John C. Gallop

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.

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.

Detection of single magnetic nanobead with a nano-superconducting quantum interference device

We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUID) to measure the hysteretic magnetization behavior of a single FePt nanobead at a temperature of around 7 K in a magnetic field of only ∼10 mT. We also show that the nanobead can be accurately positioned with respect to the SQUID loop and then removed without affecting SQUID performance. This system is capable of further development with wide applications in nanomagnetism.We report the use of an ultralow noise nano-superconducting quantum interference device (nanoSQUID) to measure the hysteretic magnetization behavior of a single FePt nanobead at a temperature of around 7 K in a magnetic field of only ∼10 mT. We also show that the nanobead can be accurately positioned with respect to the SQUID loop and then removed without affecting SQUID performance. This system is capable of further development with wide applications in nanomagnetism.

Microwave applications of high-temperature superconductors

For at least eight of the first ten years of high-temperature superconductivity it has been clear that one of the earliest areas of application and commercialization of these materials is as passive devices for microwave electronics. At the time of writing the first prototype products are entering service in the mobile communications industry and a wide range of other devices are at a relatively advanced stage of development. In the further future one can see a cryogenic integrated technology based on HTS materials and the related functional materials which have emerged within the last ten years.

Optimization of 2DEG InAs/GaSb Hall Sensors for Single Particle Detection

Magnetic sensors having high spatial and stray field resolutions are key elements in many biomedical applications. One promising magnetic detector is a microsized Hall sensor. We present our first results in realising a measurement system based on a Hall sensor made of an asymmetric 2DEG InAs/GaSb-based heterostructure. The work aims to investigate magnetotransport properties of such Hall sensors and optimize their performance. In particular, we focus on examining noise characteristics of the sensor as it allows us to determine and improve the device sensitivity. We show that in investigated devices a magnetic field sensitivity of better than 0.5 mu T/Hz1/2 (corresponding to a magnetization detection threshold of 2times105 muB/Hz1/2) should be readily achievable at room temperature and at a frequency of around 3 kHz.

SQUIDs: some limits to measurement*

SQUIDs are used extensively across the world of precision measurement in physics and many other sciences, including earth science studies and medicine, providing some of the most precise measurements possible. The paper will discuss some of the limits to SQUID measurements which emerge in these various fields. One of the newer areas of interest is the use of SQUIDs to measure the properties of nanomagnetic particles or even single spins. This is driven by the needs of future ultra high density magnetic storage and quantum information processing. The paper will focus on this issue and will propose some hard limits as well as some means by which the present state-of-the-art may be extended towards those limits.

Non-contact method for measurement of the microwave conductivity of graphene

We report a non-contact method for conductivity and sheet resistance measurements of monolayer and few layers graphene samples using a high Q microwave dielectric resonator perturbation technique, with the aim of fast and accurate measurement. The dynamic range of the microwave conductivity measurements makes this technique sensitive to a range of imperfections and impurities and can provide rapid non-contacting characterisation. As a demonstration of the power of the technique, we present results for graphene samples grown by three different methods with widely differing sheet resistance values.

Tunable microwave components based on dielectric nonlinearity by using HTS/ferroelectric thin films

This study provides the possibility of developing tunable microwave components based on the dielectric substrate nonlinearity, with the conducting surfaces made of a superconductor. The displacement vector, the dipole moment, polarization, polarizability, susceptibility, and relative permittivity concepts are used for ferroelectrics; and for superconductors, Bose statistics and the Gorter and the Casimir model for a two-fluid model, Londons equations, and the classical skin effect for the normal component of the current are used. A sinusoidal wave solution is found for a planar superconducting transmission line. This solution gives expressions for the phase velocity and attenuation coefficient which are used to characterize the tunability of microwave components. The measured data in the literature have been used to compute the relative phase velocities and phase shift per cm versus temperature and the dc bias electric field E (kV/cm). It is shown that with a ferroelectric film of thickness of 140 nm, with /spl epsiv//sub /spl tau//=2/spl times/10/sup 3/ and tan /spl delta/=10/sup -3/ phase shifts and attenuation of the order of tens of degrees per centimeter and 5.76/spl times/10/sup -3/ dB/cm, respectively, at 10 GHz, can be obtained with tens of millivolts at 4 K.

The microwave surface impedance of MgB2 thin films

In this paper we present the results of measurements of the microwave surface impedance of a powder sample and two films of MgB2. The powder sample has a Tc = 39 K and the films have Tc = 29 K and 38 K. These samples show different temperature dependences of the field penetration depth. Over a period of six months, the film with Tc = 38 K degraded to a Tc of 35 K. We compare the results on all samples with data obtained elsewhere and discuss the implications as far as is possible at this stage.

Field solution for a thin-film superconducting parallel-plate transmission line

Abstract By using Bose statistics and the Gorter and Casimir model for a two-fluid model, Londons equations, and the classical skin effect for the normal component of the current, a sinusoidal wave solution is found for a superconducting transmission line. This solution gives expressions for the phase velocity and attenuation coefficient which are used to compare the behaviour of a wave in LTSs and HTSs and to investigate the value of the Q -factor of a parallel-plate resonator. The computed values of the Q -factor are compared with measured data in the literature.