Near-zero field microwave absorption

In favorable cases, superconductivity is a sensitive and useful screen for desired crystals. Near zero field microwave absorption (95) can be used to examine very small samples of only a few micrograms. This technique has value in the search for new superconducting phases. In early attempts to identify the superconducting phase in the Pb-Sr-Y-Cu-O system, both superconducting and non-supercon-ducting crystals were obtained. Individual crystals were examined for superconductivity using near-zero field microwave absorption. Then X-ray diffraction was used to establish the structure and stoichiometry of the superconducting phase. 

At low microwave fields transitions involving only a few photons are observed, at high static fields, near the field of the avoided crossing. As the microwave field is increased more transitions are observed, at progressively lower static fields, until at the highest microwave fields the sequence of resonant transitions extends to zero static field. As shown by the scale at the top of Fig. 10.9 the 18s —> (16,3) transition nearest zero static field corresponds to the absorption of 28 photons. 

The synthesis of macroscopic amounts of C o and C70 (fullerenes) has stimuiated a variety of studies on their chemical and physical properties. We recently demonstrated that C o and C70 become conductive when doped with alkali metals. Here we describe iow-temperature studies of potassium-doped both as films and bulk samples, and demonstrate that this material becomes superconducting, Superconductivity is demonstrated by microwave, resistivity and Melssner-effect measurements. Both polycrystalline powders and thin-flim samples were studied. A thin film showed a resistance transition with an onset temperature of 16 K and essentially zero resistance near 5 K. Bulk samples showed a well-defined Meissner effect and magnetic-field-dependent microwave absorption beginning at 18 K. The onset of superconductivity at 18 K is the highest yet observed for a molecular superconductor. 

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