High-field superconductivity in Al-O-Pb and Al-O-Pb-Bi sputtered films

The sputter-quenched Al_xO_y alloys containing lead and bismuth elements, which are insoluble to aluminium; have been found to exhibit a remarkably enhanced upper critical field,H_c2, which is higher by about 6 to 66 times than those of pure lead metal and Pb₆₀Bi₄₀ alloy. The sputtered structure consists of amorphous Al_xO_y and f c c lead or h c p (lead-bismuth) phases. The lead and phases disperse homogeneously in the amorphous matrix and their particle sizes and interparticle distances are about 10 to 20 nm and 5 to 20 nm, respectively, for lead and particles. The superconducting transition temperature,T_c, upper critical magnetic field,H_c2, at 4.3 K, and residual resistivity at 10 K are 7.74 K, 3.3 T and 2.09 × 105cm, respectively, for (Al-O)_92.8 Pb_7.2 and 7.45 K, 8.2 T and 1.70 × 106 cm, respectively, for (Al-O)_86.2(Pb_0.6 Bi_0.4)_13.8. The remarkable enhancement ofH_c2 is interpreted as being mainly due to a remarkable decrease of the coherence length resulting from a large reduction of the effective mean free path of electrons. Additionally, the fluxoid pinning force under applied field has also been markedly enhanced for the duplex structure films than for sputtered lead film, probably because of the change of the dispersed lead phase into a type-II superconductor and an effective fluxoid pinning action at the interface between Al_xO_y and fine lead particles.