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

Abstract

We have examined the electrical properties of ultrathin insulating layers of cadmium arachidate (C 20), polyamic acid long alkyl amine salts (PAA) and polyimide (PI) by using tunnel junctions with structures of (Pb-Bi)/PAA (or C 20)/(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 C 20)/(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 C 20)/(Pb-Bi) junctions, although bridging filaments exist in the PAA (or C 20) 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.