Effects of disorder on voltage coherence in a modified linear Josephson array

We report numerical simulations on the effect of disorder in parallel-biased series of arrays of overdamped Josephson junctions. Our arrays feature additional parallel-conected ‘shunt’ junctions which provide strong local coupling between pairs of junctions along the series-connected ‘backbone’. We find that series arrays of 20 junctions have phase-locked and phase-coherent dynamical modes which are tolerant of more than 50% spread in critical currents,I _C , and plaquette area, S. The arrays, however, show moderate tolerance to (10% spread) disorder in bias currents,I _B . This tolerance to disorder in array parameters implies that practical RF sources might be possible using these arrays.     

Thermodynamic transition properties of highly ordered smectic phases

A series of polyethers have been synthesized from 1-(4-hydroxy-4-biphenyl)-2-(4-hydroxyphenyl)propane and , -dibromoalkanes having different numbers of methylene units [TPPs]. Both odd- and even-numbered TPPs [TPP(n=odd)s and TPP(n=even)s) exhibit multiple transitions during cooling and heating and they show little supercooling dependence, indicating close-to-equilibrium nature of these transitions. Combining the structural characterization obtainedvia wide angle X-ray diffraction powder and fiber patterns at different temperatures and the morphological observations from microscopy techniques, not only the nematic liquid crystalline phase but also highly ordered smecticF, smectic crystalG andH phases have been identified. The phase diagrams for both TPP(n=odd)s and TPP(n=even)s have been constructed [1–3]. Thermodynamic properties (enthalpy and entropy changes) during these transitions are studied based on differential scanning calorimetry experiments. The contributions of the mesogenic groups and methylene units to each ordering process can be separated and they indicate the characteristics of these processes thereby providing estimations of the transition types.This work was supported by the SZDC's Presidential Young Investigator Award from the National Science Foundation (DMR-9175538). The generous donation of a DSC-7 from Perkin Elmer Inc. is also greatly appreciated.