COMMENSURATE STATES IN DISORDERED QUASI-PERIODIC NETWORKS

In this paper, we report measurements of the normal-metal-superconduct or phase boundary T(c)(H) in disordered quasiperiodic wire networks. T he initially ordered network is an eightfold quasiperiodic array of th e type previously constructed by Ammann, and several types of disorder have been introduced. One series of arrays is areally disordered by s tretching and contracting individual worm segments in the network, and the resulting phase boundary exhibits a gradual decay of all phase-bo undary structure with increasing field. A second series is perturbed b y phason disorder, which amounts to the local flipping of small symmet ric clusters of tiles and destroys the local ordering inherent in the inflation symmetry of the ordered network. This type of disorder wipes out higher-order commensurate states but has no effect on the major s tructure of the phase boundary. A third type of array is created by a special modification of the ordered eightfold lattice and results in a fourfold geometry that is approximately inflation symmetric. Its phas e boundary exhibits commensurate states that can be related to the app roximate inflation symmetry. We use the J2 model, in which one conside rs only the kinetic energy of the supercurrents induced by fluxoid qua ntization, to describe accurately the overall behavior of the measured phase boundaries. The experimental results presented here show that c ommensurate states based on inflation symmetry are strongly favored in these particular quasicrystal geometries (even when the inflation sym metry is only approximate).