Johan Axnäs

Fluctuation conductivity of polycrystalline Hg, Tl-1223

Detailed resistivity measurements were made for polycrystalline Hg1-xTlxBa2Ca2Cu3O8+delta (x = 0 and 0.2) in zero field and above the superconducting T-c. The fluctuation conductivity Deltasigma is analyzed. With decreasing temperature two crossover temperatures could be identified for each sample; i.e. T* from two to three dimensional fluctuations in the mean field region (MFR), and T-G from the MFR to the critical region. All results are in agreement with the Lawrence-Doniach model. The c-axis coherence length and the interlayer coupling factor were obtained

Magnetoconductivity of polycrystalline Hg,Tl-1223

Abstract The magnetoconductivity of polycrystalline Hg 1− x Tl x Ba 2 Ca 2 Cu 3 O 8+ δ with x =0 and 0.2 has been measured above T c and analysed in terms of superconducting fluctuations. Such studies are numerous and successful for Y- and Bi-based materials, but still rare for Hg- and Tl-based materials. Results for the coherence lengths and the role of the Maki–Thompson contribution are briefly discussed.

Density of states fluctuations in the magnetoresistivity of YBa2Cu3O7−δ

Abstract The effects of fluctuations in the density of states (DOS) have been studied. A regularisation method was used to calculate the otherwise divergent DOS term. It was found that the DOS term can explain an observed change of sign in the c-axis magnetoconductivity but that its effect on in-plane magnetoconductivity is less significant.

Fluctuation magnetoconductivity in Fe doped YBa2Cu3O7−δ. Sign change for B and I parallel to the planes

Abstract The magnetoconductivity in Fe doped YBa 2 Cu 3 O 7−δ single crystals has been measured in the temperature region above T c where superconducting fluctuations are known to be important. For small Fe concentrations and B¦c we obtain excellent agreement with accepted fluctuation theories. However, at higher Fe concentrations this description seems to break down. For B ⊥ c we observe an anomalous dependence on the relative orientation of current and field, both applied along the ab -planes. For B¦I the magnetoconductivity is negative, as expected. For B ⊥ I we observe a cross-over to a positive magnetoconductivity above about T c + 10 K.

Density of states fluctuations in the c-axis magnetoconductivity of YBa2Cu3O7−δ

The magnetoresistivity up to 12 T of YBa2Cu3O7−δ has been measured with B//I//c fromTc+3 K toTc+40 K. A change of sign was observed, from positive nearTc to negative at temperatures above approximatelyTc+10 K. This previously not reported change of sign can be explained by recent fluctuation theories that include fluctuations in the normal density of states (DOS).

In-plane anisotropy and chain contribution from the magnetoresistivity of YBa2Cu3O7−δ

Abstract Magnetoresistivity measurements were made in untwinned YBa 2 Cu 3 O 7−δ above T c for B ∥ c and B ∥ ab , with I a or b . A fluctuation theory that allows for an in-plane coherence length anisotropy (γ =ζ b ζ a was used to determine γ. The contribution to the magnetoconductivity from the CuO-chains was also analyzed.

Different estimates of the anisotropy from resistive measurements in high-Tc superconductors

The anisotropy is one key parameter in understanding the physics of high temperature superconductors (HTSC). In addition to quantifying the variation of physical properties in different crystal directions, it contributes to strongly enhanced thermal fluctuations, and thereby to the rich variety of new phenomena observed in HTSC. Some examples are the existence of a mixed state solid-to-liquid phase transition (which depending on the amount of disorder can be a melting transition or a glass transition), the sometimes extreme broadening of the resistive transition in magnetic fields, and the strong decrease of the critical current, even far below T c for increasing temperature or magnetic field (see e.g. Cohen and Jensen, 1997).

Analysis of the high-temperature behavior of paraconductivity in HTSC

The Aslamazov-Larkin term of fluctuation conductivity in two-dimensional superconductors is extended to arbitrary temperatures for intermediate scattering times τ by including the dynamic and short wave-length contributions side by side with the non-local effects. Numerical results for a few different values of τ are presented.

Determination of In-Plane Anisotropy from Magnetoresistivity Measurements on YBa2Cu3O7−δ

AbstractThe magnetoresistance of six untwinned YBa2Cu3O7−δcrystals has been measured above TcforI⊥B‖c. Fluctuation theories allowing for an in-plane coherence length anisotropy (ξa≠ ξb) were used in the analyses. The results suggest that the anisotropy, γ = ξb/ξa, is considerably lower than that expected from the resistivity ratio, i.e.,