10.3 ferent magnitude for the two slopes
Ball-Ball Separation (A)
[15.33], electron-phonon interaction is the dominant electron-pairing mechanism, then the electron-phonon matrix elements would be expected to be similar for electrons in the tlu or tlg bands (see §14.2.2).
It is also interesting to note that alkali metal-doped C70 [15.34-38] and alkali metal-doped higher-mass fullerenes C76 [15.38,39], C78 [15.38], C82 [15.40], C84 [15.38,41,42], and C90 [15.38] do not show superconductivity. Further, the addition of C70 to C60 results in a rapid decrease in Tc for the M^(C60),_>(C70)>, compounds [15.38,43], In fact, doped C70 never becomes fully metallic, showing activated conduction even for the concentration K4C70 where the magnitude of the conductivity is a maximum (see §14.1). Furthermore, it is not expected on the basis of curvature arguments that doped carbon nanotubes would be attractive as a high-Tc superconducting material (see §19.11). It has been argued that the lower curvature of the higher-mass fullerenes suppresses the electron-phonon interaction and thus suppresses the occurrence of superconductivity. It would thus appear that doping might enhance the electron-phonon coupling for the lower-mass fullerenes more than for C60. But at the same time, the smaller lattice constant would be expected to lead to a lower density of states at the Fermi level. Thus it is not clear whether higher Tc values could be achieved by the doping of smaller-mass fullerenes.
The onset of superconductivity is typically measured in one of three ways: (1) by the loss of resistivity in temperature-dependent p(T) curves (see Fig. 14.3), (2) the temperature where the field-cooled and the zero-field-cooled magnetic susceptibilities merge (see Fig. 15.4), and (3) the onset of the decrease in microwave losses in temperature-dependent surface resistance plots RS(T). In the initial report of superconductivity in K3C60, evidence for superconductivity by all three methods was reported [15.1]. From measurements using one or more of the three techniques mentioned above, Tc is found for specific doped fullerene compounds, and the results for a variety of doped fullerene compounds are given in Table 8.3. A plot of the field-cooled and zero-field-cooled magnetic susceptibility vs. T is probably the most convenient method for the measurement of Tc (see Fig. 15.4 [15.30]), since the slope of the magnetization curve M(H) also gives the shielding fraction or superconducting fraction xsh through the relation and an ideal type I superconductor is a perfect diamagnet (i.e., xsh = 1). Since the shielding fraction is dependent on the flux exclusion, xsh is
15.2. Critical Temperature
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