The metal-intercalated bilayer graphene has a flat band
with a
high density of states near the Fermi energy and thus is anticipated
to exhibit an enhanced strong correlation effect and associated fascinating
phenomena, including superconductivity. By using a self-developed
multifunctional scanning tunneling microscope, we succeeded in observing
the superconducting energy gap and diamagnetic response of a Ca-intercalated
bilayer graphene below a critical temperature of 8.83 K. The revealed
high value of gap ratio, 2Δ/k
B
T
c ≈ 5.0, indicates a strong coupling
superconductivity, while the variation of penetration depth with temperature
and magnetic field indicates an isotropic s-wave
superconductor. These results provide crucial experimental clues for
understanding the origin and mechanism of superconductivity in carrier-doped
graphene.
The two-coil mutual inductance (TCMI) technique is a useful experimental method to derive magnetic penetration depth λ in a superconducting film after proper numerical calculations, in which various film geometries including infinite, circular and quadrangle films have been utilized. Based on previously reported reflection-type TCMI experimental data taken from NbN and K-adsorbed FeSe thin films, we investigate the validity of various numerical models with different geometries by comparing their calculation results. The calculated values of λ for various film geometries become identical only when the film size is at least three times larger than the coil size. For a rectangle film with a width comparable to the coil size, the numerical models of circular and square film geometries with proper sizes can be also adopted to obtain a similar λ value as that calculated with a rectangle film geometry. Although the true value of λ can be approximately achieved only after a complicated calibration, its calculated temperature dependence is insensitive to the choice of numerical models. With these results, a proper film geometry for the numerical calculation of λ may be selected to effectively improve the calculation efficiency.
To uncover the critical effect of disorder on superconductivity, the ideal method is to visualize the microscopic crystalline deficiencies in real space while measuring the macroscopic superconducting properties. By using a self-developed multifunctional scanning tunneling microscope, we investigated the correlation between controllably introduced disorder and superconductivity in the Si(111)-7×3-In surface reconstruction. It is revealed that not only the density but also the spatial distribution of surface vacancies makes a significant influence on the diamagnetic response of the superconducting surface reconstruction. The higher density of vacancies uniformly dispersed on terraces results in a lower critical temperature and critical magnetic field, while the presence of grooves formed by aggregation of vacancies at step edges reduces the critical supercurrent and thus weakens the diamagnetic shielding effect remarkably.
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