The effects induced on the magnetization by coherent fluctuating Cooper pairs in the normal state have been measured in Pb 1−x In x alloys up to high magnetic fields, of amplitudes above H C2 (0), the upper critical field extrapolated to T =0 K. Our results show that in dirty alloys these superconducting fluctuation effects are, in the entire H − T phase diagram above H C2 (T ), independent of the amount of impurities and that they vanish when H ∼ 1.1H C2 (0). These striking results are consistent with a phenomenological estimate that takes into account the limits imposed by the uncertainty principle to the shrinkage, when H increases, of the superconducting wave function.

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The fluctuation-induced diamagnetism ∆M , associated with the presence of precursor Cooper pairs in the normal state, has been measured in lanthanum with dilute magnetic (Pr) and nonmagnetic (Lu) impurities. It is found that while for pure La and La-Lu alloys ∆M agrees, as expected, with the theoretical predictions, it is much larger for La-Pr alloys (around a factor 5 for La-2 at.%Pr). These results suggest the existence of an indirect contribution to ∆M arising from the interaction between fluctuating Cooper pairs and magnetic impurities.

The fluctuation-diamagnetism (FD) above the superconducting transition was measured in 2H-NbSe2 single crystals. The moderate uniaxial anisotropy of this compound, and some experimental improvements, allowed to measure the superconducting fluctuation effects in the two main crystallographic directions. These results reveal that the nonlocal electrodynamic effects on the FD are highly anisotropic, and they also discard a possible contribution to the FD coming from the charge-density waves (CDW) appearing below TCDW > TC in 2H-NbSe2, in agreement with a phenomenological estimate.

Electric and magnetic characterization of NbSe2 single crystals is first presented in detail. Then, some preliminary measurements of the fluctuation-diamagnetism (FD) above the transition temperature TC are presented. The moderate uniaxial anisotropy of this compound allowed us to observe the fluctuation effects for magnetic fields H applied in the two main crystallographic orientations. The superconducting parameters resulting from the characterization suggest that it is possible to do a reliable analysis of the FD in terms of the Ginzburg-Landau (GL) theory.