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

摘要

A scheme for measuring small intrinsic critical currents I_c in nanoscale devices is described. Changes in Josephson inductance L_J are converted to frequency variations that are recorded via microwave reflection measurements at 700-800 MHz. The critical current is determined from the frequency shift of the reflection magnitude at zero phase bias assuming a sinusoidal current-phase relation. The method is used to study a multiwalled carbon nanotube transistor with Pd/Nb contacts inside a resistive on-chip environment. We observe gate-tunable critical currents up to I_c ～ 8 nA corresponding to L_J > 40 nH. The method presented is also applicable to devices shunted by closed superconducting loops.