442 research outputs found
Competition between Superconductivity and Charge Density Waves: the Role of Disorder
More recently STM experimets present firm evidence of some kind of charge
modulation in underdoped cuprates. The peculiar observations of the above
experiments are located in the so called pseudo-gap region of the phase
diagram, just over the superconducting-dome. The model that will be used
captures in a simple way the idea that the pseudo-gap phase is formed of bound
fermion pairs which are close to a CDW instability but generally do not have
long range order due to quenched disorder. Thus the charge degrees of freedom
will be modeled by an Ising order parameter in the presence of quenched
disorder, so representing a charge glassy phase. This glassy phase will be in
competition with a superconducting phase modeled by a complex order parameter.
As a first step we will study the model in one dimension without disorder to
familiarize with it and also to search for a possible explanation of the Giant
Proximity Effect (GPE). After that we will investigate numerically the ground
state properties of the 2-dimensional lattice model, focusing our attention
especially on the stiffness and magnetization of the system in the xy plane. We
will show that disorder can induce superconductivity in a CDW phase. This
picture is really interesting because it could show how an insulating system
can produce a superconducting phase thank to the interplay with impurites.Comment: 162 pages, PhD thesi
Elasticity and metastability limit in supercooled liquids: a lattice model
We present Monte Carlo simulations on a lattice system that displays a first
order phase transition between a disordered phase (liquid) and an ordered phase
(crystal). The model is augmented by an interaction that simulates the effect
of elasticity in continuum models. The temperature range of stability of the
liquid phase is strongly increased in the presence of the elastic interaction.
We discuss the consequences of this result for the existence of a kinetic
spinodal in real systems.Comment: 8 pages, 5 figure
Collective behaviour without collective order in wild swarms of midges
Collective behaviour is a widespread phenomenon in biology, cutting through a
huge span of scales, from cell colonies up to bird flocks and fish schools. The
most prominent trait of collective behaviour is the emergence of global order:
individuals synchronize their states, giving the stunning impression that the
group behaves as one. In many biological systems, though, it is unclear whether
global order is present. A paradigmatic case is that of insect swarms, whose
erratic movements seem to suggest that group formation is a mere epiphenomenon
of the independent interaction of each individual with an external landmark. In
these cases, whether or not the group behaves truly collectively is debated.
Here, we experimentally study swarms of midges in the field and measure how
much the change of direction of one midge affects that of other individuals. We
discover that, despite the lack of collective order, swarms display very strong
correlations, totally incompatible with models of noninteracting particles. We
find that correlation increases sharply with the swarm's density, indicating
that the interaction between midges is based on a metric perception mechanism.
By means of numerical simulations we demonstrate that such growing correlation
is typical of a system close to an ordering transition. Our findings suggest
that correlation, rather than order, is the true hallmark of collective
behaviour in biological systems.Comment: The original version has been split into two parts. This first part
focuses on order vs. correlation. The second part, about finite-size scaling,
will be included in a separate paper. 15 pages, 6 figures, 1 table, 5 video
Viscoelasticity and metastability limit in supercooled liquids
A supercooled liquid is said to have a kinetic spinodal if a temperature Tsp
exists below which the liquid relaxation time exceeds the crystal nucleation
time. We revisit classical nucleation theory taking into account the
viscoelastic response of the liquid to the formation of crystal nuclei and find
that the kinetic spinodal is strongly influenced by elastic effects. We
introduce a dimensionless parameter \lambda, which is essentially the ratio
between the infinite frequency shear modulus and the enthalpy of fusion of the
crystal. In systems where \lambda is larger than a critical value \lambda_c the
metastability limit is totally suppressed, independently of the surface
tension. On the other hand, if \lambda < \lambda_c a kinetic spinodal is
present and the time needed to experimentally observe it scales as
exp[\omega/(\lambda_c-\lambda)^2], where \omega is roughly the ratio between
surface tension and enthalpy of fusion
Finite-size scaling as a way to probe near-criticality in natural swarms
Collective behaviour in biological systems is often accompanied by strong
correlations. The question has therefore arisen of whether correlation is
amplified by the vicinity to some critical point in the parameters space.
Biological systems, though, are typically quite far from the thermodynamic
limit, so that the value of the control parameter at which correlation and
susceptibility peak depend on size. Hence, a system would need to readjust its
control parameter according to its size in order to be maximally correlated.
