973 research outputs found
Signatures of polaronic charge ordering in optical and dc conductivity using dynamical mean field theory
We apply dynamical mean field theory to study a prototypical model that
describes charge ordering in the presence of both electron-lattice interactions
and intersite electrostatic repulsion between electrons. We calculate the
optical and d.c. conductivity, and derive approximate formulas valid in the
limiting electron-lattice coupling regimes. In the weak coupling regime, we
recover the usual behavior of charge density waves, characterized by a transfer
of spectral weight due to the opening of a gap in the excitation spectrum. In
the opposite limit of very strong electron-lattice coupling, instead, the
charge ordering transition is signaled by a global enhancement of the optical
absorption, with no appreciable spectral weight transfer. Such behavior is
related to the progressive suppression of thermally activated charge defects
taking place below the critical temperature. At intermediate values of the
coupling within the polaronic regime, a complex behavior is obtained where both
mechanisms of transfer and enhancement of spectral weight coexist.Comment: 1 figure added, illustrating the optical sum rul
Theory and Application of Nonlinear Wave Propagation Phenomena in Combined Reaction/Separation Processes
Reaction separation processes, reactive distillation, chromatographic reactor, equilibrium theory, nonlinear waves, process control, observer design, asymptoticaly exact input/output-linearizationMagdeburg, Univ., Fak. für Elektrotechnik und Informationstechnik, Diss., 2007von Stefan Grüne
Impurity Effects on Quantum Depinning of Commensurate Charge Density Waves
We investigate quantum depinning of the one-dimensional (1D) commensurate
charge-density wave (CDW) in the presence of one impurity theoretically.
Quantum tunneling rate below but close to the threshold field is calculated at
absolute zero temperature by use of the phase Hamiltonian within the WKB
approximation. We show that the impurity can induce localized fluctuation and
enhance the quantum depinning. The electric field dependence of the tunneling
rate in the presence of the impurity is different from that in its absence.Comment: 14 pages with 13 figures. Submitted to J. Phys. Soc. Jp
Competing charge density waves and temperature-dependent nesting in 2H-TaSe2
Multiple charge density wave (CDW) phases in 2H-TaSe2 are investigated by
high-resolution synchrotron x-ray diffraction. In a narrow temperature range
immediately above the commensurate CDW transition, we observe a multi-q
superstructure with coexisting commensurate and incommensurate order
parameters, clearly distinct from the fully incommensurate state at higher
temperatures. This multi-q ordered phase, characterized by a temperature
hysteresis, is found both during warming and cooling, in contrast to previous
reports. In the normal state, the incommensurate superstructure reflection
gives way to a broad diffuse peak that persists nearly up to room temperature.
Its position provides a direct and accurate estimate of the Fermi surface
nesting vector, which evolves non-monotonically and approaches the commensurate
position as the temperature is increased. This behavior agrees with our recent
observations of the temperature-dependent Fermi surface in the same compound
[Phys. Rev. B 79, 125112 (2009)]
Giant Kohn anomaly and the phase transition in charge density wave ZrTe_3
A strong Kohn anomaly in ZrTe_3 is identified in the mostly transverse
acoustic phonon branch along the modulation vector q_P with polarization along
the a* direction. This soft mode freezes to zero frequency at the transition
temperature T_P and the temperature dependence of the frequency is strongly
affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate
superstructure shows a power law scaling of the intensity and the correlation
length that is compatible with an order parameter of dimension n = 2.Comment: 4 pages, 4 figures. accepted at Phys. Rev. Let
Self Organization and a Dynamical Transition in Traffic Flow Models
A simple model that describes traffic flow in two dimensions is studied. A
sharp {\it jamming transition } is found that separates between the low density
dynamical phase in which all cars move at maximal speed and the high density
jammed phase in which they are all stuck. Self organization effects in both
phases are studied and discussed.Comment: 6 pages, 4 figure
On Retardation Effects in Space Charge Calculations Of High Current Electron Beams
Laser-plasma accelerators are expected to deliver electron bunches with high
space charge fields. Several recent publications have addressed the impact of
space charge effects on such bunches after the extraction into vacuum.
Artifacts due to the approximation of retardation effects are addressed, which
are typically either neglected or approximated. We discuss a much more
appropriate calculation for the case of laser wakefield acceleration with
negligible retardation artifacts due to the calculation performed in the mean
rest frame. This presented calculation approach also aims at a validation of
other simulation approaches
Density Waves in a Transverse Electric Field
In a quasi-one-dimensional conductor with an open Fermi surface, a Charge or
a Spin Density Wave phase can be destroyed by an electric field perpendicular
to the direction of high conductivity. This mechanism, due to the breakdown of
electron-hole symmetry, is very similar to the orbital destruction of
superconductivity by a magnetic field, due to time-reversal symmetry.Comment: 3 pages, Latex, 2 figures, to appear in Phys. Rev. B Rapid Com
Spin-Peierls Quantum Phase Transitions in Coulomb Crystals
The spin-Peierls instability describes a structural transition of a crystal
due to strong magnetic interactions. Here we demonstrate that cold Coulomb
crystals of trapped ions provide an experimental testbed in which to study this
complex many-body problem and to access extreme regimes where the instability
is triggered by quantum fluctuations alone. We present a consistent analysis
based on different analytical and numerical methods, and provide a detailed
discussion of its feasibility on the basis of ion-trap experiments. Moreover,
we identify regimes where this quantum simulation may exceed the power of
classical computers.Comment: slightly longer than the published versio
Strictly One-Dimensional Electron System in Au Chains on Ge(001) Revealed By Photoelectron K-Space Mapping
Atomic nanowires formed by Au on Ge(001) are scrutinized for the band
topology of the conduction electron system by k-resolved photoemission. Two
metallic electron pockets are observed. Their Fermi surface sheets form
straight lines without undulations perpendicular to the chains within
experimental uncertainty. The electrons hence emerge as strictly confined to
one dimension. Moreover, the system is stable against a Peierls distortion down
to 10 K, lending itself for studies of the spectral function. Indications for
unusually low spectral weight at the chemical potential are discussed.Comment: 4 pages, 4 figures - revised version with added Fig. 2e) and
additional reference
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