310 research outputs found
Structural distortions and model Hamiltonian parameters: from LSDA to a tight-binding description of LaMnO_3
The physics of manganites is often described within an effective two-band
tight-binding (TB) model for the Mn e_g electrons, which apart from the kinetic
energy includes also a local "Hund's rule" coupling to the t_{2g} core spin and
a local coupling to the Jahn-Teller (JT) distortion of the oxygen octahedra. We
test the validity of this model by comparing the energy dispersion calculated
for the TB model with the full Kohn-Sham band-structure calculated within the
local spin-density approximation (LSDA) to density functional theory. We
analyze the effect of magnetic order, JT distortions, and "GdFeO_3-type"
tilt-rotations of the oxygen octahedra. We show that the hopping amplitudes are
independent of magnetic order and JT distortions, and that both effects can be
described with a consistent set of model parameters if hopping between both
nearest and next-nearest neighbors is taken into account. We determine a full
set of model parameters from the density functional theory calculations, and we
show that both JT distortions and Hund's rule coupling are required to obtain
an insulating ground state within LSDA. Furthermore, our calculations show that
the "GdFeO_3-type" rotations of the oxygen octahedra lead to a substantial
reduction of the hopping amplitudes but to no significant deviation from the
simple TB model.Comment: replaced with final (published) version with improved presentatio
Pseudogap Formation in Models for Manganites
The density-of-states (DOS) and one-particle spectral function of the one- and two-orbital models for manganites, the latter with
Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly
robust pseudogap (PG) features were found at low- and
intermediate-temperatures, particularly at or near regimes where
phase-separation occurs as 0. The PG follows the chemical potential
and it is caused by the formation of ferromagnetic metallic clusters in an
insulating background. It is argued that PG formation should be generic of
mixed-phase regimes. The results are in good agreement with recent
photoemission experiments for .Comment: Accepted for publication in Phys. Rev. Lett., 4 pages, Revtex, with 4
figures embedde
Temperature-Dependent Pseudogaps in Colossal Magnetoresistive Oxides
Direct electronic structure measurements of a variety of the colossal
magnetoresistive oxides show the presence of a pseudogap at the Fermi energy
E_F which drastically suppresses the electron spectral function at E_F. The
pseudogap is a strong function of the layer number of the samples (sample
dimensionality) and is strongly temperature dependent, with the changes
beginning at the ferromagnetic transition temperature T_c. These trends are
consistent with the major transport trends of the CMR oxides, implying a direct
relationship between the pseudogap and transport, including the "colossal"
conductivity changes which occur across T_c. The k-dependence of the
temperature-dependent effects indicate that the pseudogap observed in these
compounds is not due to the extrinsic effects proposed by Joynt.Comment: 5 pages, 6 figures, submitted to Phys. Rev.
Electronic excitations in BiSrCaCuO : Fermi surface, dispersion, and absence of bilayer splitting
From a detailed study, including polarization dependence, of the normal state
angle-resolved photoemission spectra for BiSrCaCuO, we find
only one CuO band related feature. All other spectral features can be
ascribed either to umklapps from the superlattice or to ``shadow bands''. Even
though the dispersion of the peaks looks like band theory, the lineshape is
anomalously broad and no evidence is found for bilayer splitting. We argue that
the ``dip feature'' in the spectrum below arises not from bilayer
splitting, but rather from many body effects.Comment: 4 pages, revtex, 3 uuencoded postscript figure
Experimental setup for low-energy laser-based angle resolved photoemission spectroscopy
A laser-based angle resolved photoemission (APRES) system utilizing 6 eV
photons from the fourth harmonic of a mode-locked Ti:sapphire oscillator is
described. This light source greatly increases the momentum resolution and
photoelectron count rate, while reducing extrinsic background and surface
sensitivity relative to higher energy light sources. In this review, the
optical system is described, and special experimental considerations for
low-energy ARPES are discussed. The calibration of the hemispherical electron
analyzer for good low-energy angle-mode performance is also described. Finally,
data from the heavily studied high T_c superconductor Bi2Sr2CaCu2O8+\delta
(Bi2212) is compared to the results from higher photon energies.Comment: Please download final version from Journal-Re
Collective Modes and the Superconducting State Spectral Function of Bi2212
Photoemission spectra of the high temperature superconductor Bi2212 near
(pi,0) show a dramatic change when cooling below Tc: the broad peak in the
normal state turns into a sharp low energy peak followed by a higher binding
energy hump. Recent experiments find that this low energy peak persists over a
significant range in momentum space. We show in this paper that these data are
well described by a simple model of electrons interacting with a collective
mode which appears only below Tc.Comment: 4 pages, revtex, 4 encapsulated postscript figure
- …