C.M. Muirhead

Copper oxide superconductors containing sulphate and phosphate groups

Abstract Following observations that carbonate anions can be substituted into perovskite structures containing Cu to yield superconducting phases, the possibility that other oxy-anions may behave similarly has been explored. Here we report the incorporation of sulphate and phosphate groups into the YBa 2 Cu 3 O 7−δ structure, to form materials such as YSr 2 Cu 2.79 (SO 4 ) 0.21 O 6.16 . The partial replacement of Y 3+ ions by Sr 2+ or Ca 2+ ions injects holes into the CuO 2 layers and results in a change from antiferromagnetic to superconducting behaviour with T c (onset) up to ≈60 K. These materials contain not only new charge reservoir layers but also novel superconducting CuO 2 sheets with both 4-coordinate Cu (as in Nd 2 CuO 4 ) and 5-coordinate Cu (as in YBa 2 Cu 3 O 7 ).

Superconductivity in the system (Y, Ca, Sr) (Ba, Sr)2 (Cu3−z(SO4)zOx

Abstract The effects of partial substitution of Ba for Sr on the superconducting transition in the sulphate containing materials [Y0.74Ca0.16Sr0.1]Sr2Cu2.78(SO4)0.22O6.12 and [Y0.84Sr0.16]Sr2Cu2,78(SO4)0.22O6.12 have been investigated. As the Ba content was increased, the superconducting transitions became sharper and Tc onset increased to reach a maximum of 78 K. The cell symmetry was also observed to change from orthorhombic to tetragonal, which is thought to be associated with randomization of the orientations of the sulphate groups in the structure.

Two hole SQUID behaviour in a bulk YBCO sample at 77K

Abstract We describe the development of a 2-hole radio frequency SQUID made from a single disc of YBa 2 Cu 3 O 7−y . The device displays single flux quantum sensitivity and an estimated noise level of 1 × 10 −3 φ o /Hz above 50Hz.

Circuit QED with a flux qubit strongly coupled to a coplanar transmission line resonator

We propose a scheme for circuit quantum electrodynamics with a superconducting flux?qubit coupled to a high-Q coplanar resonator. Assuming realistic circuit parameters, we predict that it is possible to reach the strong coupling regime. Routes to metrological applications, such as single-photon generation and quantum non-demolition measurements, are discussed.

Surface pinning as origin of high critical current in superconducting films

We give theoretical and experimental evidence that high critical currents in laser ablated Y Ba2Cu3Ox films can be due to the pinning of vortices by surface roughness. The theory, which takes into account the bending of the vortex core and its orthogonality to the film surface, predicts that any surface elevation causes an increase in the core length and therefore produces an energy barrier whose height is proportional to the film thickness and can exceed bulk pinning barriers. Comparison with experiments gives satisfactory agreement with the value of critical current density and shows that the latter can be explained by the pinning at a steplike surface relief.

Noise properties of biepitaxial HTS junctions

The authors have measured the voltage noise across a biepitaxial junction at both low current, where the noise is dominated by critical current fluctuations, and at high current, where the noise is dominated by resistance fluctuations. They have used a phase-sensitive technique to show that the noise from these two regions is not correlated. This gives support to the suggestion that the normal and supercurrent components are carried through the junction in different channels. The result suggests that the normal and supercurrent components are carried through a series of parallel tracks.<<ETX>>

Noise measurements in a two-hole radio frequency SQUID at liquid-nitrogen temperature

The authors report measurements of the noise in a two-hole RF SQUID fabricated from YBCO by an erosion technique. The noise was measured from 10 mHz to 1 kHz. Above 25 Hz the noise is essentially white with an RMS value of 1.3*10-3 phi 0 Hz-1/2 and is dominated by the noise of the amplifier. Below 1 Hz the noise power rises as 1/f1.3.

Noise performance of a composite HTC/niobium SQUID in RF and DC bias modes

The authors report preliminary measurements on a novel form of SQUID formed from a bulk YBCO ceramic with an adjustable niobium point contact. Weak links are formed at or around the YBCO/niobium interface and the device may be operated either as a DC SQUID or an RF SQUID without readjustment. This has made possible a comparison of the noise spectral density in both modes of operation by a technique which allows the contribution from magnetic flux noise to be determined. The flux noise below 100 Hz is, within experimental error, the same for both DC and RF SQUID modes of operation.

A phase sensitive technique for the study of noise properties of HTS junctions

Abstract We report a phase sensitive technique for measuring the voltage noise of junctions which enables us easily to separate out the low frequency 1/ f noise component due to critical current fluctuations. We illustrate this technique with detailed voltage noise measurements in three junctions grown by a biepitaxial process. We also provide evidence that these fluctuations are not due to motion of magnetic flux in or near to the junction.

THE DEVELOPMENT OF SQUID MAGNETOMETERS USING BULK CERAMIC SUPERCONDUCTORS

Abstract The development of granular and geometrically defined r.f. and d.c. SQUID magnetometers using high T c superconductors is reviewed. Although operation at liquid nitrogen temperatures is shown to place an upper limit on attainable sensitivities, above 100 Hz the performance of r.f. SQUIDs is already within a factor of three to five of their liquid helium cooled counterparts. However, at low frequencies high T c SQUIDs are shown to suffer from excessive 1/f noise and, under certain circumstances, “telegraph noise”, both of which are likely to be associated with thermal excitation of trapped flux.