GINZBURG-LANDAU THEORY OF THE UPPER CRITICAL-FIELD IN FILAMENTARY SUPERCONDUCTORS

The upper critical field for coupled filamentary superconductors is analyzed within the context of a Ginzburg-Landau theory similar to the Lawrence-Doniach theory for coupled layered superconductors. Upward curvature in the critical field as the temperature is lowered results from the decreased coupling of the filaments, and an ultimate divergence in the critical field at all angles occurs below a decoupling temperature T*. Unusual anomalies are predicted to occur in the Hc2a(T) curve, corresponding to a commensurate fitting of the vortices into the filament lattice. The behavior of Hc2 for coupled filaments is contrasted with that of an isolated fiber of finite diameter. The model is applied to (SN)x, to the transition-metal trichalcogenides NbSe3 and TaSe3, and to mercury embedded in asbestos. © 1979 The American Physical Society.