62 research outputs found
Order-N Density-Matrix Electronic-Structure Method for General Potentials
A new order-N method for calculating the electronic structure of general
(non-tight-binding) potentials is presented. The method uses a combination of
the ``purification''-based approaches used by Li, Nunes and Vanderbilt, and
Daw, and a representation of the density matrix based on ``travelling basis
orbitals''. The method is applied to several one-dimensional examples,
including the free electron gas, the ``Morse'' bound-state potential, a
discontinuous potential that mimics an interface, and an oscillatory potential
that mimics a semiconductor. The method is found to contain Friedel
oscillations, quantization of charge in bound states, and band gap formation.
Quantitatively accurate agreement with exact results is found in most cases.
Possible advantages with regard to treating electron-electron interactions and
arbitrary boundary conditions are discussed.Comment: 13 pages, REVTEX, 7 postscript figures (not quite perfect
Recommended from our members
Auger spectroscopy and electronically-stimulated surface processes
Electronic excitations in adsorbate layers stimulate desorption and dissociation of adsorbed molecules as well as chemical reactions between adsorbates. The highest-probability stimulated processes produce neutral desorbates and determine how surface composition is altered by electron or photon radiation. A basic understanding has emerged, due largely to laser resonance-enhanced multi-photon ionization (REMPI) experiments, which provide quantum-state resolution of the gas-phase products. Auger phenomena enter this understanding in several ways. For example, CVV Auger spectroscopy determines the screened hole-hole interaction, U, in adsorbates, which in turn provides insight into the degree of charge-transfer screening from the substrate. In those systems where screening charge is used in excitation Auger decay, screening directly determines the lifetime, which in turn can exponentially affect the yield. Reductions in screening, e.g. induced by coadsorption of electro-negative species, thus can result in giant yield enhancements. As separate issues, a finite U may prevent the fast resonant decay and thus increase the yield from two-hole excitations, as has been suggested for NO{sub 2} dissociation on Pt (111), or may assist in the localization (self-trapping) of two-hole excitations in dense adsorbate layers, as apparently is the case for NO desorption from the same surface. The latter causes the yields from one- and two-hole excitations to differ in their coverage dependence. Finally, CVV Auger spectroscopy, of course, measures the energies of two-hole excitations, which can be correlated with observed stimulated thresholds. 27 refs., 15 figs
Superconductivity from Flat Dispersion Designed in Doped Mott Insulators
Routes to enhance superconducting instability are explored for doped Mott
insulators. With the help of insights for criticalities of metal-insulator
transitions, geometrical design of lattice structure is proposed to control the
instability. A guideline is to explicitly make flat band dispersions near the
Fermi level without suppressing two-particle channels. In a one-dimensional
model, numerical studies show that our prescription with finite-ranged hoppings
realizes large enhancement of spin-gap and pairing dominant regions. We also
propose several multi-band systems, where the pairing is driven by intersite
Coulomb repulsion.Comment: 4 pages, to be published in Phys. Rev. Let
Superconductivity in the Cuo Hubbard Model with Long-Range Coulomb Repulsion
A multiband CuO Hubbard model is studied which incorporates long-range (LR)
repulsive Coulomb interactions. In the atomic limit, it is shown that a
charge-transfer from copper to oxygen ions occurs as the strength of the LR
interaction is increased. The regime of phase separation becomes unstable, and
is replaced by a uniform state with doubly occupied oxygens. As the holes
become mobile a superfluid condensate is formed, as suggested by a numerical
analysis of pairing correlation functions and flux quantization. Although most
of the calculations are carried out on one dimensional chains, it isComment: LATEX, 14 pages, 4 figures available as postcript files or hard copy,
preprint ORNL-CCIP/93/1
Superconductivity of the One-Dimensional d-p Model with p-p transfer
Using the numerical diagonalization method, we investigate the
one-dimensional - model, simulating a Cu-O linear chain with strong
Coulomb repulsions. Paying attention to the effect of the transfer energy
between the nearest neighbor oxygen-sites, we calculate the critical
exponent of correlation functions based on the Luttinger liquid
relations and the ground state energy as a function of an external
flux . We find that the transfer increases the charge
susceptibility and the exponent in cooperation with the repulsion
at Cu-site. We also show that anomalous flux quantization occurs for
. The superconducting region is presented on a phase diagram of
vs. plane.Comment: 4 pages, RevTex + 5 PS figures include
Fluctuation Exchange Analysis of Superconductivity in the Standard Three-Band CuO2 Model
The fluctuation exchange, or FLEX, approximation for interacting electrons is
applied to study instabilities in the standard three-band model for CuO2 layers
in the high-temperature superconductors. Both intra-orbital and near-neigbor
Coulomb interactions are retained. The filling dependence of the d(x2-y2)
transition temperature is studied in both the "hole-doped" and "electron-doped"
regimes using parameters derived from constrained-occupancy density-functional
theory for La2CuO4. The agreement with experiment on the overdoped hole side of
the phase diagram is remarkably good, i.e., transitions emerge in the 40 K
range with no free parameters. In addition the importance of the "orbital
antiferromagnetic," or flux phase, charge density channel is emphasized for an
understanding of the underdoped regime.Comment: REVTex and PostScript, 31 pages, 26 figures; to appear in Phys. Rev.
