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14 April 2021
  » arxiv » cond-mat/0512700

 Article overview

Anomalous precursor diamagnetism at low reduced magnetic fields in underdoped La1.9Sr0.1CuO4 and in Pb55In45 superconductors and Tc inhomogeneities
L. Cabo ; F. Soto ; M. Ruibal ; J. Mosqueira ; F. Vidal ;
Rating Members: 3.5/5 (1 reader) | Visitors: 5/5 (1 visitor)
Date 29 Dec 2005
Subject Superconductivity
AbstractThe magnetic field dependence of the magnetization was measured above the superconducting transition in a high-Tc underdoped cuprate La1.9Sr0.1CuO4 and in a low-Tc alloy (Pb55In45). Near the superconducting transition [typically for (T-Tc)/Tc<0.05] and under low applied magnetic field amplitudes [typically for H/Hc2(0)<0.01, where Hc2(0) is the corresponding upper critical field extrapolated to T=0 K] the magnetization of both samples presents a diamagnetic contribution much larger than the one predicted by the Gaussian Ginzburg-Landau (GGL) approach for superconducting fluctuations. These anomalies have been already observed in cuprate compounds by various groups and attributed to intrinsic effects associated with the own nature of these high-Tc superconductors. However, we will see here that our results in both high and low-Tc superconductors may be explained quantitatively, and consistently with the GGL behavior observed at higher fields, by just taking into account the presence in the samples of an uniform distribution of Tc inhomogeneities. These Tc inhomogeneities, which may be in turn associated with stoichiometric inhomogeneities, were estimated from independent measurements of the temperature dependence of the field-cooled magnetic susceptibility under low applied magnetic fields.
Source arXiv, cond-mat/0512700
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1. review 06070042 (3 readers)    * Rate this comment.
Review title: The role of inhomogeneities in the anomalous precursor diamagnetism in HTc
Reviewer: reviewer182
Date: 06 July 2006 at 09:30 GMT.
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.

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