Microwave reflection measurement of critical currents in a nanotube Josephson transistorwith a resistive environment

Abstract

A scheme for measuring small intrinsic critical currentsIc in nanoscale devices is described. Changes in Josephson inductanceLJ are converted to frequency variations that are recorded via microwave reflection measurementsat 700–800 MHz. The critical current is determined from the frequency shift of the reflectionmagnitude at zero phase bias assuming a sinusoidal current–phase relation. The method isused to study a multiwalled carbon nanotube transistor with Pd/Nb contacts inside aresistive on-chip environment. We observe gate-tunable critical currents up toIc ∼ 8 nA correspondingto LJ > 40 nH. The method presented is also applicable to devices shunted by closed superconductingloops.