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Condensed Matter Seminars

ics Condensed Matter
Thursday, November 2, 2006
4:00 PM
Physics Building, Room 204
Note special time.
Note special room.
Veerle Keppens [Host: Joe Poon]
University of Tennessee
"To tunnel or not to tunnel - or why clathrates are so fascinating"
Motivated by the search for improved thermoelectric materials, several compounds have attracted attention that combine the high electron mobilities found in crystals with a low thermal conductivity approaching values typical for glasses. The common structural feature of these “Electron Crystal Phonon Glasses” (ECPG) is that they contain loosely bound atoms that reside in a large crystalline “cage”. These “rattlers” scatter phonons and greatly reduce the thermal conductivity of the material. One family of ECPGs is formed by the Ge-clathrates Sr 8 Ga 16 Ge 30 and Eu 8 Ga 16 Ge 30 . The presence of the rattler significantly softens the elastic behavior. Combined with results from low-temperature ultrasonic attenuation, neutron-scattering, thermal conductivity and microwave absorption measurements, it provides clear evidence for the existence of a new type of four-well tunneling states.

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