21,850 research outputs found
Undecidability as solution to the problem of measurement: fundamental criterion for the production of events
In recent papers we put forth a new interpretation of quantum mechanics,
colloquially known as ``the Montevideo interpretation''. This interpretation is
based on taking into account fundamental limits that gravity imposes on the
measurement process. As a consequence one has that situations develop where a
reduction process is undecidable from an evolution operator. When such a
situation is achieved, an event has taken place. In this paper we sharpen the
definition of when and how events occur, more precisely we give sufficient
conditions for the occurrence of events. We probe the new definition in an
example. In particular we show that the concept of undecidability used is not
``FAPP'' (for all practical purposes), but fundamental.Comment: 10 pages, contributed to the Castagnino Festschrif
Episodic synchronization in dynamically driven neurons
We examine the response of type II excitable neurons to trains of synaptic
pulses, as a function of the pulse frequency and amplitude. We show that the
resonant behavior characteristic of type II excitability, already described for
harmonic inputs, is also present for pulsed inputs. With this in mind, we study
the response of neurons to pulsed input trains whose frequency varies
continuously in time, and observe that the receiving neuron synchronizes
episodically to the input pulses, whenever the pulse frequency lies within the
neuron's locking range. We propose this behavior as a mechanism of rate-code
detection in neuronal populations. The results are obtained both in numerical
simulations of the Morris-Lecar model and in an electronic implementation of
the FitzHugh-Nagumo system, evidencing the robustness of the phenomenon.Comment: 7 pages, 8 figure
InAs/InP single quantum wire formation and emission at 1.5 microns
Isolated InAs/InP self-assembled quantum wires have been grown using in situ
accumulated stress measurements to adjust the optimal InAs thickness. Atomic
force microscopy imaging shows highly asymmetric nanostructures with average
length exceeding more than ten times their width. High resolution optical
investigation of as-grown samples reveals strong photoluminescence from
individual quantum wires at 1.5 microns. Additional sharp features are related
to monolayer fluctuations of the two dimensional InAs layer present during the
early stages of the quantum wire self-assembling process.Comment: 4 pages and 3 figures submitted to Applied Physics Letter
Equivalence between various versions of the self-dual action of the Ashtekar formalism
Different aspects of the self-dual (anti-self-dual) action of the Ashtekar
canonical formalism are discussed. In particular, we study the equivalences and
differences between the various versions of such an action. Our analysis may be
useful for the development of an Ashtekar formalism in eight dimensions.Comment: 10 pages, Latex, minor correction
Experimental and numerical analysis of HPTE on mechanical properties of materials and strain distribution
High Pressure Torsion Extrusion (HPTE) is a novel technique which has been recently introduced to the society of Nano-SPD researchers. HPTE exploits the deformation mechanics of HPT but in a larger scale using rod-shape samples and is capable of applying high values of strain to materials in one pass. This research aims to evaluate the effect of HPTE on mechanical properties of materials and also to study the effect of geometry of HPTE die on strain distribution in deformed samples by using Finite Element Method (FEM). Commercial pure Aluminium AA1050 was used for experimental work; and eccentric dies with parallelmisaligned channels were developed for evaluation by numerical modelling. Results of this research will help us better understand the effect of process parameters and also geometry of the die on materials
Chiral phase transition in lattice QCD as a metal-insulator transition
We investigate the lattice QCD Dirac operator with staggered fermions at
temperatures around the chiral phase transition. We present evidence of a
metal-insulator transition in the low lying modes of the Dirac operator around
the same temperature as the chiral phase transition. This strongly suggests the
phenomenon of Anderson localization drives the QCD vacuum to the chirally
symmetric phase in a way similar to a metal-insulator transition in a
