377 research outputs found
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
Parameters for Systems Exhibiting Local Lattice Distortions, Charge and Spin Ordering
Keeping in mind the experimental results that indicate local lattice
distortions, charge and spin orderings, we have developed a phenomenological
approach which allows us to describe the electronic phase diagram of cuprates
and related systems in terms of few parameters.In the present work we consider
a third-order parameter theory which characterize charge, spin and
superconductivity orderings. We are thus led to a theory of three scalar
fields. By coupling these scalars to gauge fields we are naturally led to
string-like solutions, which we interpret as stripes. This ties nicely with our
quantum group conjecture that 1d systems play an important role in the physics
of cuprates and related materials. We show that this simple approach can give
rough values for two-order parameters which can be naively be interpreted as
charge and spin orderings. We also report our attempt to understand how local
lattice distortions are involved and what role they play in terms of these two
order parameters.Comment: 8 pages revtex, Published in AIP Conference Proceedings 554: Int.
Sym. on Physics in Local Lattice Distortions, July 23-26, 200
Temperature Dependent Polarized XANES Spectra for Zn-doped LSCO system
The cuprates seem to exhibit statistics, dimensionality and phase transitions
in novel ways. The nature of excitations [i.e. quasiparticle or collective],
spin-charge separation, stripes [static and dynamics], inhomogeneities,
psuedogap, effect of impurity dopings [e.g. Zn, Ni] and any other phenomenon in
these materials must be consistently understood. Zn-doped LSCO single crystal
were grown by TSFZ technique. Temperature dependent Polarized XANES [near edge
local structure] spectra were measured at the BL13-B1 [Photon Factory] in the
Flourescence mode from 10 K to 300 K. Since both stripes and nonmagnetic Zn
impurities substituted for Cu give rise to inhomogeneous charge and spin
distribution it is interesting to understand the interplay of Zn impurities and
stripes. To understand these points we have used Zn-doping and some of the
results obtained are as follows: The spectra show a strong dependence with
respect to the polarization angle, , as is evident at any temperature
by comparing the spectra where the electric field vector is parallel with
ab-plane to the one where it is parallel to the c-axis. By using the XANES
[temperature] difference spectra we have determined T* [experimentally we find,
T* 160-170 K] for this sample. The XANES difference spectra shows
that the changes in XANES features are larger in the ab-plane than the c-axis,
this trend is expected since zinc is doped in the ab-plane at the copper site.
Our study also complements the results in literature namely that zinc doping
does not affect the c-axis transport.Comment: To appear in Physica C [ISS2001 Special Issue], related talk
presented at ISS2001 as PC-16, 10 pages revtex and 7 pages of figures (pdf
XANES study of rare-earth valency in LRu4P12 (L = Ce and Pr)
Valency of Ce and Pr in LRu4P12 (L = Ce and Pr) was studied by L2,3-edge
x-ray absorption near-edge structure (XANES) spectroscopy. The Ce-L3 XANES
spectrum suggests that Ce is mainly trivalent, but the 4f state strongly
hybridizes with ligand orbitals. The band gap of CeRu4P12 seems to be formed by
strong hybridization of 4f electrons. Pr-L2 XANES spectra indicate that Pr
exists in trivalent state over a wide range in temperature, 20 < T < 300 K. We
find that the metal-insulator (MI) transition at TMI = 60 K in PrRu4P12 does
not originate from Pr valence fluctuation.Comment: 4 page
Lattice Instability in High Temperature Superconducting Cuprates and FeAs Systems: Polarons Probed by EXAFS
Carrier-induced lattice distortion (signature of polaron) in oxypnictide superconductors is found by an instantaneous local probe, extended X-ray absorption fine structure (EXAFS). Polaron formation is detected as two distinct nearest neighbor distances (FeAs), implying an incoherent local mode that develops coherence at the critical temperature. Comparing the results with the unusual lattice response in cuprate superconductors, intimate correlation between evolution of local lattice mode and superconductivity is revealed. The results suggest that strong electron-lattice interaction is present as a common ingredient in the microscopic mechanism of superconducting transition. The effect of magnetic impurity atoms in cuprates further indicates that magnetic scattering becomes diluted as long as polaron formation is conserved. We argue that polaron coherence dominates electrical conduction and magnetic interaction in oxypnictide and cuprate superconductors
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