NOISE FROM YBCO FILMS - SIZE AND SUBSTRATE DEPENDENCE

Electrical noise measurements at 10 Hz are reported for YBCO films at the resistive edge. Results are given for films with widths of 0.1, 1, and 5 mm that were deposited simultaneously on the same substrate, fo r three different substrate materials. The NET improves by approximate ly a factor of 10 as the thermometer area is increased by a factor of 2500, with fixed bias current. At temperatures giving maximum dR/dT an d with nominally 19 mA bias currents, the 5 mm samples have very low n oise equivalent temperatures of 3.1, 3.5, and 4.4 nK/root Hz for LaAlO 3, Al2O3, and Si substrates, respectively. These are the lowest values reported up to the present time. Suprisingly, noise from the sample o n si is consistent with pure Johnson noise even with bias current as l arge as 5 mA (0.28 x 10(4) A/cm(2)). For YBCO thicknesses no greater t han 50 nm excellent thermometers can be made on any of these substrate s in spite of the mechanical strains produced in the films by the subs trate.