Fig. 15.9. Temperature dependence of the magnetic susceptibility of KjC^ measured at H = 500 Oe. Open circles for heating (numbered arrows: 1, 3, 4, 6, 7, and 9); open triangles for cooling (numbered arrows: 2, 5, and 8). The inset shows field-cooled (FC) and zero-field-cooled (ZFC) susceptibility data for KjQo measured at H = 10 Oe [15.75].
the measured low-temperature ZFC and FC values of x to the ideal x value of — l/47r yields values of 85% for the shielding fraction (ZFC) and 19% for the Meissner fraction (FC), consistent with an irreversible ZFC diamagnetism that is more than a factor of 4 greater than the reversible FC diamagnetism [15.75], Referring to the main plot in Fig. 15.9 for the same I^Qo sample but now in a field of 500 Oe, we see irreversible behavior for the 1 ->■ 2, 4-»5, 7 -»■ 8 segments and reversible behavior for the 2 3, 5 -»• 6, 8 9 segments. The intersection points between the reversible and irreversible behaviors converge to a value T* for a given H [i.e., each x(J) diagram such as Fig. 15.9 yields a single (T*,H) pair], which determines a point on the phase boundary between the vortex glass and vortex fluid phases shown in Fig. 15.10; the entire phase boundary in Fig. 15.10 is constructed by measuring T* for a range of H values [15.75]. The results in Fig. 15.10 show similarities in the superconducting phase diagram for K3C60 and Rb3C60, except for differences in scale arising from the different Tc values for these compounds. The phase boundaries for both K3C60 and Rb3C60 are fit by the relation
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