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The optical reflectivity spectra were measured for single crystalline La 2 CuO 4 and La 1.8 Sr 0.2 CuO 4 over a wide energy range from 12meV to 3eV. Comparing the phonon spectrum of single crystal with that of polycrystal, the observed four phonon modes can be classified into the two—the vibrations parallel and perpendicular to the c-axis. The infrared spectrum of La 1.8 Sr 0.2 CuO 4 shows the normal Drude-type behavior without any extra excitation.
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Tajima et al. (1988) studied this question.
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