284 research outputs found
Time and Fuel Optimal Controls in the Presence of Measurement Uncertainties
Time and fuel optimal control problems associated with controlling spacecraft under noisy measurements and bounded control input
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.
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
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
Phenomenological BCS theory of the high- cuprates
A BCS model characterized by a phenomenological pair potential with on-site
(), nearest (), and next nearest () neighbour coupling
constants, and an empirical quasiparticle dispersion taken from angle-resolved
photoemission spectra is considered. The model can consistently explain the
experimental data concerning the pair state of the hole doped cuprates. Three
ingredients are required to make the interpretation possible: the existence of
flat bands, a very small effective on-site repulsion, and a slightly dominating
effective nnn attraction of the order of 60-80meV with a ratio .Comment: 13 pages, uuencoded Postscrip
Extraction of the Electron Self-Energy from Angle Resolved Photoemission Data: Application to Bi2212
The self-energy , the fundamental function which
describes the effects of many-body interactions on an electron in a solid, is
usually difficult to obtain directly from experimental data. In this paper, we
show that by making certain reasonable assumptions, the self-energy can be
directly determined from angle resolved photoemission data. We demonstrate this
method on data for the high temperature superconductor
(Bi2212) in the normal, superconducting, and pseudogap phases.Comment: expanded version (6 pages), to be published, Phys Rev B (1 Sept 99
Two-hole bound states in modified t-J model
We consider modified model with minimum of single-hole dispersion at
the points , . It is shown that two holes on
antiferromagnetic background produce a bound state which properties strongly
differs from the states known in the unmodified model. The bound state is
d-wave, it has four nodes on the face of the magnetic Brillouin zone. However,
in the coordinate representation it looks like as usual s-wave.Comment: LaTeX 9 page
Effect of non-magnetic impurities on the gap of a superconductor as seen by angle-resolved photoemission
An analysis of angle-resolved photoemission (ARPES) experiments in the
superconducting state of the high \tc copper-oxides is presented. It is based
on a phenomenological weak-coupling BCS model which incorporates the
experimental normal state dispersion extracted from ARPES, and non-magnetic
impurity scattering in the presence of a order parameter (OP). It
is shown, that already in the pure case, the broadening by finite momentum
resolution of the analyzer leads to a finite region of apparent `gaplessness'
around the true node of the OP. Non-magnetic impurities further amplify this
effect by introducing additional spectral weight around zero frequency. At
sufficiently large impurity concentrations , this results
in an extended region of `gaplessness' up to ( the
angle on the Fermi surface) around the true node for a large range of moderate
to strong impurity potential strengths. Different ways to identify the presence
of impurity scattering in the ARPES spectra are proposed.Comment: 8 pages uuencoded gzipped Postscrip
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