Simon K. H. Lam

Fabrication and characterisation of YBCO single grain boundary step edge junctions

We use ion beam etching techniques to fabricate YBCO step edge junctions (SEJ) on MgO substrates. Argon ion-beam etching (IBE) of the substrate at angles other than at normal incidence is used to define the step height and angle. Thin (/spl sim/300 nm) magnetron sputtered YBCO films are deposited over the step and patterned using microlithography and cold substrate ion-beam etching techniques. The critical current, I/sub c/ of these SEJs can be controlled by varying the angle of the step etched into the substrate. Fabrication techniques are described which produce one grain boundary at the top of the step and include a smooth return path thereby avoiding a second grain boundary at the bottom of the step. At 77 K, the current-voltage (I-V) characteristics show resistively shunted junction behaviour. These junctions routinely demonstrate reasonably large I/sub C/R/sub N/ products (0.1-0.6 mV), making them suitable for applications in high temperature SQUID devices.

Facile synthesis of graphene oxide hybrids bridged by copper ions for increased conductivity

In this article, we report a facile method for preparing graphene oxide (GO) hybrid materials consisting of copper ions (Cu2+) complexed with GO, where Cu2+ acted as bridges connecting GO sheets. The method of film formation is based on cross-linking GO using Cu2+ followed by filtration onto nanoporous supports. This binding can be rationalized due to the chemical interaction between the functional groups on GO and the metal ion. We observed that there was a decrease in charge transfer resistance through electrochemical study. It suggests that the presence of metal ions in GO films could introduce new energy levels along the electron transport pathway and open up possible conduction channels. We also found that the hybrid graphene film assembled with Cu2+ dramatically decreases resistance through flash light reduction.

The effects of step angle on step edge Josephson junctions on MgO

We have fabricated step edge junctions using MgO substrates and YBCO thin films. By varying the angle of the step edge over a range of angles up to 45/spl deg/, we have obtained 3 distinct step edge morphologies: a deep trench junction, a double junction and a single junction. We found that only the step angle and morphology affected the critical current density (I/sub c/) and that the film thickness-to-step height ratio had no effect over the range 0.2-1.1. Noise measurements indicated that the single junction steps had the lowest level of critical current fluctuations and the highest values of dynamic resistance. We have also studied the variation of I/sub c/ with temperature and found it follows the Ambergaokar-Baratoff model with a lower zero energy gap. We use this information to confirm that the junction parameters are affected by the c-axis tilt and the in-plane orientations proposed by others and consider the transport mechanisms across the junction.

A nanoscale SQUID operating at high magnetic fields

A washer-free Nb nanoSQUID has been developed for measuring magnetization changes from nanoscale objects. The SQUID loop is etched into a 250 nm wide Au/Nb bilayer track and the diameter of the SQUID hole is ∼70 nm. In the presence of a magnetic field perpendicular to the plane of the SQUID, vortex penetration into the 250 nm wide track can be observed via the critical current-applied field characteristic and the value at which vortex first penetrates is consistent with the theoretical prediction. Upon removing the applied field, the penetrated vortices escape the track and the critical current at zero field is restored.

Operation of a geophysical HTS SQUID system in sub-Arctic environments

Transient ElectroMagnetic geophysical prospecting using SQUID sensors has demonstrated potential for improved target detection at late response times compared to conventional coil sensors. We have developed a three-axis, rf SQUID sensor system which has been extensively operated in sub-Arctic conditions by a geophysics contractor. Due to the harsh environmental and operating conditions, the system is designed to operate in sub-zero temperatures (as low as minus 40/spl deg/C) and is ruggedly packaged whilst still remaining quite portable. Auto-tuning of the rf electronics has been implemented by adjusting the rf SQUID control parameters via a microprocessor controller. After a small amount of training, regular field crews have operated two of these systems in the field continuously for periods of months at a time. An example, comparing SQUID B field data to coil dB/dt data, is presented in this paper.

