Test of Alternative Electrodynamic Theory of Superconductivity by using Ultra Low Temperature Scanning Probe Spectroscopy

One of the main assumptions of London’s theory of superconductivity is the screening of an electric field inside a conventional superconductor on a short length scale, on the order of the Thomas-Fermi length. According to the alternative electrodynamic theory of conventional superconductors proposed by Hirsch[1], the electric field inside the superconductor should have a much larger decay length, similar to London penetration of the magnetic field. To test this hypothesis, we measured the force vs distance curves between a metal probe tip and superconducting Al surface. We correlated the force curve with the superconducting tunneling spectra, in and out of the superconducting state of the Al sample. The measurements were performed in a mK STM system, which has been recently upgraded to include a quartz tuning fork sensor for AFM measurement capability. The Al film was grown by MBE onto graphene/SiC substrate and transferred in the mK STM in ultra-high vacuum with a base temperature of 10 mK. The Al film was brought into the normal state with the application of a magnetic field above its critical field limit. We describe the results and compare to similar experiments performed on Nb surface.[2]

[1] Hirsch, PRB, 69, 214515 (2004)
[2] Peronio and Giessibl, PRB, 94, 094503 (2016)