651 research outputs found
Is innovation always beneficial? A meta-analysis of the relationship between innovation and performance in SMEs
The performance implications of innovation in small and medium-sized enterprises (SMEs) have attracted considerable interest among academics and practitioners. However, empirical research on the innovation–performance relationship in SMEs shows controversial results. This meta-analysis synthesizes empirical findings in order to obtain evidence whether and especially under which circumstances smaller, resource-scarce firms benefit from innovation. We find that innovation–performance relationship is context dependent. Factors such as the age of the firm, the type of innovation, and the cultural context affect the impact of innovation on firm performance to a large extent
Interacting Dark Sector and Precision Cosmology
We consider a recently proposed model in which dark matter interacts with a
thermal background of dark radiation. Dark radiation consists of relativistic
degrees of freedom which allow larger values of the expansion rate of the
universe today to be consistent with CMB data (-problem). Scattering
between dark matter and radiation suppresses the matter power spectrum at small
scales and can explain the apparent discrepancies between CDM
predictions of the matter power spectrum and direct measurements of Large Scale
Structure LSS (-problem). We go beyond previous work in two ways: 1.
we enlarge the parameter space of our previous model and allow for an arbitrary
fraction of the dark matter to be interacting and 2. we update the data sets
used in our fits, most importantly we include LSS data with full -dependence
to explore the sensitivity of current data to the shape of the matter power
spectrum. We find that LSS data prefer models with overall suppressed matter
clustering due to dark matter - dark radiation interactions over CDM
at 3-4 . However recent weak lensing measurements of the power spectrum
are not yet precise enough to clearly distinguish two limits of the model with
different predicted shapes for the linear matter power spectrum. In two
Appendices we give a derivation of the coupled dark matter and dark radiation
perturbation equations from the Boltzmann equation in order to clarify a
confusion in the recent literature, and we derive analytic approximations to
the solutions of the perturbation equations in the two physically interesting
limits of all dark matter weakly interacting or a small fraction of dark matter
strongly interacting.Comment: 29 pages + 2 Appendices; published versio
Doped carrier formulation and mean-field theory of the tt't''J model
In the generalized-tJ model the effect of the large local Coulomb repulsion
is accounted for by restricting the Hilbert space to states with at most one
electron per site. In this case the electronic system can be viewed in terms of
holes hopping in a lattice of correlated spins, where holes are the carriers
doped into the half-filled Mott insulator. To explicitly capture the interplay
between the hole dynamics and local spin correlations we derive a new
formulation of the generalized-tJ model where doped carrier operators are used
instead of the original electron operators. This ``doped carrier'' formulation
provides a new starting point to address doped spin systems and we use it to
develop a new, fully fermionic, mean-field description of doped Mott insulators
This mean-field approach reveals a new mechanism for superconductivity, namely
spinon-dopon mixing, and we apply it to the tt't''J model as of interest to
high-temperature superconductors. In particular, we use model parameters
borrowed from band calculations and from fitting ARPES data to obtain a
mean-field phase diagram that reproduces semi-quantitatively that of hole and
electron doped cuprates. The mean-field approach hereby presented accounts for
the local antiferromagnetic and d-wave superconducting correlations which, we
show, provide a rational for the role of t' and t'' in strengthening
superconductivity as expected by experiments and other theoretical approaches.
As we discuss how t, t' and t'' affect the phase diagram, we also comment on
possible scenarios to understand the differences between as-grown and oxygen
reduced electron doped samples.Comment: 17 pages, 2 figures. Homepage http://dao.mit.edu/~wen
Spin susceptibility and the pi-excitation in underdoped cuprates
The dynamical spin susceptibility chi'' at wave vector (pi, pi) and the
spectrum pi'' of the spin-triplet particle--particle excitation with center of
mass momentum (pi, pi) (pi-excitation) are considered in the slave-boson
formulation of the t--J-model. Propagators are calculated in a diagrammatic
t-matrix approximation in the d-wave superconducting state for a wide doping
range. The resulting spectra chi'' and pi'' both show a resonance at a doping
dependent energy, in qualitative agreement with recent numerical cluster
calculations. In underdoped systems, the peak position is comparable to that
found in neutron scattering experiments. The peak in chi'' as well as pi'' is
at low doping entirely caused by spin fluctuations, whereas the triplet
particle--particle channel does not contribute as a collective mode.Comment: 3 pages, 4 eps-figures included, uses revtex, eps
Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems
A new type of perturbation expansion in the mixing of localized orbitals
with a conduction-electron band in the Anderson model is
