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spin susceptibility measurements (mb = 6.5 mn) [13.157] and band structure calculations [13.158], giving hvp = 1.3 eV = 1.0 x 104 cm-1, or mb ~ 3m0, while the EELS plasmon studies yield hvp ~ 1.1 eV and mb ~ 4m0. However, as seen in Table 13.8, a value fG ~ 0.9 is obtained, indicating that almost all the oscillator strength normally assigned to the Drude term has shifted to the mid-IR band. As a consequence, values for mb ~4 are obtained, rather than much higher values for mb. If instead, the free carriers and mid-IR band are decoupled [i.e., fc = 0 in the second term in the brackets of Eq. (13.16)], then a band mass of mb ~30 mQ (K3C6n) and mb ~60 m0 (Rb3C60) would be obtained. These band masses are clearly much larger than those obtained by present band structure calculations and larger than those found in other experimental work. If this interpretation of the optical data is correct, then strong electron correlations and/or a strong electron-phonon interaction might be implicated.

Thus, it would appear that either a theoretical basis for the shared oscillator strength in Eq. (13.16) must be established, or experimental tests of the electronic band structure and the associated Fermi surface (e.g., through Shubnikov-de Haas or de Haas-van Alphen data) are needed to provide further evidence for lower effective mass values (mb ~4). If the oscillator strength of the mid-IR band is indeed borrowed from the free electrons, a more detailed study of the temperature dependence of fG might be in order. It may be of interest to note that the mid-IR bands observed in high-Tc cuprate materials have been recently identified with the photoassisted hopping of polarons [13.156], where it is pointed out that the existence of polarons does not necessarily require an activated electrical resistivity.

13.4.4. Superconducting State Optical Properties

In Fig. 13.30 we display reflectivity data in the far-infrared for K3C60 (a) and Rb3C60 (b) for several temperatures above and below Tc [13.151]. A Kramers-Kronig analysis of these data is given in Fig. 13.31 for one

13.4. Optical Properties of Doped Cm 100

13.4. Optical Properties of Doped Cm 100

50 100

frequency (cm"')

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