Superconductivity in Pb inverse opal

Type-II superconducting behavior was observed in highly periodic three-dimensional lead inverse opal prepared by infiltration of melted Pb in blue (D = 160 nm), green (D = 220 nm) and red (D = 300 nm) opals and followed by the extraction of the SiO2 spheres by chemical etching. The onset of a broad phase transition (ΔT = 0.3 K) was shifted from Tc = 7.196 K for bulk Pb to Tc = 7.325 K. The upper critical field Hc2 (3150 Oe) measured from high-field hysteresis loops exceeds the critical field for bulk lead (803 Oe) fourfold. Two well resolved peaks observed in the hysteresis loops were ascribed to flux penetration into the cylindrical void space that can be found in inverse opal structure and into the periodic structure of Pb nanoparticles. The red inverse opal shows pronounced oscillations of magnetic moment in the mixed state at low temperatures, T < 5.5 K. The shape of these oscillations and their temperature dependence indicate on phase transitions in the fluxoid state of Pb core. The weak matching effect in the field dependence of magneto-resistivity measurement at low magnetic fields and T > 0.9Tc has been observed for all of the samples studied. The magnetic field periodicity of resistivity modulation is in good agreement with the lattice parameter of the inverse opal structure. We attribute the failure to observe pronounced modulation in magneto-resistive measurement to difficulties in the precision orientation of the sample along the magnetic field.