Electrical properties of polyimide Langmuir-Blodgett films sandwiched between superconducting electrodes: detection of microwaves

We have examined the electrical properties of ultrathin insulating layers of cadmium arachidate (C₂₀), polyamic acid long alkyl amine salts (PAA) and polyimide (PI) by using tunnel junctions with structures of (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi), Au/PI/(Pb-Bi) and Au/PI/Au. Typical current-voltage (I-V) characteristics of weakly coupled superconductors were obtained for (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi) junctions with one, two or three deposited layers at a temperature below the critical temperature of a Pb-Bi alloy. On the contrary, typical I-V characteristics of tunnel junctions were obtained for Au/PI/(Pb-Bi) and Au/PI/Au junctions, when the number of deposited layers is greater than about 30. Finally, we concluded that the efficiency of detecting microwaves is excellent in (Pb-Bi)/PAA (or C₂₀)/(Pb-Bi) junctions, although bridging filaments exist in the PAA (or C₂₀) Langmuir-Blodgett (LB) layers, and that the PI LB layers become good electrical insulating spacers when the thickness of deposited layers is more than about 11 nm.