Materials withstrong electron correlation andstrong electron-phonon interaction
H. Koizumi
Institute of materials science, U. of Tsukuba

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Research Directions
1. Anomalous electric conductivity phenomena in transition
metal oxides.
a) High temperature superconductivity in cuprates (copper oxides).
b) Colossal magnetoresistance in manganites (manganese oxides).

2. Role of transition metals in biological systems(not started, yet).

2Current central topic:
High temperature superconductivity in hole-doped cuprates.
( it was found in 1986, but its mechanism is not elucidated, yet)Two major challenges for theoretical treatment
1. Strong correlation: the parent compound is a Mott insulator.
Calculations based on the local density approximation predicts
it as a metal.

2. Strong doped hole - lattice interaction: EXAFS and otherexperimental data indicates strong doped hole - lattice interaction.
Then, doped holes become small polarons, that are not good charge
carriers at low temperature; yet, superconductivity occurs.

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To deal with strong correlation
Molecular orbital cluster calculations：apply molecular orbital
method to a part of a solid (“cluster”).
Crystal environment is mimicked by an effective potential.

4Complete active space self-consistent filed method(CASSCF method)
Package programs are used to obtain wave
functions: GAMESS, MOLCAS
To see strong doped hole - lattice interaction:
total energy is calculated with deforming the cluster.
Excited states are also calculated to compare withexperiments.
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Parent cluster does not show
lattice instability. The excitationenergy agrees with that obtainedby experiment.

Hole-doped cluster shows lattice
instability. The magnitude of thedeformation agrees with the one
obtained by EXAFS experiment.
The excitationenergy seems to agree with thepeak in the energy loss function.
6Plots of natural orbitals
The lattice instability is a pseudo
Jahn-Teller type; antisymmetricorbital and symmetric orbital couple
with antisymmetric deformation.
Doped-hole is mostly in the bridging
oxygen when the cluster is notdeformed. When the deformation istaken into account the hole-occupied
orbital becomes the one constructedfrom a central copper d-orbital andsurrounding four oxygen p-orbitals. Thisorbital is a molecular orbital-like, incontrast to the Zhan-Rice singlet state. 7

Doped holes become small polarons, that are not good charge
carriers at low temperature; yet, superconductivity occurs.
Need new current flow mechanism !

A novel current flow mechanism is currently
under construction.
Main ingredient is a fictitious magnetic field froma Berry phase that acts as a magnetic field applied toelectrons. It appears when spin vortices are created.
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Calculations are done with a Hubbard model; sincein order to describe many spin vortices,a large number of clusters have to be included.
Meron: spin vortex with winding
number +1 is called “meron”.
Antimeron: spin vortex with winding
number -1 is called “antimeron”.

So far, mean field calculations have been done.
9(a) Spin configuration with twomerons and two antimerons.
(b) Current distribution; it shows
an extended current flow
region generated by thefictitious magnetic field from a
Berry phase.
(c) The simulated angle-resolved
photoemission spectrum. It
shows an arc-like fermisurface region.
(d) The simulated inelastic
neutron scattering profile. Thecentral peak is due to theantiferromagnetic spin order.
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(a) Spin configuration with fourmerons and four antimerons.
(b) Current distribution; it shows ariver of current .
(c) The simulated angle-resolved
photoemission spectrum.
(d) The simulated inelastic neutron
scattering profile; the splitting of
the antiferromagnetic peak isseen, which agrees withexperiment.

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Future plans
1.

CASSCF calculations for a larger clusters; include more than 4coppers.

2. Variational Monte Carlo calculations for a Hubbard model that
includes small polarons (holes with surrounding lattice distortion)and spin vortices.
By further increase of the cluster size, stability of spin vortices will be
examined in terms of molecular orbital calculations. We believethat spin vortices and fictitious magnetic field are the key
ingredients to elucidate cuprate superconductivity.
3. Application of new current flow mechanism to manganites.
4. Application of new current flow mechanism to biological systems.
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