, , , 1 , , ,

Cs Concentration x (CsxC6o)

Fig. 18.3. (a) Cs concentration dependence of the effective number of Bohr magnetons per CM molecule obtained from the Curie-Weiss part of the susceptibility for Cs^C«,. The full line is a guide for the eye. (b) Cs concentration dependence of the temperature-independent part of the magnetic susceptibility Xo of Cs,CM. The broken line indicates the core diamagnetic susceptibility due to filled shells up through the h'u° shell [18.18], electrical conductivity measurements [18.18], it is concluded that the compound CsC60 corresponding to the maximum Xo ¡n Fig- 18.3(b) (see §8.5.2) is metallic. From the magnitude of a density of states of 32±5 states/eV per C60 molecule at EF has been reported. Strong electron correlation is known to reduce the effective carrier concentration and give rise to a linear T term in the heat capacity and an enhancement of the Pauli susceptibility [18.21], so that the determination of N(EF) for CsC60 from measurement of xo may not be reliable.

In addition to this study on Cs^Q,,, evidence for Pauli paramagnetism has come from studies on the K^C70 system near x = 4. Transport studies on alkali metal-doped C70 [18.22-24] first show that the resistivity for K^C70 initially decreases with doping and reaches a minimum (pmin ~ 0.5 fl-cm), then increases with further doping and goes through a second minimum with lower resistivity (prain = 1.7 mil-cm) before increasing once again (see §14.1.4). Electron paramagnetic resonance (EPR) studies on KXC1Q show that the spin concentration goes through two maxima as a function of doping [18.25]. The susceptibility at the second maximum in the doping process showed Pauli-like behavior and the stoichiometry at this doping level was found to be K4C70, consistent with the band structure calculations [18.25,26], which show that C70 with D5h symmetry has two lowest unfilled levels: A" and E", which are singly and doubly degenerate, re-


Fig. 18.4. Temperature de-¡¿5 pendence of the magnetic suscep tibility of CSjQq. Full lines are the

Was this article helpful?

0 0

Post a comment