145 research outputs found
Computing joint action costs: co-actors minimize the aggregate individual costs in an action sequence
Successful performance in cooperative activities relies on efficient task distribution between co-actors. Previous research found that people often forgo individual efficiency in favor of co-efficiency (i.e., joint-cost minimization) when planning a joint action. The present study investigated the cost computations underlying co-efficient decisions. We report a series of experiments that tested the hypothesis that people compute the joint costs of a cooperative action sequence by summing the individual action costs of their co-actor and themselves. We independently manipulated the parameters quantifying individual and joint action costs and tested their effects on decision-making by fitting and comparing Bayesian logistic regression models. Our hypothesis was confirmed: people weighed their own and their partner’s costs similarly to estimate the joint action costs as the sum of the two individual parameters. Participants minimized the aggregate cost to ensure co-efficiency. The results provide empirical support for behavioral economics and computational approaches that formalize cooperation as joint utility maximization based on a weighted sum of individual action costs
Isometric Embeddings and Noncommutative Branes in Homogeneous Gravitational Waves
We characterize the worldvolume theories on symmetric D-branes in a
six-dimensional Cahen-Wallach pp-wave supported by a constant Neveu-Schwarz
three-form flux. We find a class of flat noncommutative euclidean D3-branes
analogous to branes in a constant magnetic field, as well as curved
noncommutative lorentzian D3-branes analogous to branes in an electric
background. In the former case the noncommutative field theory on the branes is
constructed from first principles, related to dynamics of fuzzy spheres in the
worldvolumes, and used to analyse the flat space limits of the string theory.
The worldvolume theories on all other symmetric branes in the background are
local field theories. The physical origins of all these theories are described
through the interplay between isometric embeddings of branes in the spacetime
and the Penrose-Gueven limit of AdS3 x S3 with Neveu-Schwarz three-form flux.
The noncommutative field theory of a non-symmetric spacetime-filling D-brane is
also constructed, giving a spatially varying but time-independent
noncommutativity analogous to that of the Dolan-Nappi model.Comment: 52 pages; v2: References adde
Generalized Fock Spaces, New Forms of Quantum Statistics and their Algebras
We formulate a theory of generalized Fock spaces which underlies the
different forms of quantum statistics such as ``infinite'', Bose-Einstein and
Fermi-Dirac statistics. Single-indexed systems as well as multi-indexed systems
that cannot be mapped into single-indexed systems are studied. Our theory is
based on a three-tiered structure consisting of Fock space, statistics and
algebra. This general formalism not only unifies the various forms of
statistics and algebras, but also allows us to construct many new forms of
quantum statistics as well as many algebras of creation and destruction
operators. Some of these are : new algebras for infinite statistics,
q-statistics and its many avatars, a consistent algebra for fractional
statistics, null statistics or statistics of frozen order, ``doubly-infinite''
statistics, many representations of orthostatistics, Hubbard statistics and its
variations.Comment: This is a revised version of the earlier preprint: mp_arc 94-43.
Published versio
Femtosecond control of electric currents at the interfaces of metallic ferromagnetic heterostructures
The idea to utilize not only the charge but also the spin of electrons in the
operation of electronic devices has led to the development of spintronics,
causing a revolution in how information is stored and processed. A novel
advancement would be to develop ultrafast spintronics using femtosecond laser
pulses. Employing terahertz (10 Hz) emission spectroscopy, we
demonstrate optical generation of spin-polarized electric currents at the
interfaces of metallic ferromagnetic heterostructures at the femtosecond
timescale. The direction of the photocurrent is controlled by the helicity of
the circularly polarized light. These results open up new opportunities for
realizing spintronics in the unprecedented terahertz regime and provide new
insights in all-optical control of magnetism.Comment: 3 figures and 2 tables in the main tex
D-branes in PP-wave light cone string field theory
The cubic interaction vertex and the dynamical supercharges are constructed
for open strings ending on D7-branes, in light-cone superstring field theory in
PP-wave background. In this context, we write down the symmetry generators in
terms of the relevant group structure: SU(2) x SU(2) x SO(2) x SO(2),
originating from the eight transverse directions in the PP-wave background and
use the expressions to explicitly construct the vertex at the level of stringy
zero modes. The results are further generalized to include all the stringy
excitations as well.Comment: 30 pages, correction in eqn. (4.28), few equations (appendix),
Comments (p.17-18) and a reference (no. 58) added, typo is corrected in eqn.
(4.5
New half supersymmetric solutions of the heterotic string
We describe all supersymmetric solutions of the heterotic string which
preserve 8 supersymmetries and show that are distinguished by the holonomy,
, of the connection, , with skew-symmetric
torsion. The solutions are principal
bundles over a 4-dimensional hyper-K\"ahler manifold equipped with a
anti-self-dual connection and fibre group which has Lie algebra,
{\mathfrak Lie} (G)=\bR^{5,1}, \mathfrak{sl}(2,\bR)\oplus \mathfrak{su}(2)
or . Some of the solutions have the interpretation as 5-branes
wrapped on with transverse space any hyper-K\"ahler 4-dimensional manifold.
