1,091 research outputs found
Charge-density-wave formation in the Edwards fermion-boson model at one-third band filling
We examine the ground-state properties of the one-dimensional Edwards
spinless fermion transport model by means of large-scale density-matrix
renormalization-group calculations. Determining the single-particle gap and the
Tomonaga-Luttinger liquid parameter () at zero temperature, we prove
the existence of a metal-to-insulator quantum phase transition at one-third
band filling. The insulator---established by strong correlation in the
background medium---typifies a charge density wave (CDW) that is commensurate
with the band filling. is very small at the quantum critical
point, and becomes in the infinitesimally doped
three-period CDW, as predicted by the bosonization approach.Comment: 6 pages, 3 figures, contributions to SCES 201
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
The pseudogap phase in (TaSe_4)_2I
We have developed the mean-field theory of coexisting charge-density waves
(CDW) and unconventional charge-density waves (UCDW). The double phase
transition manifests itself in the thermodynamic quantities and in the magnetic
response, such as spin susceptibility and spin-lattice relaxation rate. Our
theory applies to quasi-one dimensional (TaSe_4)_2I, where above the CDW
transition, thermal fluctuations die out rapidly, but robust pseudogap
behaviour is still detected. We argue, that the fluctuations are suppressed due
to UCDW, which partially gaps the Fermi surface, and causes non-Fermi-liquid
(pseudogap) behaviour.Comment: 7 pages, 6 figure
Elementary Excitations in One-Dimensional Electromechanical Systems; Transport with Back-Reaction
Using an exactly solvable model, we study low-energy properties of a
one-dimensional spinless electron fluid contained in a quantum-mechanically
moving wire located in a static magnetic field. The phonon and electric current
are coupled via Lorentz force and the eigenmodes are described by two
independent boson fluids. At low energies, the two boson modes are charged
while one of them has excitation gap due to back-reaction of the Lorentz force.
The theory is illustrated by evaluating optical absorption spectra. Our results
are exact and show a non-perturbative regime of electron transport
Mean-field quantum phase transition in graphene and in general gapless systems
We study the quantum critical properties of antiferromagnetism in graphene at
T=0 within mean-field (MF) theory. The resulting exponents differ from the
conventional MF exponents, describing finite temperature transitions. Motivated
by this, we have developed the MF theory of general gapless phases with density
of states rho(E) |E|^r, r>-1, with the interaction as control parameter. For
r>2, the conventional MF exponents \'a la Landau are recovered, while for
-1<r<2, the exponents vary significantly with r. The critical interaction is
finite for r>0, therefore no weak-coupling solution exists in this range. This
generalizes the results on quantum criticality of the gapless Kondo systems to
bulk correlated phases.Comment: 5 pages, 1 figure, 2 table
Boundary effect on CDW: Friedel oscillations, STM image
We study the effect of open boundary condition on charge density waves (CDW).
The electron density oscillates rapidly close to the boundary, and additional
non-oscillating terms (~ln(r)) appear. The Friedel oscillations survive beyond
the CDW coherence length (v_F/Delta), but their amplitude gets heavily
suppressed. The scanning tunneling microscopy image (STM) of CDW shows clear
features of the boundary. The local tunneling conductance becomes asymmetric
with respect to the Fermi energy, and considerable amount of spectral weight is
transferred to the lower gap edge. Also it exhibits additional zeros reflecting
the influence of the boundary.Comment: 7 pages, 6 figure
Downward shift of infrared conductivity spectral weight at the DDW transition: role of anisotropy
We consider the motion of conductivity spectral weight at a
finite-temperature phase transition at which density-wave (DDW)
order develops. We show that there is a shift of spectral weight to higher
frequencies if the quasiparticle lifetime is assumed to be isotropic, but a
shift to lower frequencies if the quasiparticle lifetime is assumed to be
anisotropic. We suggest that this is consistent with recent experiments on the
pseudogap phase of the cuprate superconductors and, therefore, conclude that
the observation of a downward shift in the spectral weight at the pseudogap
temperature does not militate against the DDW theory of the pseudogap.Comment: 8 pages, 7 figures. Added reference
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
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)]
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