presented. It is built on Feynman diagrams obeying standard rules. The local
correlations of the unperturbed system (the atomic limit) are included exactly,
no auxiliary particles are introduced. As a test, an infinite-order ladder-type
resummation is analytically treated in the Kondo regime, recovering the correct
energy scale. An extension to the Anderson-lattice model is obtained via an
effective-site approximation through a cumulant expansion in on the
lattice. Relation to treatments in infinite spatial dimensions are indicated.Comment: selfextracting postscript file containing entire paper (10 pages)
including 3 figures, in case of trouble contact author for LaTeX-source or
hard copies (prep0994
Short-lived brominated hydrocarbons – observations in the source regions and the tropical tropopause layer
We conducted measurements of the five important short-lived organic bromine species in the marine boundary layer (MBL). Measurements were made in the Northern Hemisphere mid-latitudes (Sylt Island, North Sea) in June 2009 and in the tropical Western Pacific during the TransBrom ship campaign in October 2009. For the one-week time series on Sylt Island, mean mixing ratios of CHBr3, CH2Br2, CHBr2Cl and CH2BrCl were 2.0, 1.1, 0.2, 0.1 ppt, respectively. We found maxima of 5.8 and 1.6 ppt for the two main components CHBr3 and CH2Br2. Along the cruise track in the Western Pacific (between 41° N and 13° S) we measured mean mixing ratios of 0.9, 0.9, 0.2, 0.1 and 0.1 ppt for CHBr3, CH2Br2, CHBrCl2, CHBr2Cl and CH2BrCl. Air samples with coastal influence showed considerably higher mixing ratios than the samples with open ocean origin. Correlation analyses of the two data sets yielded strong linear relationships between the mixing ratios of four of the five species (except for CH2BrCl). Using a combined data set from the two campaigns and a comparison with the results from two former studies, rough estimates of the molar emission ratios between the correlated substances were: 9/1/0.35/0.35 for CHBr3/CH2Br2/CHBrCl2/CHBr2Cl. Additional measurements were made in the tropical tropopause layer (TTL) above Teresina (Brazil, 5° S) in June 2008, using balloon-borne cryogenic whole air sampling technique. Near the level of zero clear-sky net radiative heating (LZRH) at 14.8 km about 2.25 ppt organic bromine was bound to the five short-lived species, making up 13% of total organic bromine (17.82 ppt). CH2Br2 (1.45 ppt) and CHBr3 (0.56 ppt) accounted for 90% of the budget of short-lived compounds in that region. Near the tropopause (at 17.5 km) organic bromine from these substances was reduced to 1.35 ppt, with 1.07 and 0.12 ppt attributed to CH2Br2 and CHBr3, respectively
Where is the pi particle?
We discuss the interplay of particle-particle and particle-hole spin-triplet
channels in high-T_c superconductors using a quasiparticle dispersion motivated
by angle-resolved photoemission. Within a generalized RPA, we find a well
defined antibound state of two holes, the pi resonance of Demler and Zhang, as
well as a bound state of a particle and a hole, the spin exciton. We show that
the energy of the pi resonance always exceeds 2 Delta, twice the maximum d-wave
gap, therefore the neutron resonance observed in the cuprates around energy
Delta is most likely a spin exciton. At the same time, we speculate that the pi
particle can exist at higher energies and might be observed in neutron
scattering around 100 meV.Comment: RevTeX, 5 pages, 4 eps figure
Magnetic Collective Mode Dispersion in High Temperature Superconductors
Recent neutron scattering experiments in the superconducting state of YBCO
have been interpreted in terms of a magnetic collective mode whose dispersion
relative to the commensurate wavevector has a curvature opposite in sign to a
conventional magnon dispersion. The purpose of this article is to demonstrate
that simple linear response calculations are in support of a collective mode
interpretation, and to explain why the dispersion has the curvature it does.Comment: 3 pages, revtex, 4 encapsulated postscript figure
Interlayer pair tunneling and gap anisotropy in YBaCuO
Recent ARPES measurement observed a large -axis gap anisotropy,
, in clean YBaCuO. This
indicates that some sub-dominant component may exist in the -wave
dominant gap. We propose that the interlayer pairing tunneling contribution can
be determined through the investigation of the order parameter anisotropy.
Their potentially observable features in transport and spin dynamics are also
studied.Comment: 4 pages, 3 figure
Predicting a Gapless Spin-1 Neutral Collective Mode branch for Graphite
Using the standard tight binding model of 2d graphite with short range
electron repulsion, we find a gapless spin-1, neutral collective mode branch
{\em below the particle-hole continuum} with energy vanishing linearly with
momenta at the and points in the BZ. This spin-1 mode has a wide
energy dispersion, 0 to and is not Landau damped. The `Dirac cone
spectrum' of electrons at the chemical potential of graphite generates our
collective mode; so we call this `spin-1 zero sound' of the `Dirac sea'.
Epithermal neutron scattering experiments, where graphite single crystals are
often used as analyzers (an opportunity for `self-analysis'!), and spin
polarized electron energy loss spectroscopy (SPEELS) can be used to confirm and
study our collective mode.Comment: 4 pages of LaTex file, 3 eps figure file
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