Phys. Scr. 99 (2024) 085948
Authors: M I Tsindlekht, V M Genkin, I Felner, F Zeides, N Katz, Š Gazi, ŠChromik and O V Dobrovolskiy
Abstract: Measurements of the ac response represent a valuable method for probing the properties of
superconductors. In the surface superconducting state (SSS), a current exceeding the surface critical
currentIc leads to breakdown of SSS and penetration of external magnetic field into the sample bulk.
An interesting free-of-bulk system in SSS is offered by thin-walled cylinders. According to the full
penetration of magnetic flux (FPMF) model, each time the instant value of an ac field is equal to a
certain critical value, the ac susceptibility χwill exhibit jumps as a function of the ac field amplitude
Hac because of the periodic destruction and restoration of SSS in the cylinder wall. Here we study the
low-frequency (128–8192 Hz) ac response of thin-walled niobium cylinders under superimposed dc
and ac magnetic fields applied parallel to the cylinder axis. In contrast to the FPMF model predictions,
experiments reveal a smooth χ(Hac) dependence. To explain the experimental observations, we
propose a phenomenological partial penetration magnetic flux (PPMF) model, which assumes that
after restoration of the superconducting state, the magnetic fields inside and outside the cylinder are
unequal and the magnitude of the penetrating flux is random for every penetration. This model fits
very well the experimental data on the temperature dependence of the first harmonic χ1 for any dc
field and ac amplitude.
