Disorder and correlations in extended superconducting nanostructures

Experiments are presented on the magnetic properties of two types of extend ed superconducting nanostructures where disorder can be introduced in a con trolled way. Magnetotransport measurements on Nh films overlaying arrays of 250-nm diameter Ni dots show that the superstructure observed at higher mu ltiples of the matching field H-o = Phi(0)/a(2), where a = 560 nm is the do t 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 quant a trapped in individual rings repel each other due to the magnetic coupling between rings, and the system is analogous to an Ising antiferromagnet. Di sorder 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 magneti c configuration of square, honeycomb, kagome, and triangular lattice arrays containing up to 10(6) micron-size A1 rings. The dynamics are dominated by a temperature-dependent energy barrier E-B and hysteresis in the flux stat e of the ring populations. This population hysteresis is directly observed in partial derivative M/partial derivative T measurements. SHM measurements at Phi(0)/2 per ring show antiferromagnetic correlations that can be suppr essed by going to higher flux fractions due to increases in the effective r andom held. (C) 2000 Elsevier Science S.A. All rights reserved.