Regensburg 2007 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 26: Electronic properties III
MM 26.4: Talk
Wednesday, March 28, 2007, 17:30–17:45, H6
Optic phonon anomaly as a precursor to polaron formation in a layered CMR manganite — •Dmitry Reznik1, Frank Weber1,2, Nadir Aliouane3, Dimitri Argyriou3, Marcus Braden4, and Winfried Reichardt1 — 1Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021 Karlsruhe, Germany — 2Physikalisches Institut, Universität Karlsruhe (TH), D-76128 Karlsruhe, Germany — 3Hahn-Meitner Institut, Glienicker Straße 100, D-14109 Berlin, Germany — 4II. Physikalisches Institut, Universität zu Köln, D-50937 Köln, Germany
We found evidence of a precursor effect to polaron formation in the bond stretching phonons in the bilayer CMR manganite La2−2xSr1+2xMn2O7 at 10K. Neutron scattering measurements of these phonons in the XX0 direction show that both the transverse and longitudinal branches broaden abruptly from X=0.15, where they are resolution limited, to X=0.25 where FWHM=13meV. They then narrow again on approach to the zone boundary (X=0.5). The shell model predicts a downward dispersion following the cosine function for the transverse branch and upward dispersion for the longitudinal branch, which is exactly what is observed in undoped cuprates. But in La2−2xSr1+2xMn2O7 both branches show a steep downward dispersion above x=0.15. The anomalous dispersion and broadening can be understood as a precursor effect to the CMR transition. This behavior is very unusual, because such precursor effects have previously been associated with phase transitions where charge order appears on cooling, not on heating as in CMR manganites.