81,526 research outputs found
On the Coulomb interaction in chiral-invariant one-dimensional electron systems
We consider a one-dimensional electron system, suitable for the description
of the electronic correlations in a metallic carbon nanotube. Renormalization
group methods are used to study the low-energy behavior of the unscreened
Coulomb interaction between currents of well-defined chirality. In the limit of
a very large number n of subbands we find a strong renormalization of the Fermi
velocity, reminiscent of a similar phenomenon in the graphite sheet. For small
n or sufficiently low energy, the Luttinger liquid behavior takes over, with a
strong wavefunction renormalization leading to a vanishing quasiparticle
weight. Our approach is appropriate to study the crossover from two-dimensional
to one-dimensional behavior in carbon nanotubes of large radius.Comment: 8 pages, 2 figures, LaTeX, PACS: 71.27.+a, 73.20.D, 05.30.F
Crossover from marginal Fermi liquid to Luttinger liquid behavior in carbon nanotubes
We study graphene-based electron systems with long-range Coulomb interaction
by performing an analytic continuation in the number of dimensions. We
characterize in this way the crossover between the marginal Fermi liquid
behavior of a graphite layer and the Luttinger liquid behavior at . The
former persists for any dimension above . However, the proximity to the
fixed-point strongly influences the phenomenology of
quasi-onedimensional systems, giving rise to an effective power-law behavior of
observables like the density of states. This applies to nanotubes of large
radius, for which we predict a lower bound of the corresponding exponent that
turns out to be very close to the value measured in multi-walled nanotubes.Comment: 4 pages, 4 postscript figure
Building health research systems to achieve better health
Health research systems can link knowledge generation with practical concerns to improve health
and health equity. Interest in health research, and in how health research systems should best be
organised, is moving up the agenda of bodies such as the World Health Organisation. Pioneering
health research systems, for example those in Canada and the UK, show that progress is possible.
However, radical steps are required to achieve this. Such steps should be based on evidence not
anecdotes.
Health Research Policy and Systems (HARPS) provides a vehicle for the publication of research, and
informed opinion, on a range of topics related to the organisation of health research systems and
the enormous benefits that can be achieved. Following the Mexico ministerial summit on health
research, WHO has been identifying ways in which it could itself improve the use of research
evidence. The results from this activity are soon to be published as a series of articles in HARPS.
This editorial provides an account of some of these recent key developments in health research
systems but places them in the context of a distinguished tradition of debate about the role of
science in society. It also identifies some of the main issues on which 'research on health research'
has already been conducted and published, in some cases in HARPS. Finding and retaining adequate
financial and human resources to conduct health research is a major problem, especially in low and
middle income countries where the need is often greatest. Research ethics and agenda-setting that
responds to the demands of the public are issues of growing concern. Innovative and collaborative
ways are being found to organise the conduct and utilisation of research so as to inform policy, and
improve health and health equity. This is crucial, not least to achieve the health-related Millennium
Development Goals. But much more progress is needed. The editorial ends by listing a wide range
of topics related to the above priorities on which we hope to feature further articles in HARPS and
thus contribute to an informed debate on how best to achieve such progress
Free Cooling Phase-Diagram of Hard-Spheres with Short- and Long-Range Interactions
We study the stability, the clustering and the phase-diagram of free cooling
granular gases. The systems consist of mono-disperse particles with additional
non-contact (long-range) interactions, and are simulated here by the
event-driven molecular dynamics algorithm with discrete (short-range shoulders
or wells) potentials (in both 2D and 3D). Astonishingly good agreement is found
with a mean field theory, where only the energy dissipation term is modified to
account for both repulsive or attractive non-contact interactions. Attractive
potentials enhance cooling and structure formation (clustering), whereas
repulsive potentials reduce it, as intuition suggests. The system evolution is
controlled by a single parameter: the non-contact potential strength scaled by
the fluctuation kinetic energy (granular temperature). When this is small, as
expected, the classical homogeneous cooling state is found. However, if the
effective dissipation is strong enough, structure formation proceeds, before
(in the repulsive case) non-contact forces get strong enough to undo the
clustering (due to the ongoing dissipation of granular temperature). For both
repulsive and attractive potentials, in the homogeneous regime, the cooling
shows a universal behaviour when the (inverse) control parameter is used as
evolution variable instead of time. The transition to a non-homogeneous regime,
as predicted by stability analysis, is affected by both dissipation and
potential strength. This can be cast into a phase diagram where the system
changes with time, which leaves open many challenges for future research.Comment: 22 pages, 15 figure
Do the gravitational corrections to the beta functions of the quartic and Yukawa couplings have an intrinsic physical meaning?
