499 research outputs found
Fundamental Differences in Mechanical Behavior between Two Types of Crystals at the Nanoscale
We present differences in the mechanical behavior of nanoscale gold and molybdenum single crystals. A significant strength increase is observed as the size is reduced to 100 nm. Both nanocrystals exhibit discrete strain bursts during plastic deformation. We postulate that they arise from significant differences in the dislocation behavior. Dislocation starvation is the predominant mechanism of plasticity in nanoscale fcc crystals, while junction formation and hardening characterize bcc plasticity. A statistical analysis of strain bursts is performed as a function of size and compared with stochastic models
Wick's Theorem and a New Perturbation Theory Around the Atomic Limit of Strongly Correlated Electron Systems
A new type of perturbation expansion in the mixing of localized orbitals
with a conduction-electron band in the Anderson model is
presented. It is built on Feynman diagrams obeying standard rules. The local
correlations of the unperturbed system (the atomic limit) are included exactly,
no auxiliary particles are introduced. As a test, an infinite-order ladder-type
resummation is analytically treated in the Kondo regime, recovering the correct
energy scale. An extension to the Anderson-lattice model is obtained via an
effective-site approximation through a cumulant expansion in on the
lattice. Relation to treatments in infinite spatial dimensions are indicated.Comment: selfextracting postscript file containing entire paper (10 pages)
including 3 figures, in case of trouble contact author for LaTeX-source or
hard copies (prep0994
Where is the pi particle?
We discuss the interplay of particle-particle and particle-hole spin-triplet
channels in high-T_c superconductors using a quasiparticle dispersion motivated
by angle-resolved photoemission. Within a generalized RPA, we find a well
defined antibound state of two holes, the pi resonance of Demler and Zhang, as
well as a bound state of a particle and a hole, the spin exciton. We show that
the energy of the pi resonance always exceeds 2 Delta, twice the maximum d-wave
gap, therefore the neutron resonance observed in the cuprates around energy
Delta is most likely a spin exciton. At the same time, we speculate that the pi
particle can exist at higher energies and might be observed in neutron
scattering around 100 meV.Comment: RevTeX, 5 pages, 4 eps figure
Magnetic Collective Mode Dispersion in High Temperature Superconductors
Recent neutron scattering experiments in the superconducting state of YBCO
have been interpreted in terms of a magnetic collective mode whose dispersion
relative to the commensurate wavevector has a curvature opposite in sign to a
conventional magnon dispersion. The purpose of this article is to demonstrate
that simple linear response calculations are in support of a collective mode
interpretation, and to explain why the dispersion has the curvature it does.Comment: 3 pages, revtex, 4 encapsulated postscript figure
In-Situ Investigation of Plasticity at Nano-Scale
Mechanical behavior of crystals is dictated by dislocation motion in response to applied force. While it is extremely
difficult to directly observe the motion of individual dislocations, several correlations can be made between the
microscopic stress-strain behavior and dislocation activity. Here, we present for the first time the differences observed
between mechanical behavior in two fundamental types of crystals: face-centered cubic, fcc (Au, Cu, AI, Ni, etc.) and
body-centered cubic, bcc (W, Cr, Mo, Nb, etc.) with sub-micron dimensions subjected to in-situ micro-compression in
SEM chamber. In a striking deviation from classical mechanics, there is a significant increase in strength as crystal size
is reduced to 100nm; however in gold crystals (fcc) the highest strength achieved represents 44% of its theoretical
strength while in molybdenum crystals (bcc) it is only 7%. Moreover, unlike in bulk where plasticity commences in a
smooth fashion, both nano-crystals exhibit numerous discrete strain bursts during plastic deformation. These remarkable
differences in mechanical response of fcc and bcc crystals to uniaxial micro-compression challenge the applicability of
conventional strain-hardening to nano-scale crystals. We postulate that they arise from significant differences in
dislocation behavior between fcc and bcc crystals at nanoscale and serve as the fundamental reason for the observed
differences in their plastic deformation. Namely, dislocation starvation is the predominant mechanism of plasticity in
nano-scale fcc crystals while junction formation and subsequent hardening characterize bcc plasticity, as confirmed by
the microstructural electron microscopy. Experimentally obtained stress-strain curves together with video frames during
deformation and cross-sectional TEM analysis are presented, and a statistical analysis of avalanche-like strain bursts is
performed for both crystals and compared with stochastic models
Predicting a Gapless Spin-1 Neutral Collective Mode branch for Graphite
Using the standard tight binding model of 2d graphite with short range
electron repulsion, we find a gapless spin-1, neutral collective mode branch
{\em below the particle-hole continuum} with energy vanishing linearly with
momenta at the and points in the BZ. This spin-1 mode has a wide
energy dispersion, 0 to and is not Landau damped. The `Dirac cone
spectrum' of electrons at the chemical potential of graphite generates our
collective mode; so we call this `spin-1 zero sound' of the `Dirac sea'.
