Interpretation of large room-temperature diamagnetism at low magnetic fields in films of oxidised atactic polypropylene in terms of superconducting current loops

TL;DR

This paper models large room-temperature diamagnetism in oxidised atactic polypropylene films as arising from superconducting current loops, providing estimates for critical fields and loop sizes consistent with experimental data.

Contribution

It introduces a simple superconducting loop model to explain room-temperature diamagnetism in OAPP films, estimating critical fields and loop dimensions from experimental results.

Findings

Estimated lower critical field H_{c1} ≈ 5260 Oe.

Loop radii b_1 and b_2 fitted to data, with a maximum cross section radius of about 0.8 micrometres.

Model aligns with previous data suggesting loop sizes below 1 micrometre.

Abstract

A simple model is used to analyse published results on large room-temperature diamagnetism for two films of oxidised atactic polypropylene (OAPP) at low magnetic fields. The model involves induced currents expected in circular closed loops of superconductors in fields below the lower critical field H_{c1} at which flux penetration would first occur if a metamagnetic transition did not intervene as in OAPP, and the assumption that resistance would be restored at H_{c1} (negligible pinning). Fits to the data for the more strongly magnetic sample with the model, allowing two different types of loops with different loop radii b_1 and b_2, but with the same cross section a of loop material yield H_{c1} is approximately equal to 5260 Oe, and fits to the data for the less strongly magnetic sample with two loop sizes and with the same value of H_{c1}, combined with the knowledge that the minimum number of closed loops of any type is one, requires that the radius a of the cross section of the material should be less than about 0.8 micrometres, in fair agreement with a maximum radius of 1 micrometre obtained previously from other data.