This readjustment, though, has never been observed experimentally. By gathering
three-dimensional data on swarms of midges in the field we find that swarms
tune their control parameter and size so as to maintain a scaling behaviour of
the correlation function. As a consequence, correlation length and
susceptibility scale with the system's size and swarms exhibit a near-maximal
degree of correlation at all sizes.Comment: Selected for Viewpoint in Physics; PRL Editor's Suggestio
Divergent Construction of Pyrazoles via Michael Addition of N‑Arylhydrazones to 1,2-Diaza-1,3-dienes
partially_open4noThe base (NaH)-promoted Michael addition of
N-arylhydrazones (AHs) with 1,2-diaza-1,3-dienes (DDs)
produces unprecedented β-azohydrazone adducts. Strategically,
the use of AHs as acyl anion equivalents (d1 synthon)
and DDs as α-electrophiles (a2 synthon) of carbonyl
compounds open the way to two important classes of pyrazole
compounds.embargoed_20160409Serena mantenuto; Fabio Mantellini; Orazio Attanasi; Gianfranco FaviMantenuto, Serena; Mantellini, Fabio; Attanasi, ORAZIO ANTONIO; Favi, Gianfranc
Efficacy of hyaluronate injections in rotator cuff disorders: a level-I meta-analysis
Background: Rotator cuff disease is the most common cause of shoulder pain and weakness. Conservative treatment is the first choice of shoulder pain management.
Viscosupplementation of hyaluronic acid (HA) seems to be effective for management of tendon disorders.
The objective of this study was to evaluate the scientific evidence reported in literature according to HA shoulder injection in rotator cuff disorders treatment.
Methods: An English-language systematic literature search was performed by two independent researchers; data sources included the following databases:
MEDLINE, Embase, CINAHL, Google scholar web, Ovid database, Physiotherapy Evidence Database (PEDro), and the Cochrane Library. We performed a broad
research for relevant study up to February 2017. Articles were included if they reported data on clinical and functional outcomes in patients who had undergone HA
injection for management of rotator cuff pathology compared to placebo, corticosteroid injection and/or physical therapies. Methodological quality was assessed with
the PEDro rating scale. The outcomes were improvement of symptoms (assessed by VAS scale) and shoulder function (assessed through DASH and ASES Score).
Results: 5 RCTs studies (990 patients) were pooled in the Meta-analysis. The PEDro rating scale ranged from 2 to 8. Two studies compared HA injection with
corticosteroid injections, patients were injected once a week for three weeks. Four studies compared HA injection with placebo injection, of which two used 3 weekly
injections and two used 5 weekly injections. Significant difference was found in pain reduction between HA and placebo group at 26 weeks follow-up (MD= -0.51,
95% CI -0.96 to -0.07), p=0.02.
Conclusion: HA injections might be a valuable safe alternative to other conservative methods for the treatment of rotator cuff disorders. Nowadays, few and low
quality randomized controlled trials have been published. Therefore, to reach an overall conclusion about the effect of HA injection in rotator cuff we need more high
quality studies.
Level of evidence: Therapeutic Level I. See Instructions for Authors for a complete description of levels of evidence.
Study design: Meta-analys
Protected superconductivity at the boundaries of charge-density-wave domains
Solid 4He may acquire superfluid characteristics due to the frustration of the solid phase at grain boundaries. Here, introducing a negative-U generalized Hubbard model and a coarse-grained semiclassical pseudospin model, we show that an analogous effect occurs in systems with competition among charge-density-waves (CDW) and superconductivity in the presence of disorder, as cuprate or dichalcogenide superconductors. The CDW breaks apart in domains with topologically protected filamentary superconductivity at the interfaces. Our transport measurements, carried out in underdoped La2-x Sr x CuO4, with the magnetic field acting as a control parameter, are shown to be in excellent agreement with our theoretical prediction. Assuming superconductivity and CDW phases have similar energies, at intermediate temperatures, the magnetic field drives the system from a fluctuating superconductor to a CDW as expected in the clean limit. Lowering the temperature, the expected clean quantum critical point is avoided and a filamentary phase appears, analogous to 'glassy' supersolid phenomena in 4He. The transition line ends at a second quantum critical point at high-fields. Within our scenario, the filamentary superconducting phase is parasitic with CDW and bulk superconducting phases playing the role of primary competing order parameters
α-Azido Ketones, Part 8: Base-Induced Coupling of α-Azido Ketones with a 1,2-Diaza-1,3-diene as a Michael Acceptor1
Carbanions generated from various acyclic and cyclic
α-azido ketones in the presence of bases were reacted with ethyl
3-[(carbamoylimino)amino]but-2-enoate as a Michael acceptor to
give the corresponding adducts. The adducts of acyclic azides were
unstable and eliminated hydrazoic acid to give the corresponding
ethyl 2-[1-[(carbamoylamino)imino]ethyl]-4-oxo-4-phenylbut-2-
enoates as (E,E/Z,E)-diastereomeric mixtures. The relative configuration of these diastereomers was determined by X-ray analysis.
Adducts of cyclic α-azido ketones were obtained in diastereomerically pure form, with the exception of 2-azidobenzosuberone
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