89,405 research outputs found
Electronic oscillations in paired polyacetylene chains
An interacting pair of polyacetylene chains are initially modeled as a couple
of undimerized polymers described by a Hamiltonian based on the tight-binding
model representing the electronic behavior along the linear chain, plus a
Dirac's potential double well representing the interaction between the chains.
A theoretical field formalism is employed, and we find that the system exhibits
a gap in its energy band due to the presence of a mass-matrix term in the
Dirac's Lagrangian that describes the system. The Peierls instability is
introduced in the chains by coupling a scalar field to the fermions of the
theory via spontaneous symmetry breaking, to obtain a kink-like soliton, which
separates two vacuum regions, i.e., two spacial configurations (enantiomers) of
the each molecule. Since that mass-matrix and the pseudo-spin operator do not
commute in the same quantum representation, we demonstrate that there is a
particle oscillation phenomenon with a periodicity equivalent to the Bloch
oscillations.Comment: 4 pages, 1 figure.to appear in Solid State Communication
Crossovers between epigenesis and epigenetics. A multicenter approach to the history of epigenetics (1901-1975)
The origin of epigenetics has been traditionally traced back to Conrad Hal Waddington's foundational work in 1940s. The aim of the present paper is to reveal a hidden history of epigenetics, by means of a multicenter approach. Our analysis shows that genetics and embryology in early XX century--far from being non-communicating vessels--shared similar questions, as epitomized by Thomas Hunt Morgan's works. Such questions were rooted in the theory of epigenesis and set the scene for the development of epigenetics. Since the 1950s, the contribution of key scientists (Mary Lyon and Eduardo Scarano), as well as the discussions at the international conference of Gif-sur-Yvette (1957) paved the way for three fundamental shifts of focus: 1. From the whole embryo to the gene; 2. From the gene to the complex extranuclear processes of development; 3. From cytoplasmic inheritance to the epigenetics mechanisms
Discontinuous Transition in a Boundary Driven Contact Process
The contact process is a stochastic process which exhibits a continuous,
absorbing-state phase transition in the Directed Percolation (DP) universality
class. In this work, we consider a contact process with a bias in conjunction
with an active wall. This model exhibits waves of activity emanating from the
active wall and, when the system is supercritical, propagating indefinitely as
travelling (Fisher) waves. In the subcritical phase the activity is localised
near the wall. We study the phase transition numerically and show that certain
properties of the system, notably the wave velocity, are discontinuous across
the transition. Using a modified Fisher equation to model the system we
elucidate the mechanism by which the the discontinuity arises. Furthermore we
establish relations between properties of the travelling wave and DP critical
exponents.Comment: 14 pages, 9 figure
The development of technology for the electrodeposition of aluminum alloys and beryllium Final report
Technology development for electrodeposition of aluminum alloys and berylliu
Gauge Field Emergence from Kalb-Ramond Localization
A new mechanism, valid for any smooth version of the Randall-Sundrum model,
of getting localized massless vector field on the brane is described here. This
is obtained by dimensional reduction of a five dimension massive two form, or
Kalb-Ramond field, giving a Kalb-Ramond and an emergent vector field in four
dimensions. A geometrical coupling with the Ricci scalar is proposed and the
coupling constant is fixed such that the components of the fields are
localized. The solution is obtained by decomposing the fields in transversal
and longitudinal parts and showing that this give decoupled equations of motion
for the transverse vector and KR fields in four dimensions. We also prove some
identities satisfied by the transverse components of the fields. With this is
possible to fix the coupling constant in a way that a localized zero mode for
both components on the brane is obtained. Then, all the above results are
generalized to the massive form field. It is also shown that in general an
effective and forms can not be localized on the brane and we have
to sort one of them to localize. Therefore, we can not have a vector and a
scalar field localized by dimensional reduction of the five dimensional vector
field. In fact we find the expression which determines what forms
will give rise to both fields localized. For , as expected, this is valid
only for the KR field.Comment: Improved version. Some factors corrected and definitions added. The
main results continue vali
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