Linewidth of josephson oscillations in yba2cu3o7-x grain-boundary junctions

We have studied the ac Josephsöh effect in YBa2Cu3O7-x grain-boundary junctions (GBJ) in the temperature range from 4 K to 90 K. The temperature dependence of the linewidth of millimeter-wave Josephsoii oscillations was measured and it is shown that the derived effective noise temperatures of GBJ might be as low as the physical temperature in the temperature range investigated. In the millimeter-wave range linewidths as low as 380 MHz were found at liquid nitrogen temperatures. © 1993 IEEE