4,438 research outputs found
Electronic Phase Separation Transition as the Origin of the Superconductivity and the Pseudogap Phase of Cuprates
We propose a new phase of matter, an electronic phase separation transition
that starts near the upper pseudogap and segregates the holes into high and low
density domains. The Cahn-Hilliard approach is used to follow quantitatively
this second order transition. The resulting grain boundary potential confines
the charge in domains and favors the development of intragrain superconducting
amplitudes. The zero resistivity transition arises only when the intergrain
Josephson coupling is of the order of the thermal energy and phase
locking among the superconducting grains takes place. We show that this
approach explains the pseudogap and superconducting phases in a natural way and
reproduces some recent scanning tunneling microscopy dataComment: 4 pages and 5 eps fig
Nonplanar integrability at two loops
In this article we compute the action of the two loop dilatation operator on
restricted Schur polynomials that belong to the su(2) sector, in the displaced
corners approximation. In this non-planar large N limit, operators that
diagonalize the one loop dilatation operator are not corrected at two loops.
The resulting spectrum of anomalous dimensions is related to a set of decoupled
harmonic oscillators, indicating integrability in this sector of the theory at
two loops. The anomalous dimensions are a non-trivial function of the 't Hooft
coupling, with a spectrum that is continuous and starting at zero at large N,
but discrete at finite N.Comment: version to appear in JHE
A Genetic Algorithm for Assembly Sequence Planning
This work presents a genetic algorithm for assembly sequence planning.
This problem is more difficult than other sequencing problems that have
already been tackled with success using these techniques, such as the classic
Traveling Salesperson Problem (TSP) or the Job Shop Scheduling Problem
(JSSP). It not only involves the arranging of tasks, as in those problems, but
also the selection of them from a set of alternative operations. Two families of
genetic operators have been used for searching the whole solution space. The
first includes operators that search for new sequences locally in a predetermined
assembly plan, that of parent chromosomes. The other family of operators introduces
new tasks in the solution, replacing others to maintain the validity of
chromosomes, and it is intended to search for sequences in other assembly
plans. Furthermore, some problem-based heuristics have been used for generating
the individuals in the population
Measurements and analysis of the upper critical field on an underdoped and overdoped compounds
The upper critical field is one of the many non conventional
properties of high- cuprates. It is possible that the
anomalies are due to the presence of inhomogeneities in the local charge
carrier density of the planes. In order to study this point, we
have prepared good quality samples of polycrystalline
using the wet-chemical method, which has demonstrated to produce samples with a
better cation distribution. In particular, we have studied the temperature
dependence of the second critical field, , through the magnetization
measurements on two samples with opposite average carrier concentration
() and nearly the same critical temperature, namely
(underdoped) and (overdoped). The results close to do not
follow the usual Ginzburg-Landau theory and are interpreted by a theory which
takes into account the influence of the inhomogeneities.Comment: Published versio
Effects of electron-phonon coupling range on the polaron formation
The polaron features due to electron-phonon interactions with different
coupling ranges are investigated by adopting a variational approach. The
ground-state energy, the spectral weight, the average kinetic energy, the mean
number of phonons, and the electron-lattice correlation function are discussed
for the system with coupling to local and nearest neighbor lattice
displacements comparing the results with the long range case. For large values
of the coupling with nearest neighbor sites, most physical quantities show a
strong resemblance with those obtained for the long range electron-phonon
interaction. Moreover, for intermediate values of interaction strength, the
correlation function between electron and nearest neighbor lattice
displacements is characterized by an upturn as function of the electron-phonon
coupling constant.Comment: 5 pages and 4 figure
Nonrelativistic Quantum Analysis of the Charged Particle-Dyon System on a Conical Spacetime
In this paper we develop the nonrelativistic quantum analysis of the charged
particle-dyon system in the spacetime produced by an idealized cosmic string.
In order to do that, we assume that the dyon is superposed to the cosmic
string. Considering this peculiar configuration {\it conical} monopole
harmonics are constructed, which are a generalizations of previous monopole
harmonics obtained by Wu and Yang(1976 {\it Nucl. Phys. B} {\bf 107} 365)
defined on a conical three-geometry. Bound and scattering wave functions are
explicitly derived. As to bound states, we present the energy spectrum of the
system, and analyze how the presence of the topological defect modifies
obtained result. We also analyze this system admitting the presence of an extra
isotropic harmonic potential acting on the particle. We show that the presence
of this potential produces significant changes in the energy spectrum of the
system.Comment: Paper accepted for publication in Classical and Quantum Gravit
When grassroots innovation movements encounter mainstream institutions: implications for models of inclusive innovation
Grassroots innovation movements (GIMs) can be regarded as initiators or advocates of alternative pathways of innovation. Sometimes these movements engage with more established science, technology and innovation (STI) institutions and development agencies in pursuit of their goals. In this paper, we argue that an important aspect to encounters between GIMs and mainstream STI institutions is the negotiation of different framings of grassroots innovation and development of policy models for inclusive innovation. These encounters can result in two different modes of engagement by GIMs; what we call insertion and mobilization. We illustrate and discuss these interrelated notions of framings and modes of engagement by drawing on three case studies of GIMs: the Social Technologies Network in Brazil, and the Honey Bee Network and People's Science Movements in India. The cases highlight that inclusion in the context of GIMs is not an unproblematic, smooth endeavour, and involves diverse interpretations and framings, which shape what and who gets included or excluded. Within the context of increasing policy interest, the analysis of encounters between GIMs and STI institutions can offer important lessons for the design of models of inclusive innovation and development
Polaron features for long-range electron-phonon interaction
The polaron features for long-range electron-phonon interaction are
investigated by extending a variational approach previously proposed for the
study of systems with local coupling. The ground-state spectral weight, the
average kinetic energy, the mean number of phonons, and the electron-lattice
correlation function are discussed for a wide range of model parameters
focusing on the adiabatic regime and comparing the results with the short-range
case (Holstein model). A strong mixing of electronic and phononic degrees of
freedom for small values of the electron-phonon coupling constant is found in
the adiabatic case due to the long-range interaction. Finally a polaron "phase
diagram" is proposed.Comment: 4 figs., to appear in J. Phys.:Condens. Matte
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