36,845 research outputs found
RKKY interaction on the surface of three-dimensional Dirac semimetals
We study the RKKY interaction between two magnetic impurities located on the
surface of a three-dimensional Dirac semimetal with two Dirac nodes in the band
structure. By taking into account both bulk and surface contributions to the
exchange interaction between the localized spins, we demonstrate that the
surface contribution in general dominates the bulk one at distances larger than
the inverse node separation due to a weaker power-law decay. We find a strong
anisotropy of the surface term with respect to the spins being aligned along
the node separation axis or perpendicular to it. In the many impurity dilute
regime, this implies formation of quasi-one-dimensional magnetic stripes
orthogonal to the node axis. We also discuss the effects of a surface
spin-mixing term coupling electrons from spin-degenerate Fermi arcs.Comment: 7,5 pages, 3 figures (+4 pages of Appendixes
Carbon fiber composites for cryogenic filament-wound vessels
Advanced unidirectional and bidirectional carbon fiber/epoxy resin composites were evaluated for physical and mechanical properties over a cryogenic to room temperature range for potential application to cryogenic vessels. The results showed that Courtaulds HTS carbon fiber was the superior fiber in terms of cryogenic strength properties in epoxy composites. Of the resin systems tested in ring composites, CTBN/ERLB 4617 exhibited the highest composite strengths at cryogenic temperatures, but very low interlaminar shear strengths at room temperature. Tests of unidirectional and bidirectional composite bars showed that the Epon 828/Empol 1040 resin was better at all test temperatures. Neither fatigue cycling nor thermal shock had a significant effect on composite strengths or moduli. Thermal expansion measurements gave negative values in the fiber direction and positive values in the transverse direction of the composites
Discrimination and synthesis of recursive quantum states in high-dimensional Hilbert spaces
We propose an interferometric method for statistically discriminating between
nonorthogonal states in high dimensional Hilbert spaces for use in quantum
information processing. The method is illustrated for the case of photon
orbital angular momentum (OAM) states. These states belong to pairs of bases
that are mutually unbiased on a sequence of two-dimensional subspaces of the
full Hilbert space, but the vectors within the same basis are not necessarily
orthogonal to each other. Over multiple trials, this method allows
distinguishing OAM eigenstates from superpositions of multiple such
eigenstates. Variations of the same method are then shown to be capable of
preparing and detecting arbitrary linear combinations of states in Hilbert
space. One further variation allows the construction of chains of states
obeying recurrence relations on the Hilbert space itself, opening a new range
of possibilities for more abstract information-coding algorithms to be carried
out experimentally in a simple manner. Among other applications, we show that
this approach provides a simplified means of switching between pairs of
high-dimensional mutually unbiased OAM bases
Critical Flavor Number in the Three Dimensional Thirring Model
We present results of a Monte Carlo simulation of the three dimensional
Thirring model with the number of fermion flavors N_f varied between 2 and 18.
By identifying the lattice coupling at which the chiral condensate peaks,
simulations are be performed at couplings g^2(N_f) corresponding to the strong
coupling limit of the continuum theory. The chiral symmetry restoring phase
transition is studied as N_f is increased, and the critical number of flavors
estimated as N_{fc}=6.6(1). The critical exponents measured at the transition
do not agree with self-consistent solutions of the Schwinger-Dyson equations;
in particular there is no evidence for the transition being of infinite order.
Implications for the critical flavor number in QED_3 are briefly discussed.Comment: 4 pages, 5 figure
Hamilton's Turns for the Lorentz Group
Hamilton in the course of his studies on quaternions came up with an elegant
geometric picture for the group SU(2). In this picture the group elements are
represented by ``turns'', which are equivalence classes of directed great
circle arcs on the unit sphere , in such a manner that the rule for
composition of group elements takes the form of the familiar parallelogram law
for the Euclidean translation group. It is only recently that this construction
has been generalized to the simplest noncompact group , the double cover of SO(2,1). The present work develops a theory of
turns for , the double and universal cover of SO(3,1) and ,
rendering a geometric representation in the spirit of Hamilton available for
all low dimensional semisimple Lie groups of interest in physics. The geometric
construction is illustrated through application to polar decomposition, and to
the composition of Lorentz boosts and the resulting Wigner or Thomas rotation.Comment: 13 pages, Late
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