65,368 research outputs found
Spin fluctuations with two-dimensional XY behavior in a frustrated S = 1/2 square-lattice ferromagnet
The spin dynamics of the layered square-lattice vanadate Pb2VO(PO4)2 is
investigated by electron spin resonance at various magnetic fields and at
temperatures above magnetic ordering. The linewidth divergence towards low
temperatures seems to agree with isotropic Heisenberg-type spin exchange
suggesting that the spin relaxation in this quasi-two dimensional compound is
governed by low-dimensional quantum fluctuations. However, a weak easy- plane
anisotropy of the g factor points to the presence of a planar XY type of
exchange. Indeed, we found that the linewidth divergence is described best by
XY-like spin fluctuations which requires a single parameter only. Therefore,
ESR-probed spin dynamics could establish Pb2VO(PO4)2 as the first frustrated
square lattice system with XY-inherent spin topological fluctuations.Comment: 5 pages, 3 figure
Massless and massive one-loop three-point functions in negative dimensional approach
In this article we present the complete massless and massive one-loop
triangle diagram results using the negative dimensional integration method
(NDIM). We consider the following cases: massless internal fields; one massive,
two massive with the same mass m and three equal masses for the virtual
particles. Our results are given in terms of hypergeometric and
hypergeometric-type functions of external momenta (and masses for the massive
cases) where the propagators in the Feynman integrals are raised to arbitrary
exponents and the dimension of the space-time D. Our approach reproduces the
known results as well as other solutions as yet unknown in the literature.
These new solutions occur naturally in the context of NDIM revealing a
promising technique to solve Feynman integrals in quantum field theories
CDO term structure modelling with Levy processes and the relation to market models
This paper considers the modelling of collateralized debt obligations (CDOs).
We propose a top-down model via forward rates generalizing Filipovi\'c,
Overbeck and Schmidt (2009) to the case where the forward rates are driven by a
finite dimensional L\'evy process. The contribution of this work is twofold: we
provide conditions for absence of arbitrage in this generalized framework.
Furthermore, we study the relation to market models by embedding them in the
forward rate framework in spirit of Brace, Gatarek and Musiela (1997).Comment: 16 page
Spin Effects in the Local Density of States of GaAs
We present spin-resolved measurements of the local density of states in Si
doped GaAs. Both spin components exhibit strong mesoscopic fluctuations. In the
magnetic quantum limit, the main features of the spin-up and spin-down
components of the local density of states are found to be identical apart from
Zeeman splitting. Based on this observation, we introduce a mesoscopic method
to measure the -factor in a material where macroscopic methods are severely
restricted by disorder. Differences between the spin-up and spin-down
components are discussed in terms of spin relaxation due to spin-orbit
coupling.Comment: 4 pages and 5 figure
Understanding co-operative R&D activity: evidence from four European countries
This paper investigates co-operative research activity by firms using data from
the 3rd Community Innovation Survey for four countries, France, Germany, Spain and the
UK. We build on the Cassiman and Veugelers (CV) (2002) study of Belgian manufacturing
firms, by incorporating information on the service sector, and considering the role of public
support in affecting firms’ decisions to co-operate. Our results support those in CV, in that
we find a positive relationship between the likelihood of undertaking co-operative R&D
and both incoming knowledge spillovers and the extent to which firms find strategic
methods important in appropriating the returns to innovative activity. We find that public
support is positively related to the probability of undertaking co-operative agreements
particularly with regard to the likelihood of co-operation with the research base. We find
some evidence, in particular for Spain, that firms carry out co-operative R&D to overcome
excessive perceived risks and financial constraints
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