite semi-nacre and fibres of brachiopod shells using nano-indentation and electron backscatter diffraction (EBSD). The nano-indentation results show that calcite semi-nacre is a harder and stiffer (H {approx} 3ā5 GPa; E = 50ā85 GPa) biomineral structure than calcite fibres (H = 0.4ā3 GPa; E = 30ā60 GPa). The integration of EBSD to these studies has revealed a relationship between the crystallography and material properties at high spatial resolution for calcite semi-nacre. The presence of crystals with the c-axis perpendicular to the plane-of-view in longitudinal section increases hardness and stiffness. The present study determines how nano-indentation and EBSD can be combined to provide a detailed understanding of biomineral structures and their analysis for application in materials science
Interactions between unidirectional quantized vortex rings
We have used the vortex filament method to numerically investigate the
interactions between pairs of quantized vortex rings that are initially
traveling in the same direction but with their axes offset by a variable impact
parameter. The interaction of two circular rings of comparable radii produce
outcomes that can be categorized into four regimes, dependent only on the
impact parameter; the two rings can either miss each other on the inside or
outside, or they can reconnect leading to final states consisting of either one
or two deformed rings. The fraction of of energy went into ring deformations
and the transverse component of velocity of the rings are analyzed for each
regime. We find that rings of very similar radius only reconnect for a very
narrow range of the impact parameter, much smaller than would be expected from
geometrical cross-section alone. In contrast, when the radii of the rings are
very different, the range of impact parameters producing a reconnection is
close to the geometrical value. A second type of interaction considered is the
collision of circular rings with a highly deformed ring. This type of
interaction appears to be a productive mechanism for creating small vortex
rings. The simulations are discussed in the context of experiments on colliding
vortex rings and quantum turbulence in superfluid helium in the zero
temperature limit
Testing Cluster Structure of Graphs
We study the problem of recognizing the cluster structure of a graph in the
framework of property testing in the bounded degree model. Given a parameter
, a -bounded degree graph is defined to be -clusterable, if it can be partitioned into no more than parts, such
that the (inner) conductance of the induced subgraph on each part is at least
and the (outer) conductance of each part is at most
, where depends only on . Our main
result is a sublinear algorithm with the running time
that takes as
input a graph with maximum degree bounded by , parameters , ,
, and with probability at least , accepts the graph if it
is -clusterable and rejects the graph if it is -far from
-clusterable for , where depends only on . By the lower
bound of on the number of queries needed for testing graph
expansion, which corresponds to in our problem, our algorithm is
asymptotically optimal up to polylogarithmic factors.Comment: Full version of STOC 201
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