Fig. 13-31. The optical conductivity rr,(a>) presented on a logarithmic frequency scale above and below Tc for (a) KjC«, and (b) RbjQo evaluated from the 5?(ck) reflectivity data of Figs. 13.28 and 13.30 [13.151]. The normal state o-,(a>) data in the low frequency limit is determined from the Hagen Rubens extrapolation.

and superconducting states is shifted to a 8 function at w = 0 for the superconducting phase [13.159]. Here, the area A is given by poo

Jo where the subscripts n and s refer, respectively, to the normal and superconducting states. Furthermore, the integral A in Eq. (13.18) is related to the London penetration depth AL [13.159] by the relation X\ = (c2/%)A, where c is the speed of light, yielding the value \L — 8000 ± 500 A for both K3C60 and Rb3C60. This value of Az is somewhat higher than values for AL obtained from other experimental probes (see §15.3).

More detailed fits of the far-infrared conductivity in the superconducting and normal states provide strong evidence for an electron-phonon coupling mechanism involving intramolecular phonon modes (see §15.4) [13.151].

13.4.5. Optical Properties o/MjC60 Compounds

Since the structural properties of M,C60 compounds are distinctly different from those of the other MXC60 alkali metal-doped fullerides, the optical properties are also expected to exhibit unique features. Depending on the heat treatment conditions of the sample, several M,C60 phases can be reached as a function of measurement temperature. Using IR transmission as a measure of the physical state of the M[C60 system, a schematic view of various phases of RbjC60 is shown in Fig. 13.32 [13.160], This model is consistent with the measurements of the optical properties shown in Fig. 13.33. Upon slow cooling, the optical transmission through a C60 film in the fee (rock salt) phase that is stable at high temperatures (225°C) is essentially independent of temperature, with a spectrum similar to that shown in Fig. 13.33 (bottom curve) for the slow-cooled sample. Upon further slow cooling, the optical transmission for RbjQo drops at the fcc-orthorhombic phase transition [13.161], indicative of a lower electrical resistivity phase, which is found to be orthorhombic in structure [13.162], where the C60 anions are closely linked along a proposed polymer chain direction (see §8.5.2). As the temperature continues to decrease, the optical transmission

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