Gate-controlled superconducting proximity effect in carbon nanotubes

The superconducting proximity effect in single-walled carbon nanotubes connected to niobium electrodes was controlled with the use of nearby gates that tune the niobium-nanotube transparency. At 4.2 kelvin, when the transparency was tuned to be high, a dip in the Low-bias differential resistance was observed, indicating a proximity effect mediated by Andreev reflection. When the transparency was tuned to be Low, signatures of Andreev reflection disappeared and only tunneling conduction was observed. Below similar to 4 kelvin, a narrow peak in differential resistance around zero bias appeared superimposed on the Andreev dip, probably as a result of electron-electron interaction competing with the proximity effect.