B (1998); only revised EPS figures 3, 4, 6a, 6b, 6c, 7 and 8 to correct
disappearance of some labels due to technical problem
On the Liaison Between Superconductivity and Phase Separation
Models of strongly correlated electrons that tend to phase separate are
studied including a long-range 1/r repulsive interaction. It is observed that
charge-density-wave states become stable as the strength of the 1/r term, , is increased. Due to this effect, the domain of stability of the
superconducting phases that appear near phase separation at is not enlarged by a 1/r interaction as naively expected. Nevertheless,
superconductivity exists in a wide region of parameter space, even if phase
separation is suppressed. Our results have implications for some theories of
the cuprates.Comment: 11 pages, 9 postscript figures are appende
Double-exchange is not the cause of ferromagnetism in doped manganites
The coexistence of ferromagnetism and metallic conduction in doped manganites
has long been explained by a double-exchange model in which the ferromagnetic
exchange arises from the carrier hopping. We evaluate the zero-temperature spin
stiffness D(0) and the Curie temperature T_{C} on the basis of the
double-exchange model using the measured values of the bare bandwidth W and the
Hund's rule coupling J_{H}. The calculated D(0) and T_{C} values are too small
compared with the observed ones even in the absence of interactions. A
realistic onsite interorbital Coulomb repulsion can reduce D(0) substantially
in the case of a 2-orbital model. Furthermore, experiment shows that D(0) is
simply proportional to x in La_{1-x}Sr_{x}MnO_{3} system, independent of
whether the ground state is a ferromagnetic insulator or metal. These results
strongly suggest that the ferromagnetism in manganites does not originate from
the double-exchange interaction. On the other hand, an alternative model based
on the d-p exchange can semi-quantitatively explain the ferromagnetism of doped
manganites at low temperatures.Comment: 6 pages, 3 figures, some modifications in scientific content
Electrons in High-Tc Compounds: Ab-Initio Correlation Results
Electronic correlations in the ground state of an idealized infinite-layer
high-Tc compound are computed using the ab-initio method of local ansatz.
Comparisons are made with the local-density approximation (LDA) results, and
the correlation functions are analyzed in detail. These correlation functions
are used to determine the effective atomic-interaction parameters for model
Hamiltonians. On the resulting model, doping dependencies of the relevant
correlations are investigated. Aside from the expected strong atomic
correlations, particular spin correlations arise. The dominating contribution
is a strong nearest neighbor correlation that is Stoner-enhanced due to the
closeness of the ground state to the magnetic phase. This feature depends
moderately on doping, and is absent in a single-band Hubbard model. Our
calculated spin correlation function is in good qualitative agreement with that
determined from the neutron scattering experiments for a metal.Comment: 21pp, 5fig, Phys. Rev. B (Oct. 98
Non Fermi-Liquid States and Pairing of a general Model of Copper-Oxide Metals
A model of copper-oxygen bonding and anti-bonding bands with the most general
two-body interactions allowable by symmetry is considered. The model has a
continuous transition as a function of hole-density x and temperature T to a
phase in which a current circulates in each unit cell. This phase preserves the
translational symmetry of the lattice while breaking time-reversal invariance
and the four-fold rotational symmetry. The product of time-reversal and
four-fold rotation is preserved. The circulating current phase terminates at a
critical point at . In the quantum-critical region about this point
the logarithm of the frequency of the current fluctuations scales with their
momentum. The microscopic basis for the marginal Fermi-liquid phenemenology and
the observed long wavelength transport anomalies near are derived from
such fluctuations. The symmetry of the current fluctuations is such that the
associated magnetic field fluctuations are absent at oxygen sites and have the
correct form to explain the anomalous copper nuclear relaxation rate.
Cross-overs to the Fermi-liquid phase on either side of and the role of
disorder are briefly considered. The current fluctuations promote
superconductive instability with a propensity towards ``D-wave" symmetry or
``extended S-wave"symmetry depending on details of the band-structure.Comment: 85 pages RevTex,15 figures available from the autho
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