Klas Engström

Performance Gain Through Dynamic Control of Device Geometry: Nanoelectromechanical Carbon Nanotube-Based Switch

Performance advantages resulting from a dynamic control of device geometry in a nanoscale nanoelectromechanical systems device are discussed. Specifically, a nanoscale three-terminal carbon nanotube-based relay in a semiclassical analysis is investigated. As an illustration, the authors estimate the optimal subthreshold swing S of the system and find that it can be made superior to that of the ideal MOSFET; values in the range S=20-60mV/dec can be obtained with typical parameter values

Influence of electron–electron interactions on supercurrent in SNS structures

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.

Phase-Dependent Charges in S–N–S Systems

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.

Temperature-dependent resistance of a finite one-dimensional Josephson junction array

We study theoretically the temperature and array-length dependences of the resistance of a finite one-dimensional array of Josephson junctions. We use both analytic approximations and numerical simulations, and conclude that within the self-charging model, all finite arrays are resistive in the low-temperature limit. A heuristic analysis shows qualitative agreement with the resistance obtained from Monte Carlo simulations, establishing a connection between resistance and the occurrence of vortices in the corresponding 1+1D XY model. We compare our results with recent experiments and conclude that while the self-charging model reproduces some of the experimental observations, it underestimates the superconducting tendencies in the experimental structures.