Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures

Abstract

We discuss the magnetic response of clean Ag coated Nb proximity cylinders in the temperature range 150 μK < T < 9 K. In the mesoscopic temperature regime, the normal metal-superconductor system shows the yet unexplained paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C. Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full Meissner screening. The logarithmic slope of the reentrant paramagnetic susceptibility chi_para(T) \propto \exp(-L/ξ_N) is limited by the condition ξ_N=n L, with ξ_N=\hbar v_F/2 πk_B T, the thermal coherence length and n=1,2,4. In wires with perimeters L=72 μm and L=130 μm, we observe integer multiples n=1,2,4. At the lowest temperatures, χ_para compensates the diamagnetic susceptibility of the \textit{whole} AgNb structure.