M. S. Colclough

Bi‐epitaxial grain boundary junctions in YBa2Cu3O7

We have developed a new way of making grain boundary junctions in YBa2Cu3O7 thin films by controlling the in‐plane epitaxy of the deposited film using seed and buffer layers. We produce 45° grain boundaries along photolithographically defined lines. The typical value of the critical current density of the junctions is 103–104 A/cm2 at 4.2 K and 102–103 A/cm2 at 77 K, while the rest of the film has a critical current density of 1–3×106 A/cm2 at 77 K. The current‐voltage characteristics of the junctions show resistively shunted junction behavior and we have used them to fabricate dc superconducting quantum interference devices (SQUIDs) which show modulation at temperatures well above 77 K. This is the first planar high Tc Josephson junction technology that appears readily extendable to high Tc integrated circuits.

Monolithic 77 K dc SQUID magnetometer

We report the first fabrication of a monolithic dc SQUID magnetometer that operates above 77 K. We have used bi‐epitaxial grain boundary Josephson junctions in YBa2Cu3O7 (YBCO) to produce the SQUID itself while the flux transformer consists of heteroepitaxial layers of YBCO/SrTiO3/LaAlO3/SrTiO3/YBCO. In the circuit fabricated, the SQUID is built on top of the epitaxial layers of the flux transformer. We have used dry etching, ion‐beam cleaning, and photolithographic processing to pattern all the layers. Via contacts and step coverage of the epitaxial wire layers have been achieved without significant degradation of the superconducting properties of any of the three YBCO layers. The magnetometer enhances the magnetic field sensitivity of the bare SQUID by a factor of 127, giving an effective area, dΦ/dB, of 1.9 mm2.

Flux focusing effects in planar thin‐film grain‐boundary Josephson junctions

We have studied the magnetic interference of the critical currents of synthetic planar thin‐film grain‐boundary Josephson junctions. We find that the effective area of these junctions scales as the square of the width w in contrast to the usual w(2λ+d) dependence of sandwich‐type Josephson junctions. This behavior is a simple consequence of the magnetic response of thin‐film superconductors to perpendicular applied fields. A model based on the London theory yields the observed behavior. In addition, we find the correction to the interference pattern due to the effect of the corners.

Extension of the bi‐epitaxial Josephson junction process to various substrates

We report an extension of the bi‐epitaxial Josephson junction process that permits the use of a variety of substrate materials and allows junctions to be placed at any level of a multilayer structure. The new materials, SrTiO3, MgO, and CeO2, serve as a base layer, a seed layer, and a buffer layer, respectively, and replace Al2O3, MgO, and SrTiO3 in the original bi‐epitaxial process. This new process offers much more flexibility in designing a circuit. Bi‐epitaxial junctions made with the new set of materials show much improved electrical properties, especially at 77 K. We attribute the improved electrical characteristics to a better thermal expansion match between the substrate and the thin‐film layers. Important junction properties such as critical currents and junction resistances are compared to other types of grain boundary junctions.

Sensitive YBa2Cu3O7−x thin‐film magnetometer

Our YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital x}} thin-film magnetometer consists of a dc superconducting quantum interference device with bi-epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux-locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83{plus minus}3. The low-frequency rms magnetic field noise scales approximately as 1/{ital f}{sup 1/2}, where {ital f} is the frequency, with a magnitude of 0.6 pT Hz{sup {minus}1/2} at 10 Hz and 0.09 pT Hz{sup {minus}1/2} at 1 kHz with the magnetometer immersed in liquid N{sub 2}.Our YBa2Cu3O7−x thin‐film magnetometer consists of a dc superconducting quantum interference device with bi‐epitaxial Josephson junctions, fabricated on one chip, and a flux transformer with a multiturn input coil, fabricated on a second chip. Photolithographic processing is used to pattern all layers. The magnetometer operates in a flux‐locked loop at temperatures up to 81 K, with the flux transformer improving the magnetic field sensitivity by a factor of 83±3. The low‐frequency rms magnetic field noise scales approximately as 1/f1/2, where f is the frequency, with a magnitude of 0.6 pT Hz−1/2 at 10 Hz and 0.09 pT Hz−1/2 at 1 kHz with the magnetometer immersed in liquid N2.

Flicker (1/f) noise in biepitaxial grain boundary junctions of YBa2Cu3O7−x

At low frequencies f the 1/f noise power in single biepitaxial junctions of YBa2Cu3 O7−x peaks sharply for bias currents just above the noise reduced critical current and increases as I2 for high bias currents I. This behavior is explained by a model in which both critical current fluctuations δI0 and resistance fluctuations δR contribute to the measured voltage noise. The magnitude of the normalized critical‐current fluctuations ‖δI0/I0‖ is always much greater than that of the normalized resistance fluctuations ‖δR/R‖. Switching the bias current between positive and negative values at 2 kHz greatly reduces the magnitude of the 1/f noise from both sources, implying that the coherence of the noise generating process is not affected by the current reversal.

Grain boundary Josephson junctions created by bi-epitaxial processes

Abstract We describe a “bi-epitaxial” process to create 45 degree grain boundary Josephson junctions in YBa 2 Cu 3 O 7 . These bi-epitaxial grain boundary Josephson junctions are defined by standard photolithographic technique, and appear to be readily extended to integrated circuits. Some transport properties are reported and compared to other types of grain boundary Josephson junctions.

Progress Towards an Integrated HTS SQUID Magnetometer

To date, thin film HTS SQUID magnetometers have been made by using two discrete components in a flip-chip arrangement. These components are the SQUID flux detector, with its two Josephson junctions, and the pick-up loop/input coil. It is very desirable to integrate these two components on a single substrate. This requires a multilayer structure that is more complex than any previously fabricated in high Tc superconductors. Our successful progress towards this goal is described briefly in this paper.

High Tc superconducting multilayers for SQUID magnetometers

Thin-film multilayers of YBa2Cu3O7−x (YBCO) and insulating materials such as SrTiO3, MgO and CeO2 are grown in situ by laser deposition. Appropriate multilayers are used to produce bi-epitaxial grain boundary Josephson junctions, insulating crossovers - two overlapping YBCO films with an intervening inulating film, and superconducting vias - a superconducting connection between the two YBCO films via a window in the insulator. The bi-epitaxial junctions are incorporated in d.c. SQUIDs (Superconducting QUantum Interference Devices), and the crossovers and vias enable one to fabricate superconducting, multiturn flux transformers. Coupled together, the SQUID and flux transformer form a magnetometer that, operating in liquid nitrogen, has achieved a sensitivity of 0.6 pT Hz−1/2 at 10 Hz. We have used such a magnetometer to obtain magnetocardiograms.

Thin Film High-Temperature Superconducting Flux Transformers Coupled to SQUIDs

Using photolithographic patterning and laser deposition, we have constructed a sensitive thin-film SQUID magnetometer from YBa2Cu3O7−x (YBCO). The device consists of a flux transformer, deposited on one substrate, which is coupled to a thin-film YBCO SQUID, made with bi-epitaxial junctions, deposited on another substrate. At 77K, the magnetometer has attained a sensitivity of 1.8pTHz−1/2 at 1Hz, and has been used to measure the magnetic signal from the human heart.