J C Macfarlane

Josephson vortices and flux penetration in high temperature superconductors

Abstract We have investigated, experimentally and theoretically, the magnetic flux which threads a YBa 2 Cu 3 O 7− x cylinder sample at 77 K when a weak low frequency ac magnetic field is applied. We explain the highly nonlinear magnetic response in great detail quantitatively, using a critical state model where during each ac cycle, intergranular (Josephson) vortices sweep in and out of the cylinder. The Josephson vortex pinning force is found to be field independent and several orders of magnitude smaller than in conventional type II superconductors. This explains the weak shielding properties and low critical current density found in this material. At magnetic fields greater than about 50 Oe, vortices start to enter the superconducting grains.

Terahertz imaging at 77 K

Terahertz (THz) technology is receiving increasing attention around the world due to its important potential in many application areas. Novel compact solid-state sources and detectors are being sought for?THz radiation and detection. We report the realization of a?THz imager based on a high- Tc superconducting (HTS) Josephson detector working above liquid nitrogen temperature (77?K). The detector, made of a YBa2Cu3O7?x (YBCO) step-edge Josephson junction, is coupled to a thin-film ring-slot antenna and a hemispheric silicon lens. Images of high visual quality are obtained which demonstrate unique properties of?THz radiation such as the sensitivity to water content and the ability to penetrate packaging materials. The results should stimulate further research leading to the development of a HTS superconducting?THz imaging system.

Terahertz imaging using a high-Tc superconducting Josephson junction detector

A high-Tc superconducting (HTS) detector based on a YBa2Cu3O7−x (YBCO) step-edge Josephson junction has been developed and applied to terahertz (THz) detection. The detector was coupled to a ring-slot antenna designed for operation at 600 GHz, and used for THz imaging. The results suggest that the characteristic voltage and frequency of our HTS step-edge junctions can be readily optimized for the chosen THz frequency range at easily achievable temperatures. The images also clearly demonstrate some of the unique properties of THz radiation, including the sensitivity to water content and the ability to penetrate packaging materials. (Some figures in this article are in colour only in the electronic version)

HTS Josephson heterodyne oscillator on a pulse-tube cryocooler

A high-temperature superconducting (HTS) Josephson heterodyne oscillator based on step-edge junction technology has recently been developed (Du et al 2008 Appl. Phys. Lett. 93 033507, Macfarlane et al 2009 IEEE Trans. Appl. Supercond. 19 920). In this work, the implementation and characterization of such a heterodyne oscillator on a compact pulse-tube cryocooler (PTC) are presented. The rf performance of the oscillator cooled by the cryocooler is compared to that of the same device when cooled in the quiet gas phase of a liquid helium Dewar. Any measurable influence of additional electromagnetic noise and mechanical vibration of the cryocooler on the oscillator performance is assessed by measuring the linewidth broadening of the heterodyne oscillation. The cryocooled oscillator demonstrated excellent performance and negligible excess noise was observed when operating the PTC.

Intergranular flux pinning in high temperature superconductors

Abstract A critical state model has been employed with an intergranular pinning force density which is independent of the local magnetic field, to calculate the critical transport current and the ac susceptibility of sintered Y Ba Cu O as a function of temperature. The assumed temperature dependence of the pinning force density is consistent with measurements on long tubular samples. The flux creep term of the effective intergranular pinning force causes a weak frequency dependence of the ac susceptibility.

Magnetic levitation force measurement on high T/sub c/ superconducting ceramic/polymer composites

An experimental study of magnetic levitation force for 0-3 and 3-3 superconducting ceramic-polymer composites is presented. A simple, inexpensive force-versus-distance measurement technique is described. The measurements of force against distance or magnetic field show strong hysteretic behavior, which is similar to the sintered superconductor ceramics and is consistent with the hysteresis in magnetization of superconductor. The volume fraction dependence and sample thickness dependence of the levitation forces are also studied for 0-3 composites. Results suggest that the new composite materials are most suitable for levitation applications. >

Issues relating to airborne applications of HTS SQUIDs

Airborne application of HTS SQUIDs is the most difficult environment for their successful deployment. In order to operate with the sensitivity required for a particular application, there are many issues to be addressed such as the need for very wide dynamic range electronics, motion noise elimination, immunity to large changing magnetic fields and cultural noise sources. This paper reviews what is necessary to achieve an airborne system giving examples in geophysical mineral exploration. It will consider issues relating to device design and fabrication, electronics, dewar design, suspension system requirements and noise elimination methods.

Towards large scale HTS Josephson detector arrays for THz imaging

We present the design and implementation of a high-temperature superconducting (HTS) Josephson junction detector array for terahertz (THz) imaging. The array device is made of ten YBa2Cu3Ox 7 (YBCO) step-edge junctions coupled to gold thin-film ring-slot antennas on a MgO substrate. The design and characterization of the detector array in response to a 0.6 THz signal are presented. The development of multi-channel biasing and read-out electronics and the system integration with a commercial cryocooler are also described. (Some figures may appear in colour only in the online journal)

Synchronous rotation of a floating magnet and flux penetration in Y-Ba-Cu-O superconductor

An experimental demonstration of induced rotation of a rectangular magnet floating above a superconducting ring is described. Synchronism between the rotating and the applied alternating magnetic field was observed under certain conditions. The stability of the levitated magnet and the mechanism of the rotation are discussed on the basis of the authors studies of flux penetration into superconductors. >

Transport properties of variable-angle YBa2Cu3O7 − δ step-edge junctions in the a–b plane

The superconductor–barrier–superconductor interface in a range of YBa2Cu3O7 − δ (YBCO) step-edge Josephson junctions was engineered to lie at angles θ other than normal to the a–b axes of the YBCO crystal. This systematic study enabled the effect of the d-wave anisotropy to be probed over the range θ = 0° ([100] direction) to θ = 45° ([110] direction) in a way not possible with junction technologies such as bicrystals. Anomalous temperature dependence of the critical current and a peak in the differential conductance at low voltage were reminiscent of features attributed elsewhere to zero-energy states in a range of [001]-tilt bicrystal and ramp-edge junctions. Moreover, IcRN products decreased with increasing θ, and conductance minima or gap-related structures were measured for large θ. The data reported here for variable-angle junctions are compared with results from studies of other grain boundary junctions (such as bicrystal and biepitaxial junctions) and provide support for theories developed by Shirai et al (2003 J. Phys. Soc. Japan 72 2299–307) in particular.