Epithermal neutron scattering experiments, where graphite single crystals are
often used as analyzers (an opportunity for `self-analysis'!), and spin
polarized electron energy loss spectroscopy (SPEELS) can be used to confirm and
study our collective mode.Comment: 4 pages of LaTex file, 3 eps figure file
Interlayer pair tunneling and gap anisotropy in YBaCuO
Recent ARPES measurement observed a large -axis gap anisotropy,
, in clean YBaCuO. This
indicates that some sub-dominant component may exist in the -wave
dominant gap. We propose that the interlayer pairing tunneling contribution can
be determined through the investigation of the order parameter anisotropy.
Their potentially observable features in transport and spin dynamics are also
studied.Comment: 4 pages, 3 figure
Possible isotope effect on the resonance peak formation in high-T cuprates
Starting from the three-band Hubbard Hamiltonian we derive an effective
model including electron-phonon interaction of quasiparticles with
optical phonons. Within the effective Hamiltonian we analyze the influence of
electronic correlations and electron-phonon interaction on the dynamical spin
susceptibility in layered cuprates. We find a huge isotope effect on the
resonance peak in the magnetic spin susceptibility, ,
seen by inelastic neutron scattering. It results from both the electron-phonon
coupling and the electronic correlation effects taken into account beyond
random phase approximation(RPA) scheme. We find at optimal doping the isotope
coeffiecient which can be further tested
experimentally.Comment: revised version, new figure is added. Phys. Rev. B 69, 0945XX (2004);
in pres
Quantum Impurities and the Neutron Resonance Peak in : Ni versus Zn
The influence of magnetic (S=1) and nonmagnetic (S=0) impurities on the spin
dynamics of an optimally doped high temperature superconductor is compared in
two samples with almost identical superconducting transition temperatures:
YBa(CuNi)O (T=80 K) and
YBa(CuZn)O (T=78 K). In the Ni-substituted
system, the magnetic resonance peak (which is observed at E40 meV in
the pure system) shifts to lower energy with a preserved E/T ratio
while the shift is much smaller upon Zn substitution. By contrast Zn, but not
Ni, restores significant spin fluctuations around 40 meV in the normal state.
These observations are discussed in the light of models proposed for the
magnetic resonance peak.Comment: 3 figures, submitted to PR
El estudio de los riesgos con origen hidrológico en el su-reste de la Península Ibérica. Enfoque geográfico y plani-ficación territorial
This paper shows the current studies on hydrological risks in the outh-east Spain, a region specially producing hazard situations. This area is one of the most arid regions of Europe, characterized by extraordinary rains of high hourly intensity causing bulky discharges and overflows, but also by the installation of dense urban nucleus in coastal areas and fertile plains. The scanty one, or in occasions unfortunate, dialog between environment and man has done of this one an area of notable interest for numerous researchers on the risk processes associated with hydrological extreme events. The ephemeral flow regime and the hydraulic effects of the linear infrastructures and drainage works complicate the estimation of the flood discharges, increasing the above mentioned degree of complexity. There are several models, programs and projects focused on these questions, but a major knowledge is still absent on this type of risk processes, particularly in relation with the generation of flood areas. For it is necessary a cartographic information base of more detail and the incorporation of factors till now very little considered as those hydro-morphological or the indicators of vulnerability.El presente artículo muestra el estado de la cuestión en el análisis de los riesgos hidrológicos de una región natural española especialmente productora de situaciones de peligro: el Sureste de la Península Ibérica. Los graves impactos que este tipo de peligros naturales tienen en la economía, la sociedad y el medio ambiente de esta región hacen que el enfoque geográfico revista aquí especial importancia en el estudio de sus riesgos derivados, particularmente en relación con la evaluación y gestión de los mismos, y su indudable implicación en las políticas de planificación territorial. Ésta es una de las regiones más áridas de Europa, caracterizada por lluvias extraordinarias de alta intensidad horaria capaces provocar abultados caudales y desbordamientos de cauces, pero también por la instalación de núcleos densos de poblamiento en sus áreas litorales y de vega. El escaso, o en ocasiones desafortunado, diálogo entre me-dio natural y hombre ha hecho de ésta una zona de notable interés para gran número de investigadores en temas de riesgos asociados a fenómenos hidrológicos extremos. El carácter efímero de las corrientes y los efectos hidráulicos de las infraestructuras lineales y obras de drenaje complican la estimación de los cau-dales de avenida, aumentando dicho grado de complejidad. Aun cuando existen ya varios modelos, pro-gramas y proyectos enfocados a estas cuestiones todavía falta un mayor conocimiento sobre este tipo de procesos de riesgo, particularmente en relación con la generación de áreas inundables, para la que se ne-cesita una información cartográfica base de mayor detalle y la incorporación de factores hasta ahora muy poco considerados como los hidromorfológicos o los indicadores de vulnerabilidad
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