Paramagnetic Reentrance Effect in NS Proximity Cylinders

A scenario for the unusual paramagnetic reentrance behavior at ultra-low temperatures in Nb-Ag, Nb-Au, and Nb-Cu cylinders is presented. For the diamagnetic response down to temperatures of the order 15 mK, the standard theory (quasi-classical approximation) for superconductors appears to work very well, assuming that Ag, Au, and Cu remain in the normal state except for the proximity-induced superconductivity. Here it is proposed that these noble metals may become p-wave superconductors with a transition temperature of order 10 mK. Below this temperature, p-wave triplet superconductivity emerges around the periphery of the cylinder. The diamagnetic current flowing in the periphery is compensated by a quantized paramagnetic current in the opposite direction, thus providing a simple explanation for the observed increase in the susceptibility at ultra-low temperatures. In 1990 Visani et al. [1] reported a surprising paramagnetic reentrance phenomenon at ultra-low temperatures. When a Nb cylinder of diameter ∼ 20-100 μm, covered with a thin film of Ag with a thickness of a few μm, is cooled below the superconducting transition temperature of Nb, the systems initially exhibits the expected diamagnetic response down to temperatures around 10 mK. However, when the temperature is further lowered, the uniform magnetic susceptibility starts to increase again, indicating a decreasing diamagnetic response