Regensburg 2004 – wissenschaftliches Programm
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TT: Tiefe Temperaturen
TT 33: Amorphe- und Tunnelsysteme, Gl
äser
TT 33.5: Vortrag
Freitag, 12. März 2004, 11:15–11:30, H19
Four level tunneling systems and electronic structure in clathrates — •Ivica Zerec1, Alexander Yaresko2, and Peter Thalmeier1 — 1Max-Planck-Institut fuer Chemische Physik fester Stoffe, Noethnitzer Str. 40, 01187 Dresden — 2Max-Planck-Institut fuer Physik komplexer Systeme, Noethnitzer Str. 38, 01187 Dresden
The clathrates are cage compounds, interesting because of their structural and thermoelectrical properties. In the oversized cages formed by Si, Ge, Ga, etc. atoms, the No-dqguestNo-dq atoms rattle and strongly scatter phonons, reducing the phonon dominated thermal conductivity.
The elastic response of the Eu clathrate provides clear evidence for the existence of a new type of four-well tunneling states, described by two nearly degenerate four level systems (FLS). The FLS’s are closely linked with the fourfold split positions of Eu known from neutron diffraction density profiles. Using a realistic potential we estimate the tunneling frequencies and show that the energy gap between the two FLS’s is of the same order as the Einstein oscillator frequency. This explains why the observed harmonic oscillator type specific heat is not modified by tunneling states. The quadrupolar interaction of FLS’s with elastic strains explains the pronounced dip observed in the elastic constant measurements.
We present also the electronic band structure of Ba6Ge25 clathrate-like compound and investigate the modifications due to the structural phase transitions, reflected in the shifting of some Ba and Ge atoms to the split sites. The corresponding modifications in optical conductivity is calculated. It agrees well with experiment.