We study the beta functions of the quartic and Yukawa couplings of General
Relativity and Unimodular Gravity coupled to the and Yukawa
theories with masses. We show that the General Relativity corrections to those
beta functions as obtained from the 1PI functional by using the standard MS
multiplicative renormalization scheme of Dimensional Regularization are gauge
dependent and, further, that they can be removed by a non-multiplicative,
though local, field redefinition. An analogous analysis is carried out when
General Relativity is replaced with Unimodular Gravity. Thus we show that any
claim made about the change in the asymptotic behaviour of the quartic and
Yukawa couplings made by General Relativity and Unimodular Gravity lack
intrinsic physical meaning.Comment: 6 pages, 7 figure
Consistent Approximations for the Optimal Control of Constrained Switched Systems
Though switched dynamical systems have shown great utility in modeling a
variety of physical phenomena, the construction of an optimal control of such
systems has proven difficult since it demands some type of optimal mode
scheduling. In this paper, we devise an algorithm for the computation of an
optimal control of constrained nonlinear switched dynamical systems. The
control parameter for such systems include a continuous-valued input and
discrete-valued input, where the latter corresponds to the mode of the switched
system that is active at a particular instance in time. Our approach, which we
prove converges to local minimizers of the constrained optimal control problem,
first relaxes the discrete-valued input, then performs traditional optimal
control, and then projects the constructed relaxed discrete-valued input back
to a pure discrete-valued input by employing an extension to the classical
Chattering Lemma that we prove. We extend this algorithm by formulating a
computationally implementable algorithm which works by discretizing the time
interval over which the switched dynamical system is defined. Importantly, we
prove that this implementable algorithm constructs a sequence of points by
recursive application that converge to the local minimizers of the original
constrained optimal control problem. Four simulation experiments are included
to validate the theoretical developments
Large N Effects and Renormalization of the Long-Range Coulomb Interaction in Carbon Nanotubes
We develop a dimensional regularization approach to deal with the low-energy
effects of the long-range Coulomb interaction in 1D electron systems. The
method allows us to avoid the infrared singularities arising from the
long-range Coulomb interaction at D = 1, providing at the same time insight
about the fixed-points of the theory. We show that the effect of increasing the
number N of subbands at the Fermi level is opposite to that of approaching the
bare Coulomb interaction in the limit D --> 1. Then, we devise a double scaling
limit, in which the large N effects are able to tame the singularities due to
the long-range interaction. Thus, regular expressions can be obtained for all
observables right at D = 1, bearing also a dependence o the doping level of the
system. Our results imply a variation with N in the value of the exponent for
the tunneling density of states, which is in fair agreement with that observed
in different transport experiments involving carbon nanotubes. As the doping
level is increased in nanotubes of large radius and multi-walled nanotubes, we
predict a significant reduction of order N^{-1/2} in the critical exponent of
the tunneling density of states.Comment: 16 pages, 5 figures, PACS codes: 73.40, 11.10.
Copper(I)-Phosphinite Complexes in Click Cycloadditions: Three-Component Reactions and Preparation of 5-Iodotriazoles
© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.The remarkable activity displayed by copper(I)–phosphinite complexes of general formula [CuBr(L)] in two challenging cycloadditions is reported: a) the one-pot azidonation/cycloaddition of boronic acids, NaN3, and terminal alkynes; b) the cycloaddition of azides and iodoalkynes. These air-stable catalysts led to very good results in both cases and the expected triazoles could be isolated in pure form under ‘Click-suitable’ conditions
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