Thermally activated charge carriers and mid-infrared optical
excitations in quarter-filled CDW systems |
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Authors: | I Kup?i? |
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Institution: | (1) Department of Physics, Faculty of Science, University of Zagreb, POB 331, 10 002 Zagreb, Croatia |
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Abstract: | The optical properties of the quarter-filled single-band CDW systems
have been reexamined in the model with the electron-phonon coupling
related to the variations of electron site energies.
It appears that the indirect, electron-mediated coupling
between phase phonons and external electromagnetic fields
vanishes for symmetry reasons, at variance with the infrared selection
rules used in the generally accepted microscopic theory.
It is shown that the phase phonon modes and the electric fields
couple directly, with the coupling constant proportional to the
magnitude of the charge-density wave.
The single-particle contributions to the optical
conductivity tensor are determined for the ordered CDW state
and the related weakly doped metallic state by means of the Bethe-Salpeter
equations for elementary electron-hole excitations.
It turns out that this gauge-invariant approach establishes
a clear connection between the effective numbers of residual,
thermally activated and bound charge carriers.
Finally, the relation between these numbers and the activation energy of dc
conductivity and the optical CDW gap scale is explained in the way consistent
with the conductivity sum rules. |
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Keywords: | 71 45 Lr Charge-density-wave systems 78 30 -j Infrared and Raman spectra 78 20 Bh Theory models and numerical simulation |
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