Millimeter-wave response in NbN(g)/Al nanobridges

The testing of NbN(g) nanobridges with Al direct shunt resistors for use as millimeter-wave Josephson mixers is discussed. The NbN(g)/Al nanobridges have a nearly sinusoidal current-phase relation, high resistance, well-defined gap structure at ~4 mV, large I_J R_N products of ~3 mV, and nonhysteresis I -V curves. By means of the Al direct shunt, the effective noise temperature T_N^eff of the NbN(g) nanobridges, which was calculated by transition-state theory, was reduced to 4.2 K as a result of reducing the self-heating and noise rounding effects. The NbN(g)/Al nanobridges show almost ideal Josephson response to millimeter-wave radiation at 106 GHz. The well-pronounced Josephson steps were observed up to ~4 mV. The observed systematic decreasing of the step heights with increasing the RF current can be accounted for by the effect of the thermal noise if an effective noise temperature of T_N=15 K (at i_RF=3) is assumed. The IF peaks in the Josephson mixing were obtained up to the bias voltage of ~1.2 mV. The NEP was about 3×10^-20 W/Hz at 4.2 K