12,425 research outputs found
Electrical spin injection from an organic-based ferrimagnet in a hybrid organic/inorganic heterostructure
We report the successful extraction of spin polarized current from the
organic-based room temperature ferrimagnetic semiconductor V[TCNE]x (x~2, TCNE:
tetracyanoethylene; TC ~ 400 K, EG ~ 0.5 eV, s ~ 10-2 S/cm) and its subsequent
injection into a GaAs/AlGaAs light-emitting diode (LED). The spin current
tracks the magnetization of V[TCNE]x~2, is weakly temperature dependent, and
exhibits heavy hole / light hole asymmetry. This result has implications for
room temperature spintronics and the use of inorganic materials to probe spin
physics in organic and molecular systems
Explicit Zeta Functions for Bosonic and Fermionic Fields on a Noncommutative Toroidal Spacetime
Explicit formulas for the zeta functions corresponding to
bosonic () and to fermionic () quantum fields living on a
noncommutative, partially toroidal spacetime are derived. Formulas for the most
general case of the zeta function associated to a quadratic+linear+constant
form (in {\bf Z}) are obtained. They provide the analytical continuation of the
zeta functions in question to the whole complex plane, in terms of series
of Bessel functions (of fast, exponential convergence), thus being extended
Chowla-Selberg formulas. As well known, this is the most convenient expression
that can be found for the analytical continuation of a zeta function, in
particular, the residua of the poles and their finite parts are explicitly
given there. An important novelty is the fact that simple poles show up at
, as well as in other places (simple or double, depending on the number of
compactified, noncompactified, and noncommutative dimensions of the spacetime),
where they had never appeared before. This poses a challenge to the
zeta-function regularization procedure.Comment: 15 pages, no figures, LaTeX fil
Investment under ambiguity with the best and worst in mind
Recent literature on optimal investment has stressed the difference between the impact of risk and the impact of ambiguity - also called Knightian uncertainty - on investors' decisions. In this paper, we show that a decision maker's attitude towards ambiguity is similarly crucial for investment decisions. We capture the investor's individual ambiguity attitude by applying alpha-MEU preferences to a standard investment problem. We show that the presence of ambiguity often leads to an increase in the subjective project value, and entrepreneurs are more eager to invest. Thereby, our investment model helps to explain differences in investment behavior in situations which are objectively identical
Existential questions in (relatively) hyperbolic groups {\it and} Finding relative hyperbolic structures
This arXived paper has two independant parts, that are improved and corrected
versions of different parts of a single paper once named "On equations in
relatively hyperbolic groups".
The first part is entitled "Existential questions in (relatively) hyperbolic
groups". We study there the existential theory of torsion free hyperbolic and
relatively hyperbolic groups, in particular those with virtually abelian
parabolic subgroups. We show that the satisfiability of systems of equations
and inequations is decidable in these groups.
In the second part, called "Finding relative hyperbolic structures", we
provide a general algorithm that recognizes the class of groups that are
hyperbolic relative to abelian subgroups.Comment: Two independant parts 23p + 9p, revised. To appear separately in
Israel J. Math, and Bull. London Math. Soc. respectivel
Dynamical Determination of the Metric Signature in Spacetime of Nontrivial Topology
The formalism of Greensite for treating the spacetime signature as a
dynamical degree of freedom induced by quantum fields is considered for
spacetimes with nontrivial topology of the kind , for varying . It is shown that a dynamical origin for the Lorentzian
signature is possible in the five-dimensional space with small torus radius (periodic boundary conditions), as well as in
four-dimensional space with trivial topology. Hence, the possibility exists
that the early universe might have been of the Kaluza-Klein type, \ie
multidimensional and of Lorentzian signature.Comment: 10 pages, LaTeX file, 4 figure
Gravitational Waves from Mergin Compact Binaries: How Accurately Can One Extract the Binary's Parameters from the Inspiral Waveform?
The most promising source of gravitational waves for the planned detectors
LIGO and VIRGO are merging compact binaries, i.e., neutron star/neutron star
(NS/NS), neutron star/black hole (NS/BH), and black hole/black-hole (BH/BH)
binaries. We investigate how accurately the distance to the source and the
masses and spins of the two bodies will be measured from the gravitational wave
signals by the three detector LIGO/VIRGO network using ``advanced detectors''
(those present a few years after initial operation). The combination of the masses of the two bodies is
measurable with an accuracy . The reduced mass is measurable
to for NS/NS and NS/BH binaries, and for BH/BH
binaries (assuming BH's). Measurements of the masses and spins are
strongly correlated; there is a combination of and the spin angular
momenta that is measured to within . We also estimate that distance
measurement accuracies will be for of the detected
signals, and for of the signals, for the LIGO/VIRGO
3-detector network.Comment: 103 pages, 20 figures, submitted to Phys Rev D, uses revtex macros,
Caltech preprint GRP-36
Physically meaningful and not so meaningful symmetries in Chern-Simons theory
We explicitly show that the Landau gauge supersymmetry of Chern-Simons theory
does not have any physical significance. In fact, the difference between an
effective action both BRS invariant and Landau supersymmetric and an effective
action only BRS invariant is a finite field redefinition. Having established
this, we use a BRS invariant regulator that defines CS theory as the large mass
limit of topologically massive Yang-Mills theory to discuss the shift k \to
k+\cv of the bare Chern-Simons parameter in conncection with the Landau
supersymmetry. Finally, to convince ourselves that the shift above is not an
accident of our regularization method, we comment on the fact that all BRS
invariant regulators used as yet yield the same value for the shift.Comment: phyzzx, 21 pages, 2 figures in one PS fil
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