Fluctuation superconductivity at high magnetic fields

Apparent superconductive fluctuation conductivity ("paraconductivity") $\Delta {\sigma }_f$ has been observed in very "dirty" (short electron-mean-free-path), bulk, type-II superconductors in applied magnetic fields $H$ up to twice the zero-temperature upper critical field $H_{c20}$. Peaks observed in the isomagnetic field paraconductivity as a function of temperature at $H>\mathrm{}{H}_{c20}$ are attributed to the suppression of paraconductivity as $T$ approaches either zero or values high in comparison with the zero-$H$ transition temperature $T_c$. In the ($H-T$) region well beyond the upper-critical-field curve $H_{c2}\mathrm{}\left(T\right)\mathrm{}$, the experimentally derived $\Delta {\sigma }_f\mathrm{}(H,T)\mathrm{}$ is smaller, less dependent on the orientation of $H$ with respect to the measuring current density $J$, and decreases more rapidly with $H$ than suggested by current theory. As $H$ is increased isothermally in the 80-140-kG region, the positive magnetoresistance associated with the $H$ quenching of paraconductivity gives way to a small negative magnetoresistance which is probably not explicable on an ordinary static-localized-magnetic-moment basis.