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26 May 2017
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SCIENCE-ADVISOR - review - "The role of inhomogeneities in the anoma..." review 06070042 (3 readers)    * Rate this comment.
Article reference: Anomalous precursor diamagnetism at low reduced magnetic fields in underdoped La1.9Sr0.1CuO4 and in Pb55In45 superconductors and Tc inhomogeneities.
Review title: The role of inhomogeneities in the anomalous precursor diamagnetism in HTc
Reviewer: reviewer182
Date: 06 July 2006 at 09:30 GMT.
Read this article?: To see a concrete example where material (chemical) features may mask or alter the underlying physical properties and make its interpretation more difficult
The work reported in this article deals with the fluctuation-induced precursor diamagnetism above Tc in high-temperature superconductors (HTS), and in particular with its magnetic field dependance. Understanding this effect allows to make useful statements about the nature of the puzzling normal phase of the HTS, in particular for the underdoped compounds. The results from Cabo et al. are both theoretical and experimental and support the interpretation of the magnetization anomaly (i.e. non-monotonicity of the H dependence of M(T>Tc,H->0)) in terms of Tc inhomogeneities in the sample that was measured.

The model is based on the "Gaussian Ginzburg-Landau" approach where fluctuations of the order parameter above Tc are treated at the Gaussian level. Analytic expreesions of M(T,H) in the T-H phase diagram have been calculated previously by the group of the authors. Here the new ingredient are Eqs. 1&2 that allow to calculate M(T,H) for the inhomogeneous case. The superconductor is modelled as an ensemble of independant grains the Tc distribution of which follows a Gaussian, the parameters of which (Tc,m and dTc,m) having to be extracted from the fit to the experimemtal data. The key point of the author' methodology is the comparison between a high-Tc sample (underdoped LSCO) and the inhomogeneous low-Tc SC Pb (with various In impurities levels). The latter indeed displays an anomaly in M(T>Tc,H->0) that vanishes in the pure Pb case. The qualitative behaviour of the magnetization M(T,H) in both high and low-Tc compounds is well described by the unique model that nicely reproduces the non-monotonicity of M(T>Tc,H->0)(Figs. 2 and 5), while providing meaningful values for the fit parameters Tc,m and dTc,m (Table I). At the more quantitative level, some discrepancies appear, although they cannot invalidate the full approach. For instance the anomaly peak in M(T>Tc,H->0) is sharper (thiner and higher) in the theoretical curves than in the experimental ones, that is probably the reason why the authors did not merge parts a&c and b&d in Fig.2.

From a more general point of view, this work provides strong arguments in favor of an interpretation of the anamalous precursor diamagnetism in term of Tc inhomogeneities. However this treatment only applies to samples constituted of grains. It would be interesting to see if the anomaly is still present in the case of a monocrystal. The authors also pretend that their approach (assuming a Gaussian fluctuation regime) is sufficient to explain the precursor diamagnetism, and that an interpretation of this phenomenon m in term of critical (or pre-critical) XY-like phase fluctuations, as proposed by other groups, is not necessary. I don't agree totally, maybe the above-mentioned
quantitative discrepancies observed for the diamagnetic peak is due to an incorrect treatment of the fluctuation regime for T->Tc, H->0, which should contain a phase contribution. This question may be answered by making similar experiments with other dopings, so that the intrinsic fluctuation contribution is different.

important: 3.5,    broad interest: 2.5,    readable: 3.5,    new: 4,    correct: 4,   Global: 3.5
Article rating: 3.5/5
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