3,681 research outputs found
Carrier-density effects in many-polaron systems
Many-polaron systems with finite charge-carrier density are often encountered
experimentally. However, until recently, no satisfactory theoretical
description of these systems was available even in the framework of simple
models such as the one-dimensional spinless Holstein model considered here. In
this work, previous results obtained using numerical as well as analytical
approaches are reviewed from a unified perspective, focussing on spectral
properties which reveal the nature of the quasiparticles in the system. In the
adiabatic regime and for intermediate electron-phonon coupling, a
carrier-density driven crossover from a polaronic to a rather metallic system
takes place. Further insight into the effects due to changes in density is
gained by calculating the phonon spectral function, and the fermion-fermion and
fermion-lattice correlation functions. Finally, we provide strong evidence
against the possibility of phase separation.Comment: 13 pages, 6 figures, accepted for publication in J. Phys.: Condens.
Matter; final versio
Optical conductivity of polaronic charge carriers
The optical conductivity of charge carriers coupled to quantum phonons is
studied in the framework of the one-dimensional spinless Holstein model. For
one electron, variational diagonalisation yields exact results in the
thermodynamic limit, whereas at finite carrier density analytical
approximations based on previous work on single-particle spectral functions are
obtained. Particular emphasis is put on deviations from weak-coupling,
small-polaron or one-electron theories occurring at intermediate coupling
and/or finite carrier density. The analytical results are in surprisingly good
agreement with exact data, and exhibit the characteristic polaronic excitations
observed in experiments on manganites.Comment: 23 pages, 11 figure
Two electrons on a hypersphere: a quasi-exactly solvable model
We show that the exact wave function for two electrons, interacting through a
Coulomb potential but constrained to remain on the surface of a
-sphere (), is a polynomial in the
interelectronic distance for a countably infinite set of values of the
radius . A selection of these radii, and the associated energies, are
reported for ground and excited states on the singlet and triplet manifolds. We
conclude that the model bears the greatest similarity to normal
physical systems.Comment: 4 pages, 0 figur
Interplay of charge and spin correlations in nickel perovskites
Analyzing the motion of low--spin holes in a high--spin
background, we derive a sort of generalized t--J Hamiltonian for the planes of Sr--doped nickelates. In addition to the rather complex
carrier--spin and spin--spin couplings we take into account the coupling of the
doped holes to in--plane oxygen breathing modes by a Holstein--type interaction
term. Because of strong magnetic confinement effects the holes are nearly
entirely prelocalized and the electron--phonon coupling becomes much more
effective in forming polarons than in the isostructural cuprates. In the light
of recent experiments on we discuss how the variety of
the observed transport and charge/spin--ordering phenomena can be qualitatively
understood in terms of our model Hamiltonian.Comment: 2 pages, LTpaper.sty, Proc. XXI Int. Conf. on Low Temp. Phys. Prague
9
Invariance of the correlation energy at high density and large dimension in two-electron systems
We prove that, in the large-dimension limit, the high-density correlation
energy \Ec of two opposite-spin electrons confined in a -dimensional space
and interacting {\em via} a Coulomb potential is given by \Ec \sim -1/(8D^2)
for any radial confining potential . This result explains the observed
similarity of \Ec in a variety of two-electron systems in three-dimensional
space.Comment: 4 pages, 1 figure, to appear in Phys. Rev. Let
Phonon affected transport through molecular quantum dots
To describe the interaction of molecular vibrations with electrons at a
quantum dot contacted to metallic leads, we extend an analytical approach that
we previously developed for the many-polaron problem. Our scheme is based on an
incomplete variational Lang-Firsov transformation, combined with a perturbative
calculation of the electron-phonon self-energy in the framework of generalised
Matsubara functions. This allows us to describe the system at weak to strong
coupling and intermediate to large phonon frequencies. We present results for
the quantum dot spectral function and for the kinetic coefficient that
characterises the electron transport through the dot. With these results we
critically examine the strengths and limitations of our approach, and discuss
the properties of the molecular quantum dot in the context of polaron physics.
We place particular emphasis on the importance of corrections to the concept of
an antiadiabatic dot polaron suggested by the complete Lang-Firsov
transformation.Comment: 30 pages, 15 figures, revised version including new figure
Sub-picosecond compression by velocity bunching in a photo-injector
We present an experimental evidence of a bunch compression scheme that uses a
traveling wave accelerating structure as a compressor. The bunch length issued
from a laser-driven radio-frequency electron source was compressed by a factor
>3 using an S-band traveling wave structure located immediately downstream from
the electron source. Experimental data are found to be in good agreement with
particle tracking simulations.Comment: 19 pages, 9 figures, submitted to Phys. Rev. Spec. Topics A&
An ultrashort pulse ultra-violet radiation undulator source driven by a laser plasma wakefield accelerator
Narrow band undulator radiation tuneable over the wavelength range of 150–260 nm has been produced by short electron bunches from a 2 mm long laser plasma wakefield accelerator based on a 20 TW femtosecond laser system. The number of photons measured is up to 9 × 106 per shot for a 100 period undulator, with a mean peak brilliance of 1 × 1018 photons/s/mrad2/mm2/0.1% bandwidth. Simulations estimate that the driving electron bunch r.m.s. duration is as short as 3 fs when the electron beam has energy of 120–130 MeV with the radiation pulse duration in the range of 50–100 fs
Photoemission spectra of many-polaron systems
The cross over from low to high carrier densities in a many-polaron system is
studied in the framework of the one-dimensional spinless Holstein model, using
unbiased numerical methods. Combining a novel quantum Monte Carlo approach and
exact diagonalization, accurate results for the single-particle spectrum and
the electronic kinetic energy on fairly large systems are obtained. A detailed
investigation of the quality of the Monte Carlo data is presented. In the
physically most important adiabatic intermediate electron-phonon coupling
regime, for which no analytical results are available, we observe a
dissociation of polarons with increasing band filling, leading to normal
metallic behavior, while for parameters favoring small polarons, no such
density-driven changes occur. The present work points towards the inadequacy of
single-polaron theories for a number of polaronic materials such as the
manganites.Comment: 15 pages, 13 figures; final version, accepted for publication in
Phys. Rev.
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