Disorder and correlations in extended superconducting nanostructures

Experiments are presented on the magnetic properties of two types of extended superconducting nanostructures where disorder can be introduced in a controlled way. Magnetotransport measurements on Nb films overlaying arrays of 250-nm diameter Ni dots show that the superstructure observed at higher multiples of the matching field H0=Φ0/a2, where a=560 nm is the dot lattice constant, are systematically suppressed as disorder is introduced into the dot arrays. In arrays of superconducting rings in external fields corresponding to half-integral numbers of flux quanta per ring, flux quanta trapped in individual rings repel each other due to the magnetic coupling between rings, and the system is analogous to an Ising antiferromagnet. Disorder enters through small, random variations in ring sizes, and plays the role of a random field in the Ising model. SQUID magnetometry and scanning Hall microscopy (SHM) were used to probe the dynamics and specific magnetic configuration of square, honeycomb, kagomé, and triangular lattice arrays containing up to 106 micron-size Al rings. The dynamics are dominated by a temperature-dependent energy barrier EB and hysteresis in the flux state of the ring populations. This population hysteresis is directly observed in ∂M/∂T measurements. SHM measurements at Φ0/2 per ring show antiferromagnetic correlations that can be suppressed by going to higher flux fractions due to increases in the effective random field.