Abstract
From the C 1s and K 2p photoelectron holograms, we directly reconstructed atomic images of the cleaved surface of a bimetal-intercalated graphite superconductor, (Ca, K)C8, which differed substantially from the expected bulk crystal structure based on x-ray diffraction (XRD) measurements. Graphene atomic images were collected in the in-plane cross sections of the layers 3.3 Å and 5.7 Å above the photoelectron emitter C atom and the stacking structures were determined as AB- and AA-type, respectively. The intercalant metal atom layer was found between two AA-stacked graphenes. The K atomic image revealing 2 × 2 periodicity, occupying every second centre site of C hexagonal columns, was reconstructed, and the Ca 2p peak intensity in the photoelectron spectra of (Ca, K)C8 from the cleaved surface was less than a few hundredths of the K 2p peak intensity. These observations indicated that cleavage preferentially occurs at the KC8 layers containing no Ca atoms.
Highlights
Other than the confirmation of Tc at 65 mK for BaC613, the members of the GIC superconductor family have remained unchanged for the past nine years
We directly obtained the images of C and metal atomic arrangements at the (Ca, K)C8 cleaved surface reconstructed from the C 1s and K 2p photoelectron holograms
The magnetic susceptibility, χg(cm[3] g−1), of the present (Ca, K)C8 sample is shown in Fig. 1a; χg MV (G cm3), W (g) and H (Oe) are the superconducting quantum interference device (SQUID) magnetic moment and weight of the sample and the applied magnetic field, respectively
Summary
Other than the confirmation of Tc at 65 mK for BaC613, the members of the GIC superconductor family have remained unchanged for the past nine years. We demonstrate the effectiveness of photoelectron holography in the study of the surface of superconducting materials prepared by intercalation with metal atoms. The cleaved surface region of GIC was demonstrated to have a stage 2 structure by photoelectron holography, which differed substantially from the expected bulk (Ca, K)C8 crystal structure elucidated from the x-ray diffraction (XRD) measurements. These observations revealed that the cleavage preferentially occurs at the stage-2 type K atom intercalated layers containing no Ca atoms, which is likely the most fragile part of the crystal sample
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