Diamagnetism above the superconducting transition in underdoped La1.9Sr0.1CuO4: Chemical disorder versus phase incoherent superconductivity

Abstract

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 Tc 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 La1.9Sr0.1CuO4 [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 Tc in underdoped cuprates, here we have extended our magnetization measurements in La1.9Sr0.1CuO4 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 La1.9Sr0.1CuO4 crystal). For this sample, the corresponding Tc inhomogeneities may be approximated as symmetric around the average Tc. In contrast, the most disordered sample presents a pronounced asymmetric Tc 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.

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