Condensed Matter Seminars

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.