Critical currents and scaling laws in sputtered copper molybdenum sulfide

We have measured critical current densities J_c, of superconducting copper molybdenum sulfide prepared by sputtering onto heated sapphire substrates. In addition, we have examined the samples with SEM, TEM, electron microprobe, and x-ray diffraction techniques. Critical currents were measured as a function of external magnetic field and as a function of temperature and sample preparation conditions. We find that a scaling law P = CB_c2ⁿf(b) describes our results, where P is the pinning force per unit volume, B_c2 is the upper critical field, b is the reduced field b ≡ B/B_c2, f(b) is a function of b only, and C is a constant of proportionality. Our results fit the flux shearing model of Kramer from relatively low b (≃0.5) to b = 1. Furthermore, applying this model to PbMo₆S₈, we estimate the critical current density as a function of magnetic field. For example, we predict J_c10⁸ A/m² at 26 T. From analysis of all experimental data on our samples, we conclude that flux pinning is mainly on excess Mo metal. The Mo is present as a second phase in the predominantly Chevrel-phase Cu_xMo₆S₈, while all the Cu metal is included in the Chevrel phase.