4,546 research outputs found
Many-Body Perturbation Theory (MBPT) and Time-Dependent Density-Functional Theory (TD-DFT): MBPT Insights About What is Missing in, and Corrections to, the TD-DFT Adiabatic Approximation
In their famous paper Kohn and Sham formulated a formally exact
density-functional theory (DFT) for the ground-state energy and density of a
system of interacting electrons, albeit limited at the time by certain
troubling representability questions. As no practical exact form of the
exchange-correlation (xc) energy functional was known, the xc-functional had to
be approximated, ideally by a local or semilocal functional. Nowadays however
the realization that Nature is not always so nearsighted has driven us up
Perdew's Jacob's ladder to find increasingly nonlocal density/wavefunction
hybrid functionals. Time-dependent (TD-) DFT is a younger development which
allows DFT concepts to be used to describe the temporal evolution of the
density in the presence of a perturbing field. Linear response (LR) theory then
allows spectra and other information about excited states to be extracted from
TD-DFT. Once again the exact TD-DFT xc-functional must be approximated in
practical calculations and this has historically been done using the TD-DFT
adiabatic approximation (AA) which is to TD-DFT very much like what the local
density approximation (LDA) is to conventional ground-state DFT. While some of
the recent advances in TD-DFT focus on what can be done within the AA, others
explore ways around the AA. After giving an overview of DFT, TD-DFT, and
LR-TD-DFT, this article will focus on many-body corrections to LR-TD-DFT as one
way to building hybrid density-functional/wavefunction methodology for
incorporating aspects of nonlocality in time not present in the AA.Comment: 56 pages, 17 figure
Nonlinear dynamics of flexural wave turbulence
The Kolmogorov-Zakharov spectrum predicted by the Weak Turbulence Theory
remains elusive for wave turbulence of flexural waves at the surface of an thin
elastic plate. We report a direct measurement of the nonlinear timescale
related to energy transfer between waves. This time scale is extracted
from the space-time measurement of the deformation of the plate by studying the
temporal dynamics of wavelet coefficients of the turbulent field. The central
hypothesis of the theory is the time scale separation between dissipative time
scale, nonlinear time scale and the period of the wave (). We
observe that this scale separation is valid in our system. The discrete modes
due to the finite size effects are responsible for the disagreement between
observations and theory. A crossover from continuous weak turbulence and
discrete turbulence is observed when the nonlinear time scale is of the same
order of magnitude as the frequency separation of the discrete modes. The
Kolmogorov-Zakharov energy cascade is then strongly altered and is frozen
before reaching the dissipative regime expected in the theory.Comment: accepted for publication in Physical Review
Exit times in non-Markovian drifting continuous-time random walk processes
By appealing to renewal theory we determine the equations that the mean exit
time of a continuous-time random walk with drift satisfies both when the
present coincides with a jump instant or when it does not. Particular attention
is paid to the corrections ensuing from the non-Markovian nature of the
process. We show that when drift and jumps have the same sign the relevant
integral equations can be solved in closed form. The case when holding times
have the classical Erlang distribution is considered in detail.Comment: 9 pages, 3 color plots, two-column revtex 4; new Appendix and
references adde
Parrondo-like behavior in continuous-time random walks with memory
The Continuous-Time Random Walk (CTRW) formalism can be adapted to encompass
stochastic processes with memory. In this article we will show how the random
combination of two different unbiased CTRWs can give raise to a process with
clear drift, if one of them is a CTRW with memory. If one identifies the other
one as noise, the effect can be thought as a kind of stochastic resonance. The
ultimate origin of this phenomenon is the same of the Parrondo's paradox in
game theoryComment: 8 pages, 3 figures, revtex; enlarged and revised versio
Un nuevo yacimiento de Vertebrados en el Burdigaliense de Martorell (Provincia de Barcelona)
Se describe en la nota presente un nuevo yacimiento inédito
del Burdigaliense del Vallés-Penedés, cerca de Martorell y de la falla con la cadena litoral. Se da un resumen de la fauna encontrada y un corte estratigráfico detallado de la nueva localidadPeer reviewe
A quantum gate array can be programmed to evaluate the expectation value of any operator
A programmable gate array is a circuit whose action is controlled by input
data. In this letter we describe a special--purpose quantum circuit that can be
programmed to evaluate the expectation value of any operator acting on a
space of states of dimensions. The circuit has a program register whose
state encodes the operator whose expectation value is to be
evaluated. The method requires knowledge of the expansion of in a basis of
the space of operators. We discuss some applications of this circuit and its
relation to known instances of quantum state tomography.Comment: 4 pages, 3 figures include
Is Dark Energy Dynamical? Prospects for an Answer
Recent data advances offer the exciting prospect of a first look at whether
dark energy has a dynamical equation of state or not. While formally theories
exist with a constant equation of state, they are nongeneric -- Einstein's
cosmological constant is a notable exception. So limits on the time variation,
w', directly tell us crucial physics. Two recent improvements in supernova data
from the Hubble Space Telescope allow important steps forward in constraining
the dynamics of dark energy, possessing the ability to exclude models with
w'\ga 1, if the universe truly has a cosmological constant. These data bring us
much closer to the ``systematics'' era, where further improvements will come
predominantly from more accurate, not merely more, observations. We examine the
possible gains and point out the complementary roles of space and ground based
observations in the near future. To achieve the leap to precision understanding
of dark energy in the next generation will require specially designed space
based measurements; we estimate the confidence level of detection of dynamics
(e.g. distinguishing between and ) will be ~1.8\sigma after the
ongoing generation, improving to more than 6.5\sigma in the dedicated space
generation.Comment: 6 pages, 2 figures; version accepted to Phys. Rev.
Sarcoma sinovial sobre grave traumatismo de pie
El sarcoma sinovial es una neoplasia mesenquimal maligna poco frecuente, apareciendo como una tumoración profunda de crecimiento lento, con nulo o escaso dolor y poca alteración funcional por lo que el diagnóstico suele ser tardÃo. El antecedente traumático de ha registrado en algún caso sin probar relación etiológica. Se presenta el caso clÃnico de un varón de 56 años de edad, que sufrió un grave traumatismo en los pies con fractura de ambos calcáneos y II, III y IV MTT pie derecho más luxación articulación metatarso falángica del primer dedo. Tras la intervención mediante enclavijado sin exposición de los focos de lesión, la evolución de las fracturas fue buena, observando a los diez meses una tumoración en el dorso del pie derecho diagnosticada como sarcoma sinovial por biopsia. Tras el estudio de extensión de la neoplasia de objetivó afectación multisitémica y a pesar de los tratamientos instaurados al paciente falleció tres meses después. Se discute la dificultad del diagnóstico diferencialSynovial sarcoma is an inusual mesenchymal neoplasm, appearing as a deep tumour of show growth, with none or rare pain and few functional disturbances. These features usually lead to a late diagnosis. Previous injury is present in some cases but there in no proven ethilogical relation. We present a male, age 56, who suffered a serious feet injury leading to fracture of both calcaneus and II, III and IV MTT right foot with dislocation MTT-falangic of first finger. After surgery with with closed Kirschner wires, the evolution was good. Ten months later a tumour appeared on the right foot, diagnosed as a synovial sarcoma after biopsy. The stating study showed multisystemic damage and the patient died three months in spite of treatment. We address the difficulty of the differential diagnose
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