Ofer Naaman

Tunable graphene dc superconducting quantum interference device.

Graphene exhibits unique electrical properties on account of its reduced dimensionality and relativistic band structure. When contacted with two superconducting electrodes, graphene can support Cooper pair transport, resulting in the well-known Josephson effect. We report here the fabrication and operation of a two junction dc superconducting quantum interference device (SQUID) formed by a single graphene sheet contacted with aluminum/palladium electrodes in the geometry of a loop. The supercurrent in this device can be modulated not only via an electrostatic gate but also by an applied magnetic fielda potentially powerful probe of electronic transport in graphene and an ultrasensitive platform for nanomagnetometry.

Phase-locking transition in a chirped superconducting Josephson resonator.

We observe a sharp threshold for dynamic phase locking in a high-Q transmission line resonator embedded with a Josephson tunnel junction, and driven with a purely ac, chirped microwave signal. When the drive amplitude is below a critical value, which depends on the chirp rate and is sensitive to the junction critical current I0, the resonator is only excited near its linear resonance frequency. For a larger amplitude, the resonator phase locks to the chirped drive and its amplitude grows until a deterministic maximum is reached. Near threshold, the oscillator evolves smoothly in one of two diverging trajectories, providing a way to discriminate small changes in I0 with a nonswitching detector, with potential applications in quantum state measurement.

Quantum fluctuations in the chirped pendulum

A study of autoresonant behaviour in a superconducting quantum pendulum reveals that fluctuations, both quantum and classical, only determine the initial oscillator motion, not its subsequent dynamics.

Interplay between interface structure and magnetism in NiFe/Cu/Ni-based pseudo-spin valves

Magnetic pseudo spin valves (PSVs) with superconducting Nb electrodes, have been leading candidates for an energy-efficient memory solution compatible with cryogenic operation of ultra-low power superconducting logic. Integration of these PSV Josephson junctions in a standard multi-layer Nb process requires growing high-quality thin magnetic films on a thick Nb bottom electrode (i.e. ≥1.5kA, to achieve bulk superconducting properties). However, as deposited, 1.5kA Nb exhibits a rough surface with a characteristic rice grain morphology, which severely degrades the switching properties of subsequently deposited PSVs. Therefore, in order to achieve coherent switching throughout a PSV, the Nb interface must be modified. Here, we demonstrate that the Nb surface morphology and PSV crystallinity can be altered with the incorporation of separate 50A Cu or 100A Al/50A Cu non-magnetic seed layers, and demonstrate their impact on the magnetic switching of a 15A Ni80Fe20/50A Cu/20A Ni PSV, at both room temperature an...