MILLIMETER-WAVE RESPONSE IN NBN(G)/AL NANOBRIDGES

We report on the testing of NbN(g) nanobridges with Al "direct shunt" resistors for use as millimeter-wave Josephson mixers. The NbN(g)/Al nanobridges have nearly sinusoidal current-phase relation, high resistance, well-defined gap structure at approximately 4mV, large I(j) R(n) products of approximately 3mV 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 4.2K as a result of reducing the self-heating and noise rounding effects. The present NbN(g)/Al nanobridges showed almost ideal Josephson response to millimeter-wave radiation at 106GHz. The well-pronounced Josephson steps were observed up to approximately 4mV. 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) = 15K(at i(R F) = 3) is assumed. The IF peaks in Josephson mixing were obtained up to the bias voltage of approximately 1.2mV. The NEP was about 3 x 10(-20)W/Hz at 4.2K.