We construct new solutions for {\mathfrak Lie} (G)=\mathfrak{sl}(2,\bR)\oplus
\mathfrak{su}(2) and show that are characterized by 3 integers and have
continuous moduli. There is also a smooth family in this class with one
asymptotic region and the dilaton is bounded everywhere on the spacetime. We
also demonstrate that the worldvolume theory of the backgrounds with holonomy
SU(2) can be understood in terms of gauged WZW models for which the gauge
fields are composite. The {\rm hol}(\hat\nabla) \subseteq\bR^8 solutions are
superpositions of fundamental strings and pp-waves in flat space, which may
also include a null rotation. The heterotic
string backgrounds which preserve 8 supersymmetries are Lorentzian group
manifolds.Comment: 31 pages, minor corrections, analysis improved and more references
adde
Versatile Coordination of Cyclopentadienyl-Arene Ligands and Its Role in Titanium-Catalyzed Ethylene Trimerization
Cationic titanium(IV) complexes with ansa-(η5-cyclopentadienyl,η6-arene) ligands were synthesized and characterized by X-ray crystallography. The strength of the metal-arene interaction in these systems was studied by variable-temperature NMR spectroscopy. Complexes with a C1 bridge between the cyclopentadienyl and arene moieties feature hemilabile coordination behavior of the ligand and consequently are active ethylene trimerization catalysts. Reaction of the titanium(IV) dimethyl cations with CO results in conversion to the analogous cationic titanium(II) dicarbonyl species. Metal-to-ligand backdonation in these formally low-valent complexes gives rise to a strongly bonded, partially reduced arene moiety. In contrast to the η6-arene coordination mode observed for titanium, the more electron-rich vanadium(V) cations [cyclopentadienyl-arene]V(NiPr2)(NC6H4-4-Me)+ feature η1-arene binding, as determined by a crystallographic study. The three different metal-arene coordination modes that we experimentally observed model intermediates in the cycle for titanium-catalyzed ethylene trimerization. The nature of the metal-arene interaction in these systems was studied by DFT calculations.
T-folds, doubled geometry, and the SU(2) WZW model
The SU(2) WZW model at large level N can be interpreted semiclassically as
string theory on S^3 with N units of Neveu-Schwarz H-flux. While globally
geometric, the model nevertheless exhibits an interesting doubled geometry
possessing features in common with nongeometric string theory
compactifications, for example, nonzero Q-flux. Therefore, it can serve as a
fertile testing ground through which to improve our understanding of more
exotic compactifications, in a context in which we have a firm understanding of
the background from standard techniques. Three frameworks have been used to
systematize the study of nongeometric backgrounds: the T-fold construction,
Hitchin's generalized geometry, and fully doubled geometry. All of these double
the standard description in some way, in order to geometrize the combined
metric and Neveu Schwarz B-field data. We present the T-fold and fully doubled
descriptions of WZW models, first for SU(2) and then for general group.
Applying the formalism of Hull and Reid-Edwards, we indeed recover the physical
metric and H-flux of the WZW model from the doubled description. As additional
checks, we reproduce the abelian T-duality group and known semiclassical
spectrum of D-branes.Comment: 69 pages; uses amslatex; v4 minor revision
Magnetisation switching of FePt nanoparticle recording medium by femtosecond laser pulses
Manipulation of magnetisation with ultrashort laser pulses is promising for information storage device applications. The dynamics of the magnetisation response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage are FePt nanoparticles, for which switching of the magnetisation with optical angular momentum was demonstrated recently. The mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetisation switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to switch the magnetisation of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.EC under Contract No. 281043, FemtoSpin. The work at Greifswald University
was supported by the German research foundation (DFG), projects MU MU 1780/8-1, MU 1780/10-1. Research
at Göttingen University was supported via SFB 1073, Projects A2 and B1. Research at Uppsala University was
supported by the Swedish Research Council (VR), the Röntgen-Ångström Cluster, the Knut and Alice Wallenberg
Foundation (Contract No. 2015.0060), and Swedish National Infrastructure for Computing (SNIC). Research at
Kiel University was supported by the DFG, projects MC 9/9-2, MC 9/10-2. P.N. acknowledges support from EU
Horizon 2020 Framework Programme for Research and Innovation (2014-2020) under Grant Agreement No.
686056, NOVAMAG. The work in Konstanz was supported via the Center for Applied Photonics
Lectures on Branes in Curved Backgrounds
These lectures provide an introduction to the microscopic description of
branes in curved backgrounds. After a brief reminder of the flat space theory,
the basic principles and techniques of (rational) boundary conformal field
theory are presented in the second lecture. The general formalism is then
illustrated through a detailed discussion of branes on compact group manifolds.
In the final lecture, many more recent developments are reviewed, including
some results for non-compact target spaces.Comment: 109 pages, 11 figures, Lectures presented at the third RTN school on
`The Quantum Structure of Spacetime and the Geometric Nature of Fundamental
Interactions', Utrecht, January 200
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