458 research outputs found
Relevant coherent states method for the quantum adiabatic dynamics of lattice-coupled charge carriers
A new numerical method is proposed for determining the low-frequency dynamics
of the charge carrier coupled to the deformable quantum lattice. As an example,
the polaron band structure is calculated for the one-dimensional Holstein
model. The adiabatic limit on the lattice, which cannot be reached by other
approaches, is investigated. In particular, an accurate description is obtained
of the crossover between quantum small adiabatic polarons, pinned by the
lattice, and large adiabatic polarons, moving along the continuum as classical
particles. It is shown how the adiabatic contributions to the polaron
dispersion, involving spatial correlations over multiple lattice sites, can be
treated easily in terms of coherent states.Comment: 6 pages, 2 figure
Central peak in the pseudogap of high T_c superconductors
We study the effect of antiferromagnetic (AF) correlations in the three-band
Emery model, with respect to the experimental situation in weakly underdoped
and optimally doped BSCCO. In the vicinity of the vH singularity of the
conduction band there appears a central peak in the middle of a pseudogap,
which is in an antiadiabatic regime, insensitive to the time scale of the
mechanism responsible for the pseudogap. We find a quantum low-temperature
regime corresponding to experiment, in which the pseudogap is created by
zero-point motion of the magnons, as opposed to the usual semiclassical
derivation, where it is due to a divergence of the magnon occupation number.
Detailed analysis of the spectral functions along the (pi,0)-(pi,pi) line show
significant agreement with experiment, both qualitative and, in the principal
scales, quantitative. The observed slight approaching-then-receding of both the
wide and narrow peaks with respect to the Fermi energy is also reproduced. We
conclude that optimally doped BSCCO has a well-developed pseudogap of the order
of 1000 K. This is only masked by the narrow antiadiabatic peak, which provides
a small energy scale, unrelated to the AF scale, and primarily controlled by
the position of the chemical potential.Comment: Final version as accepted in EPJ B, 13 pages, 8 figure
Dielectric properties of multiband electron systems: II - Collective modes
Starting from the tight-binding dielectric matrix in the random phase
approximation we examine the collective modes and electron-hole excitations in
a two-band electronic system. For long wavelengths (), for
which most of the analysis is carried out, the properties of the collective
modes are closely related to the symmetry of the atomic orbitals involved in
the tight-binding states. In insulators there are only inter-band charge
oscillations. If atomic dipolar transitions are allowed, the corresponding
collective modes reduce in the asymptotic limit of vanishing bandwidths to
Frenkel excitons for an atomic insulator with weak on-site interactions. The
finite bandwidths renormalize the dispersion of these modes and introduce a
continuum of incoherent inter-band electron-hole excitations. The possible
Landau damping of collective modes due to the presence of this continuum is
discussed in detail.Comment: 25 pages, LaTeX, to appear in Z.Phys.
Dielectric properties of multiband electron systems: I - Tight-binding formulation
The screened electron-electron interaction in a multi-band electron system is
calculated within the random phase approximation and in the tight-binding
representation. The obtained dielectric matrix contains, beside the usual
site-site correlations, also the site-bond and bond-bond correlations, and thus
includes all physically relevant polarization processes. The arguments are
given that the bond contributions are negligible in the long wavelength limit.
We analyse the system with two non-overlapping bands in this limit, and show
that the corresponding dielectric matrix reduces to a form. The
intra-band and inter-band contributions are represented by diagonal matrix
elements, while the off-diagonal elements contain the mixing between them. The
latter is absent in insulators but may be finite in conductors. Performing the
multipole expansion of the bare long-range interaction, we show that this
mixing is directly related to the symmetry of the atomic orbitals participating
in the tight-binding electronic states. In systems with forbidden atomic
dipolar transitions, the intra-band and inter-band polarizations are separated.
However, when the dipolar transitions are allowed, the off-diagonal elements of
the dielectric matrix are of the same order as diagonal ones, due to a finite
monopole-dipole interaction between the intra-band and inter-band charge
fluctuations.Comment: 32 pages, LaTeX, to appear in Z.Phys.
Theory of stripes in quasi two dimensional rare-earth tritellurides
Even though the rare-earth tritellurides are tetragonal materials with a
quasi two dimensional (2D) band structure, they have a "hidden" 1D character.
The resultant near-perfect nesting of the Fermi surface leads to the formation
of a charge density wave (CDW) state. We show that for this band structure,
there are two possible ordered phases: A bidirectional "checkerboard" state
would occur if the CDW transition temperature were sufficiently low, whereas a
unidirectional "striped" state, consistent with what is observed in experiment,
is favored when the transition temperature is higher. This result may also give
some insight into why, in more strongly correlated systems, such as the
cuprates and nickelates, the observed charge ordered states are generally
stripes as opposed to checkerboards.Comment: Added contents and references, changed title and figures. Accepted to
PR
Slave-Boson Three-Band Model with O-O Hopping for High-Tc Superconductors
Slave boson mean-field approximation is carried out analytically for weakly
doped CuO_2 conduction planes, characterized by Cu-O charge transfer energy
\Delta_{pd}, Cu-O hopping t_0, O-O hopping t' and repulsion U_d between holes
on Cu site taken as infinite. At zero doping \delta, finite negative
t',|t'|<t_0/2, expands the range of stability of the covalent, conducting state
on the expense of the insulating state which, however, remains stable at larger
\Delta_{pd}. For sufficiently large \Delta_{pd} the renormalized charge
transfer energy saturates at 4|t'| instead of decreasing to zero, as at t'=0
case. In contrast to t', finite \delta suppresses the insulating state nearly
symmetrically with respect to the sign of \delta. The regime with charge
transfer energy renormalized close to 4|t'| fits remarkably well the ARPES
spectra of Bi2212 and LSCO, and, in the latter case, explains the observed
strong doping dependence of the Cu-O hopping.Comment: 4 pages, 2 figure
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