700 research outputs found
A simple variational approach to the quantum Frenkel-Kontorova model
We present a simple and complete variational approach to the one-dimensional
quantum Frenkel-Kontorova model. Dirac's time-dependent variational principle
is adopted together with a Hatree-type many-body trial wavefunction for the
atoms. The single-particle state is assumed to have the Jackiw-Kerman form. We
obtain an effective classical Hamiltonian for the system which is simple enough
for a complete numerical solution for the static ground state of the model.
Numerical results show that our simple approach captures the essence of the
quantum effects first observed in quantum Monte Carlo studies.Comment: 12 pages, 2 figure
Composite absorbing potentials
The multiple scattering interferences due to the addition of several
contiguous potential units are used to construct composite absorbing potentials
that absorb at an arbitrary set of incident momenta or for a broad momentum
interval.Comment: 9 pages, Revtex, 2 postscript figures. Accepted in Phys. Rev. Let
Tunable linear and quadratic optomechanical coupling for a tilted membrane within an optical cavity: theory and experiment
We present an experimental study of an optomechanical system formed by a
vibrating thin semi-transparent membrane within a high-finesse optical cavity.
We show that the coupling between the optical cavity modes and the vibrational
modes of the membrane can be tuned by varying the membrane position and
orientation. In particular we demonstrate a large quadratic dispersive
optomechanical coupling in correspondence with avoided crossings between
optical cavity modes weakly coupled by scattering at the membrane surface. The
experimental results are well explained by a first order perturbation treatment
of the cavity eigenmodes.Comment: 10 pages, 6 figure
Time-Resolved Studies of Stick-Slip Friction in Sheared Granular Layers
Sensitive and fast force measurements are performed on sheared granular
layers undergoing stick-slip motion, along with simultaneous imaging. A full
study has been done for spherical particles with a +-20% size distribution.
Stick-slip motion due to repetitive fluidization of the layer occurs for low
driving velocities. Between major slip events, slight creep occurs that is
variable from one event to the next. The effects of changing the stiffness k
and velocity V of the driving system are studied in detail. The stick-slip
motion is almost periodic for spherical particles over a wide range of
parameters, but becomes irregular when k is large and V is relatively small. At
larger V, the motion becomes smoother and is affected by the inertia of the
upper plate bounding the layer. Measurements of the period T and amplitude A of
the relative motion are presented as a function of V. At a critical value Vc, a
transition to continuous sliding motion occurs that is discontinuous for k not
too large. The time dependence of the instantaneous velocity of the upper plate
and the frictional force produced by the granular layer are determined within
individual slipping events. The force is a multi-valued function of the
instantaneous velocity, with pronounced hysteresis and a sudden drop prior to
resticking. Measurements of vertical displacement reveal a small dilation of
the material (about one tenth of the mean particle size in a layer 20 particles
deep) associated with each slip event. Finally, optical imaging reveals that
localized microscopic rearrangements precede (and follow) each slip event. The
behavior of smooth particles is contrasted with that of rough particles.Comment: 20, pages, 17 figures, to appear in Phys. Rev.
A single-mode quantum transport in serial-structure geometric scatterers
We study transport in quantum systems consisting of a finite array of N
identical single-channel scatterers. A general expression of the S matrix in
terms of the individual-element data obtained recently for potential scattering
is rederived in this wider context. It shows in particular how the band
spectrum of the infinite periodic system arises in the limit . We
illustrate the result on two kinds of examples. The first are serial graphs
obtained by chaining loops or T-junctions. A detailed discussion is presented
for a finite-periodic "comb"; we show how the resonance poles can be computed
within the Krein formula approach. Another example concerns geometric
scatterers where the individual element consists of a surface with a pair of
leads; we show that apart of the resonances coming from the decoupled-surface
eigenvalues such scatterers exhibit the high-energy behavior typical for the
delta' interaction for the physically interesting couplings.Comment: 36 pages, a LaTeX source file with 2 TeX drawings, 3 ps and 3 jpeg
figures attache
Alternative oxidase-mediated respiration prevents lethal mitochondrial cardiomyopathy
Alternative oxidase (AOX) is a non-mammalian enzyme that can bypass blockade of the complex III-IV segment of the respiratory chain (RC). We crossed a Ciona intestinalis AOX transgene into RC complex III (cIII)-deficient Bcs1l(p.S78G) knock-in mice, displaying multiple visceral manifestations and premature death. The homozygotes expressing AOX were viable, and their median survival was extended from 210 to 590 days due to permanent prevention of lethal cardiomyopathy. AOX also prevented renal tubular atrophy and cerebral astrogliosis, but not liver disease, growth restriction, or lipodystrophy, suggesting distinct tissue-specific pathogenetic mechanisms. Assessment of reactive oxygen species (ROS) production and damage suggested that ROS were not instrumental in the rescue. Cardiac mitochondrial ultrastructure, mitochondrial respiration, and pathological transcriptome and metabolome alterations were essentially normalized by AOX, showing that the restored electron flow upstream of cIII was sufficient to prevent cardiac energetic crisis and detrimental decompensation. These findings demonstrate the value of AOX, both as a mechanistic tool and a potential therapeutic strategy, for cIII deficiencies.Peer reviewe
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