3,703 research outputs found
Scaling behavior of crystalline membranes: an -expansion approach
We study the scaling behavior of two-dimensional (2D) crystalline membranes
in the flat phase by a renormalization group (RG) method and an
-expansion. Generalization of the problem to non-integer dimensions,
necessary to control the -expansion, is achieved by dimensional
continuation of a well-known effective theory describing out-of-plane
fluctuations coupled to phonon-mediated interactions via a scalar composite
field, equivalent for small deformations to the local Gaussian curvature. The
effective theory, which will be referred to as Gaussian curvature interaction
(GCI) model, is equivalent to theories of elastic -dimensional manifolds
fluctuating in a -dimensional embedding space in the physical case
for arbitrary . For , instead, the GCI model is not
equivalent to a direct dimensional continuation of elastic membrane theory and
it defines an alternative generalization to generic internal dimension . We
calculate explicitly RG functions at two-loop order and determine the exponent
characterizing the long-wavelength scaling of correlation functions to
order in an -expansion. The self-consistent
screening approximation (SCSA) for the GCI model is shown to be exact to
O(). For , the O() correction is
suppressed by a small numerical prefactor. As a result, despite the large value
of , extrapolation of the first and second order results to leads to very close numbers, and . The
calculated exponent values are close to earlier reference results obtained by
non-perturbative RG, the SCSA and numerical simulations. These indications
suggest that a perturbative analysis of the GCI model could provide an useful
framework for accurate quantitative predictions of the scaling exponent even at
.Comment: 15 pages, 4 figure
Effect of dimensionality on the charge-density-wave in few-layers 2H-NbSe
We investigate the charge density wave (CDW) instability in single and double
layers, as well as in the bulk 2H-NbSe. We demonstrate that the density
functional theory correctly describes the metallic CDW state in the bulk
2H-NbSe. We predict that both mono- and bilayer NbSe undergo a CDW
instability. However, while in the bulk the instability occurs at a momentum
, in free-standing layers it
occurs at . Furthermore, while
in the bulk the CDW leads to a metallic state, in a monolayer the ground state
becomes semimetallic, in agreement with recent experimental data. We elucidate
the key role that an enhancement of the electron-phonon matrix element at
plays in forming the CDW ground state.Comment: 4 pages 5 figure
Effects of magnetism and doping on the electron-phonon coupling in BaFeAs
We calculate the effect of local magnetic moments on the electron-phonon
coupling in BaFeAs using the density functional perturbation
theory. We show that the magnetism enhances the total electron-phonon coupling
by , up to , still not enough to explain the
high critical temperature, but strong enough to have a non-negligible effect on
superconductivity, for instance, by frustrating the coupling with spin
fluctuations and inducing order parameter nodes. The enhancement comes mostly
from a renormalization of the electron-phonon matrix elements. We also
investigate, in the rigid band approximation, the effect of doping, and find
that versus doping does not mirror the behavior of the density of
states; while the latter decreases upon electron doping, the former does not,
and even increases slightly.Comment: 4 pages, 3 figure
Assaig sobre l'evolució del coneixement. Algunes relacions psicodinàmiques
El coneixement en l’ésser humà, en tant que espècie,apareixeria, en estrats successius de l’evolució, en forma decategories diferents de coneixement; les categories serien:fabril, artística, religiosa, filosòfica i científica, i eventualmentuna categoria emergent, intuïcional.L’assoliment i expressió de les categories tindria un aspectenominal i un altre factual (com en les fases de la libido). Lescategories de coneixement podrien explicar la disposiciódiferent dels individus, i estar en el substrat de les castessocials; també seria possible una transposició entre categoriesi estats evolutius, en termes psicodinàmics, de l’espèciehumana i l’individu humà
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