The interplay between superconducting fluctuations and inhomogeneities presents a renewed interest due to recent works of different groups, which apparently support an intrinsically anomalous (beyond the conventional Gaussian-Ginzburg-Landau scenario) diamagnetism above T c in underdoped cuprates. This conclusion, mainly based in the observation of new anomalies in the isothermal magnetization curves at low-field amplitudes, is in contradiction with our earlier results in the underdoped La 1.9 Sr 0.1 CuO 4 [Phys. Rev. Lett. 84, 3157 (2000)]. These seemingly intrinsic anomalies are being presented in various influential works as a "thermodynamic evidence" for phase incoherent superconductivity in the pseudogap regime, this last being at present a central and debated issue of the cuprate superconductors' physics. To further probe the diamagnetism above T c in underdoped cuprates, here we have extended our magnetization measurements in La 1.9 Sr 0.1 CuO 4 to two samples with the same nominal composition but, due to different growth procedures, with different chemical disorder, in one of the samples this disorder being close to the intrinsic-like one, associated with the unavoidable random distribution of the Sr ions (which will be then present even in an ideal La 1.9 Sr 0.1 CuO 4 crystal). For this sample, the corresponding T c inhomogeneities may be approximated as symmetric around the average T c . In contrast, the most disordered sample presents a pronounced asymmetric T c distribution. The comparison between the magnetization measured in both samples provides a crucial check of the chemical disorder origin of the observed diamagnetism anomalies, which are similar to those claimed as due to phase fluctuations by other authors. This conclusion applies also to the sample affected only by the intrinsic-like chemical disorder, providing then a further check that, for all applied magnetic field amplitudes, the intrinsic diamagnetism above the superconducting transition of underdoped cuprates is not affected by the opening of a pseudogap in the normal state. It is also shown here that once these disorder effects are overcome, the remaining precursor diamagnetism may be accounted at a quantitative level in terms of the Gaussian-Ginzburg-Landau approach under a total energy cutoff.

The magnetization around the superconducting transition was recently measured in a high-quality Ba 1-x K x Fe 2 As 2 single crystal with magnetic fields applied along and transverse to the crystal Felayers [J. Mosqueira et al., Phys. Rev. B 83, 094519 (2011)]. Here we extend this study to the finite field (or Prange) regime, in which the magnetic susceptibility is expected to be strongly dependent on the applied magnetic field. These measurements are analyzed in the framework of the threedimensional anisotropic Ginzburg Landau (3D-aGL) approach generalized to the short wavelength regime through the introduction of a total-energy cutoff in the fluctuation spectrum. The results further confirm the adequacy of GL approaches to describe the fluctuation effects close to the superconducting transition of these materials.

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High-resolution magnetization measurements performed in a high-quality Ba 1−x K x Fe 2 As 2 single crystal allowed to determine the diamagnetism induced above the superconducting transition by thermally activated Cooper pairs. These data, obtained with magnetic fields applied along and transverse to the crystal ab layers, demonstrate experimentally that the superconducting transition of iron pnictides may be explained at a phenomenological level in terms of the Gaussian Ginzburg-Landau approach for three-dimensional anisotropic superconductors.

The new results summarized here, including a brief comparison with the paraconductivity, further suggest that the anomalous precursor (above T c ) diamagnetism recently observed in the underdoped La 1.9 Sr 0.1 CuO 4 superconductor could be attributed to the presence, in addition to the conventional superconducting pair fluctuations, of T c -inhomogeneities with long characteristic lengths associated with chemical disorder .

We report on magnetization measurements performed on a single crystal of Bi 2+x Sr 2−x CuO 6+δ with T c ≈ 7 K for H c-axis. The isofield M (T ) curves show a large reversible region, with a pronounced rounding effect as M approaches zero which prevents the determination of T c (H). Deviations from the linear behavior of magnetisation near T c (H) are studied through the asymptotic behavior of −M (T ) ∝ (T − T a ) m , where m is an exponent and T a an apparent temperature transition. Values of m deviates from the expected mean field value, 1/2, suggesting the importance of phase fluctuations. Resulting values of T a , interpreted as the onset of phase correlations, decrease as field increases showing an upward curvature. Values of T c (H) are obtained through a two-dimensional critical scaling analysis obeyed by many M (T ) curves. The resulting phase diagram do not show upward curvature and lies below the T a (H) line. The value of H c2 (0) estimated from the initial slope dH c2 /dT is twice the value suggested by the phase diagram. Amplitude fluctuations above T a (H) are explained in terms of a Ginzburg-Landau approach extended to high reduced temperatures and magnetic fields by the introduction of a total-energy cutoff in the fluctuation spectrum.

The magnetization around the superconducting transition was measured in YBa 2 Cu 3 O 7−δ with magnetic impurities in the CuO 2 layers (Cu substituted by Zn or Ni) or between them (Y substituted by Gd or Pr). While some of these impurities have an important effect on the superconducting transition temperature (T c ), the precursor diamagnetism observed above T c is not appreciably affected. This result contrasts with recent observations in a conventional BCS superconductor (La), in which the precursor diamagnetism was found to increase several orders of magnitude with the addition of a small amount (a few atomic per cent) of Gd or Pr magnetic impurities.

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