28,164 research outputs found
Universality in quantum chaos and the one parameter scaling theory
We adapt the one parameter scaling theory (OPT) to the context of quantum
chaos. As a result we propose a more precise characterization of the
universality classes associated to Wigner-Dyson and Poisson statistics which
takes into account Anderson localization effects. Based also on the OPT we
predict a new universality class in quantum chaos related to the
metal-insulator transition and provide several examples. In low dimensions it
is characterized by classical superdiffusion or a fractal spectrum, in higher
dimensions it can also have a purely quantum origin as in the case of
disordered systems. Our findings open the possibility of studying the metal
insulator transition experimentally in a much broader type of systems.Comment: 4 pages, 2 figures, acknowledgment added, typos correcte
On the role of the nonlocal Hartree-Fock exchange in ab initio quantum transport: Hydrogen in Platinum nanocontacts revisited
We propose a practical way to overcome the ubiquitous problem of the
overestimation of the zero-bias and zero-temperature conductance, which is
associated to the use of local approximations to the exchange-correlation
functional in Density-Functional Theory when applied to quantum transport. This
is done through partial substitution of the local exchange term in the
functional by the nonlocal Hartree-Fock exchange. As a non-trivial example of
this effect we revisit the smallest molecular bridge studied so far: a Hydrogen
molecule placed in between Platinum nanocontacts. When applied to this system
the value of the conductance diminishes as compared to the local-exchange-only
value, which is in close agreement with results predicted from Time-Dependent
Current-Density-Functional Theory. Our results issue a warning message on
recent claims of perfect transparency of a Hydrogen molecule in Platinum
nanocontacts
Plasmonic amplifier of the evanescent field of free electrons
We show experimentally for the first time that free electron evanescent fields can be amplified by a plasmonic nanolayer in much that same way as optical evanescent fields are amplified in the poor-man's super-lens
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.
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
Topological and Entanglement Properties of Resonating Valence Bond wavefunctions
We examine in details the connections between topological and entanglement
properties of short-range resonating valence bond (RVB) wave functions using
Projected Entangled Pair States (PEPS) on kagome and square lattices on
(quasi-)infinite cylinders with generalized boundary conditions (and perimeters
with up to 20 lattice spacings). Making use of disconnected topological sectors
in the space of dimer lattice coverings, we explicitly derive (orthogonal)
"minimally entangled" PEPS RVB states. For the kagome lattice, we obtain, using
the quantum Heisenberg antiferromagnet as a reference model, the finite size
scaling of the energy separations between these states. In particular, we
extract two separate (vanishing) energy scales corresponding (i) to insert a
vison line between the two ends of the cylinder and (ii) to pull out and freeze
a spin at either end. We also investigate the relations between bulk and
boundary properties and show that, for a bipartition of the cylinder, the
boundary Hamiltonian defined on the edge can be written as a product of a
highly non-local projector with an emergent (local) su(2)-invariant
one-dimensional (superfluid) t--J Hamiltonian, which arises due to the symmetry
properties of the auxiliary spins at the edge. This multiplicative structure, a
consequence of the disconnected topological sectors in the space of dimer
lattice coverings, is characteristic of the topological nature of the states.
For minimally entangled RVB states, it is shown that the entanglement spectrum,
which reflects the properties of the edge modes, is a subset (half for kagome
RVB) of the spectrum of the local Hamiltonian, providing e.g. a simple argument
on the origin of the topological entanglement entropy S0=-ln 2 of Z2 spin
liquids. We propose to use these features to probe topological phases in
microscopic Hamiltonians and some results are compared to existing DMRG data.Comment: 15 pages, 19 figures. Large extension of the paper. Finite size
scaling of the (topological) ground state energy splittings added (for the
Kagome quantum antiferromagnet
Probing the electron-phonon coupling in ozone-doped graphene by Raman spectroscopy
We have investigated the effects of ozone treatment on graphene by Raman
scattering. Sequential ozone short-exposure cycles resulted in increasing the
doping levels as inferred from the blue shift of the 2 and peak
frequencies, without introducing significant disorder. The two-phonon 2 and
2 Raman peak intensities show a significant decrease, while, on the
contrary, the one-phonon G Raman peak intensity remains constant for the whole
exposure process. The former reflects the dynamics of the photoexcited
electrons (holes) and, specifically, the increase of the electron-electron
scattering rate with doping. From the ratio of 2 to 2 intensities, which
remains constant with doping, we could extract the ratio of electron-phonon
coupling parameters. This ratio is found independent on the number of layers up
to ten layers. Moreover, the rate of decrease of 2 and 2 intensities
with doping was found to slowdown inversely proportional to the number of
graphene layers, revealing the increase of the electron-electron collision
probability
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
Precise dispersive data analysis of the f0(600) pole
We review how the use of recent precise data on kaon decays together with
forward dispersion relations (FDR) and Roy's equations allow us to determine
the sigma resonance pole position very precisely, by using only experimental
input. In addition, we present preliminary results for a modified set of
Roy-like equations with only one subtraction, that show a remarkable
improvement in the precision around the sigma region. We also improve the
matching between the parametrizations at low and intermediate energy of the S0
wave, and show that the effect of this on the sigma pole position is
negligible.Comment: 4 pages, 1 figure. To appear in the proceedings of the Meson 2008
conference, June 6-10, Cracow, Polan
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