High-temperature operation of oxide SFQ-circuit-elements

Single flux quantum (SFQ) circuit components such as an SFQ-dc converter and a confluence buffer have been fabricated by using an YBa₂Cu₃O_7-δ ramp-edge junction technology and their logic operations at temperatures up to near 60 K were investigated. The SFQ-dc converter was correctly operated in a wide temperature range from 4.2 K to 56 K and found to be useful for detecting output signals from other SFQ circuit components at any operating temperatures. The basic function that a signal from either of two input Josephson transmission lines (JTLs) was transmitted to an output JTL was confirmed for the confluence buffer and finite operating margins were obtained at temperatures from 42 K to 61 K. The narrowest margin of dc supply current obtained at temperatures from 55 K to 60 K was ±20% and was consistent with the simulation. Margin reduction due to thermal noise was also evaluated. According to the analytical calculation, the operating margin to keep the bit-error rate less than 10^-5 was as large as ±20% even at 50 K when the value of junction critical-current I_c was kept near 0.4 mA.