Intrinsic noise temperatures of YBa₂Cu₃O_7-δ Josephson devices on bicrystal substrates and the upper frequency limit for their operation

Following a method proposed by Divin and Modovets [Sov. Tech. Phys. Lett. 9, 108 (1983)], we have measured at millimeter waveband the intrinsic noise temperatures T_N of YBa₂Cu₃O_7-δ Josephson junctions or dc superconducting quantum interference devices (SQUIDs) fabricated on SrTiO₃, yttria-stabilized ZrO₂, or Si bicrystal substrates. Over wide ranges of physical temperatures T_p and the junction's normal resistance R_N, it was found that T_N follows T_P pretty well. This indicates that the intrinsic noise in the devices is dominated by Johnson noise. T_N was also measured in cases where there is external magnetic field applied, or where there is another microwave radiation like the local oscillator in a mixer. The magnetic field or microwave radiation does not seem to affect T_N in any appreciable way. To estimate the high frequency performance of the junctions on Si bicrystal substrates, direct irradiation by a far infrared laser at 1.81 THz is carried out and the clear first Shapiro step is observed.