disordered conductor. We also discuss how Anderson localization affects the
usual phenomenological treatment of phase transitions a la Ginzburg-Landau.Comment: 7 pages, 6 figures, references added, typos corrected, journal
versio
Crossing the phantom divide in an interacting generalized Chaplygin gas
Unified generalized Chaplygin gas models assuming an interaction between dark
energy and dark matter fluids have been previously proposed. Following these
ideas, we consider a particular relation between dark densities, which allows
the possibility of a time varying equation of state for dark energy that
crosses the phantom divide at a recent epoch. Moreover, these densities decay
during all the evolution of the Universe, avoiding a Big Rip. We find also a
scaling solution, i.e. these densities are asymptotically proportional in the
future, which contributes to the solution of the coincidence problem.Comment: Improved version, 10 pages, 4 figures, References adde
Comment on "X-ray resonant scattering studies of orbital and charge ordering in Pr1-xCaxMnO3"
In a recent published paper [Phys. Rev. B 64, 195133 (2001)], Zimmermann et
al. present a systematic x-ray scattering study of charge and orbital ordering
phenomena in the Pr1-xCaxMnO3 series with x= 0.25, 0.4 and 0.5. They propose
that for Ca concentrations x=0.4 and 0.5, the appearance of (0, k+1/2, 0)
reflections are originated by the orbital ordering of the eg electrons in the
a-b plane while the (0, 2k+1, 0) reflections are due to the charge ordering
among the Mn3+ and Mn4+ ions. Moreover, for small Ca concentrations (x<0.3),
the orbital ordering is only considered and it occurs at (0, k, 0) reflections.
A rigorous analysis of all these resonance reflections will show the inadequacy
of the charge-orbital model proposed to explain the experimental results. In
addition, this charge-orbital model is highly inconsistent with the electronic
balance. On the contrary, these reflections can be easily understood as arising
from the anisotropy of charge distribution induced by the presence of local
distortions, i.e. due to a structural phase transition.Comment: 10 pages, 2 figures.To be published Phys. Rev.
Universal Conductance Distributions in the Crossover between Diffusive and Localization Regimes
The full distribution of the conductance in quasi-one-dimensional
wires with rough surfaces is analyzed from the diffusive to the localization
regime. In the crossover region, where the statistics is dominated by only one
or two eigenchannels, the numerically obtained P(G) is found to be independent
of the details of the system with the average conductance as the only
scaling parameter. For < e^2/h, P(G) is given by an essentially
``one-sided'' log-normal distribution. In contrast, for e^2/h <= 2e^2/h,
the shape of P(G) remarkable agrees with those predicted by random matrix
theory for two fluctuating transmission eigenchannels.Comment: Accepted for publication in Phys. Rev. Let
Magnetic and electronic Co states in layered cobaltate GdBaCo2O5.5-x
We have performed non-resonant x-ray diffraction, resonant soft and hard
x-ray magnetic diffraction, soft x-ray absorption and x-ray magnetic circular
dichroism measurements to clarify the electronic and magnetic states of the
Co3+ ions in GdBaCo2O5.5. Our data are consistent with a 3+ Py Co HS state at
the pyramidal sites and a 3+ Oc Co LS state at the octahedral sites. The
structural distortion, with a doubling of the a axis (2ap x 2ap x 2ap cell),
shows alternating elongations and contractions of the pyramids and indicates
that the metal-insulator transition is associated with orbital order in the t2g
orbitals of the 3+ Py Co HS state. This distortion corresponds to an
alternating ordering of xz and yz orbitals along the a and c axes for the 3+ Py
Co . The orbital ordering and pyramidal distortion lead to deformation of the
octahedra, but the 3+ Oc Co LS state does not allow an orbital order to occur
for the 3+ Oc Co ions. The soft x-ray magnetic diffraction results indicate
that the magnetic moments are aligned in the ab plane but are not parallel to
the crystallographic a or b axes. The orbital order and the doubling of the
magnetic unit cell along the c axis support a non-collinear magnetic structure.
The x-ray magnetic circular dichroism data indicate that there is a large
orbital magnetic contribution to the total ordered Co moment
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