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Local Lattice Instability and Superconductivity in LaSrCuMO (M=Mn, Ni, and Co)
Local lattice structures of LaSrCuMO (M=Mn,
Ni, and Co) single crystals are investigated by polarized extended x-ray
absorption fine structure (EXAFS). The local lattice instability at low
temperature is described by in-plane Cu-O bond splitting. We find that
substitution of Mn for Cu causes little perturbation of local lattice
instability while Ni and Co substitution strongly suppresses the instability.
The suppression of superconductivity by Cu-site substitution is related to the
perturbation of lattice instability, indicating that local lattice instability
(polaron) plays an important role in superconductivity
Once again: Instanton method vs. WKB
A recent analytic test of the instanton method performed by comparing the
exact spectrum of the Lam potential (derived from representations
of a finite dimensional matrix expressed in terms of generators) with
the results of the tight--binding and instanton approximations as well as the
standard WKB approximation is commented upon. It is pointed out that in the
case of the Lam potential as well as others the WKB--related method
of matched asymptotic expansions yields the exact instanton result as a result
of boundary conditions imposed on wave functions which are matched in domains
of overlap.Comment: 10 pages, no figures. References list revised according to JHE
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