Fluctuation-induced in-plane conductivity, magnetoconductivity, and diamagnetism ofBi2Sr2

Abstract

We report detailed experimental results on the fluctuation-induced in-plane conductivity ⌬ ab , magnetoconductivity ⌬ ab , and diamagnetism ⌬ ab , of high-quality Bi 2 Sr 2 CaCu 2 O 8 crystals. The data were obtained with magnetic fields H applied perpendicularly to the superconducting ͑CuO 2 ͒ planes and up to 0 Hϭ5 T, which not too close to the transition ͓for reduced temperatures ϵ(TϪT C0 )/T C0 տ10 Ϫ2 ͔ may be considered in the weak magnetic field limit. In the mean field region ͑MFR͒ above the transition, these data are analyzed in terms of thermal fluctuations of the superconducting order parameter amplitude ͑OPF͒, on the grounds of the existing theoretical approaches for layered superconductors that take into account the presence of two superconducting layers in the layer periodicity length, s, which for these compounds is equal to one-half the crystallographic unit-cell length in the c direction. These results show that, due to its strong dependence, ⌬ ab is dramatically affected by the presence of small T C inhomogeneities, associated with small oxygen content inhomogeneities uniformly distributed in the crystals. These inhomogeneity effects are taken into account, consistently with our ⌬ ab and ⌬ ab results, by using an effective medium approach proposed by Maza and Vidal. In this way, the amplitude and the behavior of the three observables studied here are explained in terms of the direct OPF effects, at a quantitative level, confirming then the absence of appreciable indirect contributions ͓as, for instance, the Maki-Thompson and the density-of-states ͑DOS͒ terms͔. These last results may suggest unconventional ͑non 1 s 0 ͒, pair breaking, wave pairing in these compounds, as first proposed from OPF analyses by Veira and Vidal. The resulting values of the in-plane and out-of-plane coherence length amplitudes are, respectively, ab ͑0͒ϭ͑0.9Ϯ0.1͒ nm and c ͑0͒Շ0.05 nm. These coherence length amplitudes are consistent with the values that we have obtained before for other Bi-based crystals by analyzing the effects of the vortex position fluctuations on the magnetization below the transition. These results also confirm at a quantitative level that in Bi-2212 compounds the effective number of fluctuating CuO 2 planes per periodicity length above the superconducting transition is N e Ϸ2 and that the OPF's are essentially two dimensional over the entire MFR. ͓S0163-1829͑96͒07833-2͔

Fluctuation conductivity in cuprate superconductors

“…The fluctuation conductivity obtained for BSCCO in the present work agrees in magnitude with that obtained by Pomar et al [6] in a similar study. However, they have been able to explain their data in terms of the LD model (eq.…”

“…The DOS or other indirect contributions were not required. Similarly, Pomar et al [6] did not require to invoke these terms in the paraconductivity of Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO). On the other hand, Livanov et al [7] found significant amounts of σ DOS to be present in their studies on thin films of BSCCO.…”

The role of density of states fluctuations in the normal state properties of highTcsuperconductors

“…A further re® nement of the theory of the in-plane¯uctuation conductivity was carried out considering two di erent interlayer distances and di erent strengths of the tunnelling coupling between adjacent layers for YBCO [142,143] and BSCCO 2212 [144]. This approach led to the same qualitative results as those obtained by the conventional single layer approach, but to a better quantitative agreement between the theory and experiments.…”

Fluctuations aboveTcand the kinetic energy of Cooper pairs inLa1.9Sr0.1

“…The samples of La-SrCuO are grain aligned with a total mass of 48 mg, thus at least 2 orders of magnitude larger than those crystals used in other fluctuation measurements. [25][26][27][28][29] Such a big mass makes it possible to better detect fluctuations because of the larger temperature range employed. The grains have diameters around 5-10 m, a dimension much bigger than the superconducting coherence length, which determines the thermal fluctuations of Cooper pairs above the transition.…”