A superconductor–normal quantum dot–superconductor structure where the number of electrons in the dot can be controlled by a gate voltage is considered. The effect of electron–electron interactions on the supercurrent between the two superconductors is studied. Using an analytic model and numerical density functional calculations it is found that Coulomb interactions can make the system quantum-mechanically more “rigid,” i.e. increase its sensitivity to phase gradients, thereby enhancing the supercurrent through the structure, especially for small phase differences. Accordingly, it is found that the supercurrent in this structure can be controlled by the gate voltage.
We consider a mesoscopic superconductor–normal metal–superconductor (SNS) structure and analyze the dependence of the charge in the normal part on the phase difference between the superconductors. This charge originates from both localized states inside the superconducting gap (Andreev states) and propagating states in the continuous spectrum outside the gap. We find that depending on the electrostatic potential in the normal part, which may be tuned using a gate electrode, either of the two charge species may dominate. Depending on this balance, the magnitude of the associated Josephson current through the structure may be significantly changed and even its direction reversed. A geometry suitable for detection of the phase-dependent charge is proposed.
Carbon-based nanoelectromechanical devices are approaching applications in electronics. Switches based on individual carbon nanotubes deliver record low off-state leakage currents. Arrays of vertically aligned carbon nanotubes or nanofibers can be fabricated to constitute varactors. Very porous, low density arrays of quasivertically aligned arrays of carbon nanotubes behave mechanically as a single unit with very unusual material properties.
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