Abstract:
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Abstract: |
We have performed 69Ga, 71Ga, and 137Ba NMR on Ba8Ga16 Ge 30, a clathrate
semiconductor which has been of considerable interest due to its large
figure of merit for thermoelectric applications. In measurements from 4 K
to 450 K, we used measurements on the two Ga nuclei to separate the
magnetic and electric quadrupole hyperfine contributions and thereby
gain information about the metallic and phonon behavior. The results
show the presence of a pseudogap in the Ga electronic states within the
conduction band, superposed upon a large Ba contribution to the conduction
band. Meanwhile the phonon contributions to the Ga relaxation rates are
large and increase more rapidly with temperature than typical
semiconductors. These results provide evidence for enhanced anharmonicity
of the propagative phonon modes over a wide range, providing experimental
evidence for enhanced phonon-phonon scattering as a mechanism for the
reduced thermal conductivity.
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