Comment on ''Field-Enhanced Diamagnetism in the Pseudogap State of the Cuprate Bi2Sr2CaCu2O8+\delta Superconductor in an Intense Magnetic Field''

In the above mentioned letter by Wang et al. [Phys. Rev. Lett, 95, 247002 (2005)], magnetization measurements on two Bi_2Sr_2caCu_2O_8+delta samples are reported. They claim that these experimental results support the vortex scenario for the loss of phase coherence at Tc. On the contrary, we show in this comment that they can be explained by means of the Ginzburg Landau theory (under a total-enery cutoff) for the superconducting fluctuations above Tc.Comment: Final versio

Breakdown by a magnetic field of the superconducting fluctuations in the normal state: A simple phenomenological explanation

We first summarize our recent observations, through magnetization measurements in different low-Tc superconductors, of a rather sharp disappearance of the superconducting fluctuations in the normal state when the magnetic field approaches Hc2(0), the upper critical field extrapolated to T=0K. We propose that a crude phenomenological description of the observed effects may be obtained if the quantum limits associated with the uncertainty principle are introduced in the Gaussian-Ginzburg-Landau description of the fluctuation-induced magnetization.Comment: LaTeX, 8 pages, including 2 eps figures. Proceedings of SNS'04, Sitges, Spai

On the dilemma between percolation processes and fluctuating pairs as the origin of the enhanced conductivity above the superconducting transition in cuprates

The confrontation between percolation processes and superconducting fluctuations to account for the observed enhanced in-plane electrical conductivity above but near $T_c$ in cuprates is revisited. The cuprates studied here, La$_{1.85}$Sr$_{0.15}$CuO$_4$, Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, and Tl$_2$Ba$_2$Ca$_2$Cu$_3$O$_{10}$, have a different number of superconducting CuO$_2$ layers per unit-cell length and different Josephson coupling between them, and are optimally-doped to minimize $T_c$-inhomogeneities. The excellent chemical and structural quality of these samples also contribute to minimize the effect of extrinsic $T_c$-inhomogeneities, a crucial aspect when analyzing the possible presence of intrinsic percolative processes. Our analyses also cover the so-called high reduced-temperature region, up to the resistivity rounding onset $\varepsilon_{onset}$. By using the simplest form of the effective-medium theory, we show that possible emergent percolation processes alone cannot account for the measured enhanced conductivity. In contrast, these measurements can be quantitatively explained using the Gaussian-Ginzburg-Landau (GGL) approach for the effect of superconducting fluctuations in layered superconductors, extended to $\varepsilon_{onset}$ by including a total energy cutoff, which takes into account the limits imposed by the Heisenberg uncertainty principle to the shrinkage of the superconducting wavefunction. Our analysis confirms the adequacy of this cutoff, and that the effective periodicity length is controlled by the relative Josephson coupling between superconducting layers. These conclusions are reinforced by analyzing one of the recent works that allegedly discards the superconducting fluctuations scenario while supporting a percolative scenario for the enhanced conductivity above $T_c$ in cuprates.Comment: 13 pages, 7 figure