John C. Macfarlane

Ballistic thermal and electrical conductance measurements on individual multiwall carbon nanotubes

We report thermal measurements on individual carbon nanotubes using a temperature sensing scanned microscope probe. An arc-grown bundle of multiwalled nanotubes (MWNTs) is mechanically attached to a thermal probe. The heat flow down individual MWNTs is recorded as a function of the temperature difference across them. Simultaneous measurements of thermal and electrical conductance are recorded. The size of the conductance steps observed at room temperature and the correlation between electrical and thermal conductance steps are discussed and we present evidence for ballistic transport of both phonons and electrons in these tubes.

Using a 77 K SQUID to measure magnetic fields for NDE

A bare HTS SQUID of commercial design was used in 77 K experiments concerning NDE. The SQUID was operated with flux-locked instrumentation to provide a noise floor of 80 pT//spl radic/Hz. The effective sensor area was measured to be approximately 70 /spl mu/m/sup 2/ equivalent to an ideal point detector for NDE. The SQUID was used unshielded in a normal laboratory environment in a special purpose LN/sub 2/ cryostat positioned above a motorized computer-controlled scanning system. We measured magnetic fields associated with current flowing in wires and compared them with calculations. We also detected a simulated flaw in an aluminum plate using an eddy current technique and made a preliminary depth assessment by frequency sweeping. Although developments in electronic gradiometers and gradiometric SQUIDs should make the use of single bare magnetometer SQUIDs unnecessary, we show that these already have sufficient sensitivity for NDE research, even without flux-focusing washers or pick-up coils.<<ETX>>

Excess noise in thin film grain boundary Josephson junctions and devices

The subject of electronic noise in high- superconducting Josephson devices and in their applications is considered. Several types of grain boundary junctions, prepared in different ways by four separate international laboratories, are fully characterized in terms of their electrical and noise properties at a range of temperatures, frequencies, magnetic fields and in the presence of microwaves. Similar characterization is carried out for multijunction Josephson flux-flow arrays, and bi-epitaxial SQUIDs. The theory of Likharev and Semenov for thermal noise in low- junctions is adopted as a reference against which excess noise can be identified in high- junctions. It is combined with theoretical models of excess noise based on fluctuations of critical current and resistance to provide accurate fitting of the observed noise curves. It is shown that the normalized levels of critical current and normal resistance fluctuations for all of the junction types examined are remarkably close (typically around and at 100 Hz) and are nearly independent of temperature in the range T = 30 - 80 K. The frequency dependence is close to the universal 1/f law with some deviations due to trapping of charge carriers at discrete states in the tunnelling barrier. Measurements of voltage noise levels and critical current fluctuations in multijunction flux-flow amplifiers, reported for the first time, are consistent with levels in single junctions of the same type. Excess noise is studied as a function of external magnetic field, in a way that has not been previously described. A new interpretation of the magnetic field-dependent noise data indicates that absolute levels of critical current fluctuations are nearly independent of applied magnetic field. This observation does not appear to have been reported elsewhere. It has important implications for the operation of grain boundary Josephson devices in real applications. Voltage noise in the presence of microwave irradiation, at operating points between Shapiro steps, is consistent with a modified form of the Likharev - Semenov equation which takes into account the known level of critical current fluctuations as measured in field-free conditions. The implications of these results for further improvements in device performance are discussed, and directions for further work are suggested.

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.

Simulation and Measurement of HTS Josephson Heterodyne Oscillator

We report continuing investigations into practical applications of the AC Josephson effect as the basis for a voltage-tunable radio-frequency oscillator. We have previously demonstrated experimentally that useful power levels (10 s of nW) and linewidths of a few kHz can be achieved in the heterodyne output from a high-temperature-superconducting resistive SQUID (HTS-RSQUID) operating in the frequency range 1-50 MHz. Those results were achieved with 2-junction R-SQUIDs incorporating current-biased shunt resistors of a few micro-ohms. We have now modified the fabrication procedures, and adjusted the shunt resistors and bias current values so that higher frequencies can be achieved. The Josephson junctions are of step-edge type, rather than the bi-crystal type used in our earlier work. The step-edge technique permits much more flexibility in the geometrical lay-out and utilizes the more cost-effective single-crystal MgO substrates. In the present paper, we report numerical simulations and experimental measurements on these devices in the frequency range up to 2 GHz.

SQUID-based nondestructive evaluation of carbon fiber reinforced polymer

Recent work with HTS SQUIDs in nondestructive evaluation has concentrated on the detection of flaws in aircraft-grade aluminum, with particular emphasis on surface-breaking tears beside rivets. More complex materials are now also being used in aircraft manufacture, with carbon fiber reinforced polymer (CFRP) being one of the most common. Existing technologies such as ultrasound are particularly well suited to the detection of impact-damaged sites and until now there have only been a few reports of eddy current examination of CFRP samples. Here we present results on samples with regions of heat damage, impact damage and with nonmagnetic inserts using eddy current detection techniques. We compare the signal to noise ratio and spatial resolution for a variety of sensors including HTS SQUIDs and gradiometers and conventional induction coils, and discuss variations in detection efficiency with field component measured.

Miniature dc SQUID devices for the detection of single atomic spin-flips

Abstract We report progress towards a superconducting quantum interference device (SQUID) based system capable of detecting single atomic spin-flips. The scaling of the flux sensitivity with SQUID loop dimension of miniature Nb dc SQUIDs is examined and shown experimentally to vary as predicted. Our smallest device, with loop size 3×3 μm 2 , is capable of detecting a few spins in a 1 Hz bandwidth. We address the task of depositing a sample, of nanoscale dimension, within the SQUID loop.

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.

Design and performance of an HTS current comparator for charged-particle-beam measurements

Superconducting direct current comparators are well established for operation at liquid helium temperatures. We have begun investigations into HTS Cryogenic Current Comparator designs, which incorporate a flux concentrator and readout SQUID also made from HTS. This work aims to produce a system capable of high accuracy, non-invasive, measurements of an ion or electron beam current in the range /spl sim/10 nA to 1 mA. We outline a proposal for a precise determination of the faraday fundamental constant, using purely physical systems, and employing an HTS CCC for charged particle beam current measurement. In addition estimates of the potential current sensitivity are reported, based on measurement results of a prototype system, and proof-of-principle measurements of ion beam currents are described. The design and construction of a robust, compact cryogenic unit suitable for further, more realistic, field trials are discussed.