Attachment of magnetic molecules on a nanoSQUID

A novel procedure combining monolayer self-assembly with electron beam lithography has been developed for attaching ferritin nanoparticles to a submicron thin-film SQUID (superconducting quantum interference device). After opening a window in the PMMA (polymethylmethacrylate) resist, organic linker molecules are used to attach ferritin to the exposed parts of the gold overlayer of a Nb nanoSQUID. This allows the magnetic nanoparticles to be located optimally as far as magnetic coupling to the nanoSQUID is concerned.

Confirmation of the INRiM and PTB Determinations of the Si Lattice Parameter

As metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth A is much greater than the film thickness and is comparable with the lateral dimensions of the deviceAs metrology extends toward the nanoscale, a number of potential applications and new challenges arise. By combining photolithography with focused ion beam and/or electron beam methods, superconducting quantum interference devices (SQUIDs) with loop dimensions down to 200 nm and superconducting bridge dimensions of the order 80 nm have been produced. These SQUIDs have a range of potential applications. As an illustration, we describe a method for characterizing the effective area and the magnetic penetration depth of a structured superconducting thin film in the extreme limit, where the superconducting penetration depth A is much greater than the film thickness and is comparable with the lateral dimensions of the device

Inductive superconducting transition-edge detector for single-photon and macro-molecule detection

We present a new type of transition-edge sensor for single-photon and macro-molecule detection. In our detector the absorber element is an isolated, passive absorber of extremely low thermal mass, maintained close to, but below, its superconducting–normal transition, and strongly inductively coupled to a SQUID sensor. Incoming particles or photons are sensed in terms of a transient change in the inductive coupling, rather than a change in resistance. The detectors energy sensitivity and response time can be defined by the thermal mass of the absorber and its thermal contact with a substrate, independently of any electrical connections. We have modelled the energy sensitivity of our inductive superconducting transition-edge sensor using a sub-micron SQUID as an inductive read-out device. An ultimate energy resolution of order 10−25 J Hz−1 is theoretically estimated based on the properties of the read-out SQUID and the dimensions of the absorber. We also report our initial work on fabrication of the Nb nanoscale SQUID where we have used the same material deposited on top of the SQUID as a thin-film absorber.

Metallization and interconnection of HTS YBCO thin film devices and circuits

A comprehensive study of specific contact resistivity and ultrasonic wire bonding yield and strength was carried out on noble metal-YBa2Cu3O7-x (YBCO) thin film contacts prepared by a variety of methods and with different YBCO surface conditions. The metallization techniques investigated include in situ and ex situ deposition of gold or silver on YBCO films. The ex situ contacts were made with and without lithographic processes. Contact resistivities of less than 5×10-8 Ω cm2 at 77 K were achieved for contacts made by the rapid ex situ deposition of gold or silver on fresh YBCO films with smooth surfaces. These contacts also gave a high wire bonding yield and strength of 10-19 g. High contact resistivities in excess of 5×10-4 Ω cm2 and poor wire bonding yield and strength were observed for the contacts made by standard lithographic lift-off processes on old YBCO films. Surface treatments using either argon ion beam etching or rf O2 plasma cleaning prior to metallization were found to be useful in reducing the contact resistivity and improving the wire bonding results for the lift-off contacts. The influence of YBCO film morphology on the contact resistance and wire bonding yield and strength was also studied.

Geophysical exploration using magnetic gradiometry based on HTS SQUIDs

Magnetic tensor gradiometry provides gradient components of true potential fields which enables unique depth estimates and improves analytic signal methods as well as providing a number of other advantages. A high temperature SQUID (HTS) gradiometer can provide measurements of the components of the earths field tensor creating a new tool for mineral exploration. A successful comparison between a HTS SQUID gradiometer and a Cs-vapour gradiometer under survey conditions has been conducted. Both instruments were configured vertically. The HTS gradiometer measured the B/sub zz/ component of the gradient tensor, while the Cs-vapor gradiometer measured the vertical gradient of the total magnetic intensity. The HTS gradient measurement was the difference in output between two coaxial SQUID sensors. Effective noise levels achieved were 0.16-0.3 nT/m RMS, compared with 0.1-0.5 nT/m RMS for the Cs-vapor system. The SQUID noise was dominated by vibration with additional contributions from the multiplexed sampling between the SQUIDs. This paper reports on the system development, design issues, trial results and the implications for geophysical exploration.