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26 May 2019
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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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5 messages found in this forum:
Subject: Anomalous precursor diamagnetism
Author: Riesling
Date: 13 July 2006 at 12:52 GMT.
Message: In connection with this article by Cabo et al., I would like to start a discussion about various aspects of precursor diamagnetism in high-Tc superconductors. Which fluctuation regime of the order parameter is most suitable to describe the experimental data (Gaussian or critical-phase-dominated), what is the importance of the material, and so on. Experts from both experimental and theoretical side should contribute to the exchange.
To start with, I would like to consider the anomalous behaviour observed in the magnetization M(T,H) for low magnetic field H just above Tc. The above article claims that it is an exclusive effect of the Tc inhomogeities. Former analyses by Varlamov and colleagues were able to reproduce the non-monotonicity of M(T=const,H) by phase-fluctuation model. So what are the strongest evidences that would support/infirm each the various approaches?

Date: 20 July 2006 at 09:36 GMT.

The first observation of an anomalous “precursor” diamagnetism (fluctuating diamagnetism, FD) related to superconducting fluctuations (SF) and superimposed to the “conventional” Ginzburg-Landau (GL) contribution, in LSCO underdoped single crystal, was achieved by A. Lascialfari, A. Rigamonti, L. Romano’, A. A. Varlamov and I. Zucca (Phys. Rev. B 68, 100505 (2003) ). The justification of the experimental finding was given on the basis of a theory involving the presence, above the bulk Tc , of mesoscopic superconducting (SC) regions, lacking of phase coherence (see also A. Lascialfari, A. Rigamonti, L. Romano’, P. Tedesco, A. Varlamov, and D. Embriaco, Phys. Rev. B 65, 144523 (2002)), with the generation of vortex-antivortex pairs, implying phase fluctuations of the superconducting order parameter. This theoretical picture was given (see also L.Romanò, Inter. J. Mod. Phys. 17,423 (2003)) by extending , with some integrations, the inspiring paper by A.Sewer and H. Beck ( Phys. Rev. B 64, 014510 (2001) ).
The anomalous diamagnetism and local SC “islands” in underdoped LSCO had been independently proposed on the basis of scanning superconducting quantum interference microscopy (I. Iguchi, T. Yamaguchi, and A. Sagimoto, Nature 412, 420 (2001) ).
Additional effect implied in the picture accounting for the observed FD had to be Nernst effect. Successfull detection of Nernst-related effects , with a comprehensive analysis, has been given indeed ( Y. Wang, L. Li, and N. P. Ong, Phys. Rev. B 73, 024510 (2006) and References therein).

Date: 20 July 2006 at 09:40 GMT.
Message: It should be remarked that in a study of precursor diamagnetism in underdoped (aligned powders) LSCO, before the work by Lascialfari et al. already mentioned, ( C. Carballeira, J. Mosqueira, A. Revcolevschi, and F. Vidal, Phys. Rev. Lett. 84, 3157-3160 (2000)), had found discrepancies between “conventional” Prange theory and the experimental data. However, no low- field diamagnetic effect ( with un upturn in the magnetization curves) was noticed in this latter paper, probably because the measurements had carried out at relatively strong magnetic fields. In fact the upturn field Hup in isothermal magnetization M(H) curves (see again A.L. et al., Phys. Rev. B 68, 100505 (2003) ) in LSCO is noticeable for H smaller than about 100 Oe, value above which only the conventional GL diamagnetism is active.
Recently Cabo et al. (L.Cabo, F. Soto, M. Ruibal, J. Mosqueira, and F. Vidal
Phys. Rev. B 73, 184520 (2006)) by means of detailed measurements in the low-field regime did observe the anomalous FD in underdoped LSCO, showing data in substantial agreement with the previous ones by Lascialfari et al. (Phys. Rev. B 68, 100505 (2003) ). Cabo et al. explained their experimental results by invoking a distribution of local transition temperatures, in other words in terms of a narrow Tc-inhomogeneity. Then the magnetization curves above the bulk Tc would not be related to SC fluctuations, rather being the standard magnetization curves for II- type SC’s , the upturn field basically coinciding with Hc1

Date: 20 July 2006 at 09:41 GMT.
Message: It is well known that Tc inhomogeneity is a source of anomalous precursor diamagnetism (see for some experimental evidence A. Lascialfari, T. Mishonov, A. Rigamonti, I. Zucca, G. Behr, W. Loser, S.L. Drechsler, Eur. Phys. J. B 35, 325 (2003), for similar effects due to disorder and Tc distribution). Our picture of mesoscopic SC “islands” above Tc actually does imply a certain distribution of transition temperatures. Thus the interpretation given by Cabo et al. ( Phys. Rev. B 73, 184520 (2006)) is certainly suitable to account for the experimental observations. However our interpretation relates the unconventional magnetization curves M= M(H) in underdoped LSCO and YBCO to phase fluctuations of the order parameter among SC mesoscopic zones above Tc, mostly on the basis of the temperature dependence of the upturn field. ( See for a discussion A.Rigamonti, A.Lascialfari, L. Romanò, A.Varlamov and I. Zucca, J. Superconductivity, ISSN: 0896-1107 (Paper) 1572-9605 (Online), DOI :10.1007/s10948-005-0077-z (2005)), where samples with “vanishing” inhomogeneities (underdoped LSCO) are compared to samples where strong inhomogeneity is present (Al- doped MgB2 ).
In conclusion, it is proper to say that both effects (distribution of local Tc’s and strong phase fluctuations ) in principle are present in underdoped cuprates and generate the “anomalous” diamagnetism. To discern between the vortex-antivortex fluctuations and the somewhat more trivial Tc-inhomogeneity contribution to FD , is the temperature behaviour of the upturn field Hup for T>Tc bulk : for precursor diamagnetism due to Tc inhomogeneties Hup(T) decreases as T is increased, while in case of FD due to vortex-antivortex pairs the theory predicts that Hup(T) increases as T is increased.

Date: 20 July 2006 at 09:43 GMT.
Message: A final word of caution is in order, in regards of the measurements in grains ( Cabo et al.) rather than in single crystal as we did (Phys. Rev. B 68, 100505 (2003)). For grains, it is conceivable that surface superconductivity and Tc –corrections do occur. Further measurements in YBCO , where the anomalous diamagnetism is stronger than in LSCO, would certainly clarify the issue , once that single crystal of reasonable size for the former should be available.

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