Nanoscale Superconducting Quantum Interference Devices Research Articles

Characteristics of focused ion beam nanoscale Josephson devices

The requirements of quantum metrology and nanoscience are driving the need forsingle-particle detection capability across a wide variety of physics areas, includingquantum information processing, single-photon detection, nanoelectromechanicalsystems, nanomagnetism and spintronics. The single particles to be detected includeatoms, molecules, photons, spins and even phonons, in the future. Nanoscalesuperconducting quantum interference devices (nanoSQUIDs) represent a newmanifestation of an old but exciting superconducting technology which addresses someof these requirements. In this paper we describe a straightforward approach tofabricating Nb microbridge weak links using combined optical lithography andfocused ion beams which may be used to fabricate nanoSQUIDs. The devices shownon-hysteretic current–voltage characteristics and demonstrate very low noise,even at operating temperatures above 4.2 K. To improve our understanding of thesuperconducting properties of the Josephson microbridge/nanobridge junctions which haveproved very successful in realizing low noise nanoSQUIDs we have carried out acombination of investigations. These include cryogenic resistance versus temperature (R(T)) and current–voltage characteristic measurements, atomic force microscope scans,controlled gallium (Ga) ion milling and implantation, and ion beam track modelling ofthese Josephson devices and Nb thin films.

NanoSQUID sensitivity for isolated dipoles and small spin populations

The sensitivity of nanoscale SQUIDs (nanoSQUIDs; SQUID: superconducting quantuminterference device) to single and small populations of magnetic dipoles is considered. Thesimple estimate given previously for the atomic spin sensitivity of a nanoSQUIDcoupled to an isolated magnetic dipole at its centre is confirmed. It is demonstratedthat the sensitivity is constrained in most practical situations by the finite sizeof the SQUID loop and nanobridges. An exact analytic result is obtained for ananoSQUID composed of an idealized filamentary circular loop. The issue of optimumplacement and orientation of the dipole with respect to the nanoSQUID hole is alsoconsidered, and it is shown that the optimum position for the dipole depends upon theheight of the dipole above the plane of the nanoSQUID. It is pointed out that theconclusion quoted by previous authors, that the optimum position is above one of theJosephson junctions or Dayem bridges, although true in the limit of very narrowbridges with tight magnetic coupling, is not true in general, and estimates of thepotential sensitivity when the dipole is placed in this region based on simplefilamentary models are likely to overestimate the sensitivity achievable in practice.

Fabrication and characterization ofhigh-Tc YBa2Cu3O7−x nanoSQUIDs made by focused ion beam milling

We have fabricated high-Tc nanoscale superconducting quantum interference devices (nanoSQUIDs) with a hole size of250 nm × 250 nm based on a 100 nm bridge at 77 K by focused ion beam milling and ionimplantation. At 78 K, the curve of the voltage branch became roughly linearand agreed with the Josephson-like behavior. The sample exhibited strong fluxflow behavior at temperatures under 76 K. The voltage flux characteristic curves,V –Imod,of the nanoSQUID at different bias currents at 78 K were observed. Typically, critical currents of15 µA and peak-to-peak values of the voltage flux transfer function of3.7 µV were measured. The measured data strongly suggest that the weak linkstructure could be a superconducting metal with a critical temperatureTc′ smallerthan that (Tc) of other YBa2Cu3O7−x (YBCO) films. This fabrication method of combining a nanobridge and ion implantationcan improve the yield of nanojunctions and nanoSQUIDs.

Measurement and noise performance of nano-superconducting-quantum-interference devices fabricated by focused ion beam

Science and industry demand ever more sensitive measurements on ever smaller systems, as exemplified by spintronics, nanoelectromechanical system, and spin-based quantum information processing, where single electronic spin detection poses a grand challenge. Superconducting quantum interference devices (SQUIDs) have yet to be effectively applied to nanoscale measurements. Here, we show that a simple bilayer deposition route, combining photolithography with focused ion beam patterning, produces high performance nanoscale SQUIDs. We present results of noise measurements on these nanoSQUIDs which correspond to a magnetic flux sensitivity of around 0.2μΦ0∕Hz1∕2. This represents one of the lowest noise values achieved for a SQUID device operating above 1K.