A. Eulenburg

Pulsed laser deposition of YBa2Cu3O7-δ and NdBa2Cu3O7-δ thin films : A comparative study

Abstract Thin films of YBa2Cu3O7−δ and NdBa2Cu3O7−δ were grown by pulsed laser deposition. The statistical methods of experimental design and regression analysis were used to correlate the electrical and morphological properties of the films to the growth conditions. For optimised transition temperatures the deposition parameter settings of target–substrate distance, oxygen pressure and laser energy density were found to differ significantly for YBa2Cu3O7−δ and NdBa2Cu3O7−δ. While the maximum transition temperatures obtained were similar (∼90 K) for the two materials, all NdBa2Cu3O7−δ films had a roughness comparable to the c-axis unit cell dimension whereas the YBa2Cu3O7−δ films had a surface roughness which varied between 5–17 nm depending on the growth conditions. In addition, there were differences in the average size and density of non-stoichiometric outgrowths on the two types of film. These differences we relate to a difference in growth mode. Our atomic force microscope and scanning tunnelling microscope studies suggest a 3D screw dislocation mediated growth for YBa2Cu3O7−δ and a 2D layer-by-layer process for NdBa2Cu3O7−δ.

Highly balanced long-baseline single-layer high-Tc superconducting quantum interference device gradiometer

We describe a direct-current superconducting quantum interference device (SQUID) first-order gradiometer fabricated from a single layer of YBa2Cu3O7 on a 30×10 mm2 bicrystal substrate. The device has a baseline of 13 mm and an intrinsic balance of ∼10−3. The gradient sensitivity at 77 K and 1 kHz is 50 fT/(cmHz) in magnetic shielding and 260 fT/(cmHz) when operated unshielded in our laboratory. An antiparallel two-SQUID coupling scheme is employed to optimize the device’s balance to at least 3×10−5.

High-Tc gradiometric superconducting quantum interference device and its incorporation into a single-layer gradiometer

We describe a first-order gradiometric dc superconducting quantum interference device (SQUID) and its incorporation into a first-order directly coupled single-layer gradiometer. The gradiometric SQUIDs were fabricated from a single layer of YBa2Cu3O7, with a silicon dioxide insulating layer and a gold crossover structure. For several gradiometric SQUIDs, with estimated inductances of order 67 pH, we measured parasitic effective areas in the range 1–2 μm2, approximately two orders of magnitude lower than for conventional narrow linewidth SQUIDs of similar inductance. For a single-layer gradiometer incorporating a gradiometric SQUID, we measured a parasitic effective area of 95 μm2. We demonstrate that for this device, the SQUID itself makes a negligible contribution to the overall parasitic effective area. We show that the improved balance leads to better performance in an unshielded environment.

High-temperature superconducting YBCO dc SQUID gradiometers fabricated on STO bicrystal substrates

HTS dc gradiometers have been fabricated with slots and flux dams in their SQUID washers. Using a single layer of YBCO, the gradient sensitivity is limited by the sensing magnetometer in the centre of the structure. The spatial response of such devices has been measured experimentally indicating that single layer devices, in terms of gradient sensitivity, have characteristics that deviate slightly from idealized first order gradiometers. The flux noise of these devices is discussed with particular emphasis on their unshielded low frequency noise properties. We also discuss the effect of different cooldown procedures on the flux noise.

First-order high-T/sub c/ single-layer gradiometers: Parasitic effective area compensation and system balance

We have previously reported an anti-parallel two-SQUID coupling scheme which can significantly improve the balance of first-order HTS single-layer gradiometers, and therefore improve the device performance in unshielded environments. We report on the behaviour of such devices fabricated on 10/spl times/10 mm/sup 2/ and 30/spl times/10 mm/sup 2/ STO bicrystal substrates. We present detailed studies of the balance of such devices, with particular emphasis on the time-dependent behaviour in unshielded environments, and with the device in motion.

Pulsed laser deposition of YBCO and NBCO using experimental design

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) and NdBa/sub 2/Cu/sub 3/O/sub 7-x/ (NBCO) were grown by pulsed laser deposition (PLD). The experiment was planned using experimental design to investigate the relative importance of the individual growth parameters for the properties of the films. For both materials critical temperatures close to 90 K were obtained. The electrical and morphological properties of the films were correlated to the growth conditions using regression analysis. For NBCO, regions where both electrical and morphological properties were optimised could be found, while for YBCO we found that the main factors that lead to high critical temperatures also lead to a substantial increase in the surface roughness.

High-temperature single-layer SQUID gradiometers with long baseline and parasitic effective area compensation

First-order HTS SQUID gradiometers were fabricated on 30 × 10 mm2 bicrystal substrates. These devices have a baseline of 13 mm, intrinsic balance levels of ~1/700 and a typical gradient sensitivity at 1 kHz of 79 fT cm-1 Hz-1/2. A two-SQUID coupling scheme is discussed that further enhances the devices ability to reject uniform fields.

Step-edge Josephson junctions and their use in HTS single-layer gradiometers

We describe reproducible technology for fabricating YBa/sub 2/Cu/sub 3/O/sub 7/ step edge Josephson junctions on SrTiO/sub 3/ substrates. We report on the electrical characteristics of the junctions at 77 K and the dependence on the ratio of film thickness to step height. Single-layer gradiometers incorporating step-edge junction dc SQUIDs have been fabricated. We compare the performance of identical SQUID gradiometers based on our step-edge junctions and on 24/spl deg/ SrTiO/sub 3/ bicrystal junctions. Gradiometric SQUIDs (G-SQUIDs) incorporating step-edges have also been fabricated. We measured the effective area of several G-SQUIDs to be in the range of 1-2 /spl mu/m/sup 2/, approximately two orders of magnitude lower than for conventional SQUIDs of similar inductance. We demonstrate that when incorporated into a gradiometer the G-SQUID leads to an improved performance in an unshielded environment.

NdBa/sub 2/Cu/sub 3/O/sub 7/ bicrystal Josephson junctions and SQUIDs operating at 77 K

Recently there has been considerable interest in the use of NdBa/sub 2/Cu/sub 3/O/sub 7/ (NBCO) as an alternative thin-film material to YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) for device applications. This is due to the its potentially superior surface and structural properties. However the development of NBCO Josephson junctions and superconducting quantum interference devices (SQUIDs) has been found difficult worldwide due to the degradation in the transition temperature upon patterning. This effect appears to be target dependent. By careful patterning and annealing of samples we have been able to obtain working NBCO Josephson junctions and SQUIDs at 77 K. Here we report on their electrical and noise characteristics.