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Condensed Matter Seminars

ics Condensed Matter
Monday, March 12, 2007
3:30 PM
Physics Building, Room 204
Note special date.
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Bella Lake [Host: Seunghun Lee]
Hahn-Meitner Institut
"Neutron Scattering Investigation of Excitations Spectrum in Doped and Undoped Spin-Ladders"
ABSTRACT:
The ‘telephone number compound’ family, (La,Sr,Ca) 14 Cu 24 O 41 , is composed of copper oxide planes which form alternating layers of ladders and chains. In the absence of Sr these materials are intrinsically hole doped with a valence of +2.25 per Cu.In contrast, for La 4 Sr 10 Cu 24 O 41 the hole doping is much reduced and magnon heat conductivity measurements show that the ladders are free of holes [1]. A third member of the family, Sr 2.5 Ca 11.5 Cu 24 O 41 , is hole-doped with ~2 holes per seven rungs of the ladder (the exact doping is debated). This compound is of particular interest because it shows a number of phenomena in common with high-Tc cuprates, including linear temperature dependence of resistivity above 130K, charge ordering below 60K and superconductivity under applied pressure [2]. Our neutron scattering measurements on the undoped ladder La 4 Sr 10 Cu 24 O 41 reveal gapped one-magnon and multi-magnon excitations. The results have been modelled to find accurate values of the exchange constants [3]. The doped ladder Sr 2.5 Ca 11.5 Cu 24 O 41 has also been measured. The holes give rise to a number of changes in the excitation spectrum which will be described and discussed.

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