Mizushi Matsuda

Growth behavior and surface morphology of homoepitaxial YBa2Cu3O7−δ thin films on flux‐grown single crystals

Atomically flat terraces and steps with a height of one c‐axis lattice parameter have been observed with atomic force microscopy (AFM) on the surface of a flux‐grown YBa2Cu3O7−δ (YBCO) single crystal. Homoepitaxial growth of YBCO on the flux‐grown single crystal has been investigated using reflection high‐energy electron diffraction (RHEED). Initial stage of the RHEED intensity oscillation without anomalous peaks usually observed in heteroepitaxial growth suggests epitaxial growth of YBCO on the substrate surface from the beginning of deposition. A surface image of AFM for the homoepitaxial film reveals coalescent step‐like structures and two‐dimensional islands of one c‐axis height on the growing surface of YBCO. The observed RHEED oscillation and AFM image are consistent with the two‐dimensional nucleation growth of the film.

Fabrication of magnetometers with multiple-SQUID arrays

We have designed and fabricated magnetometers with multiple-SQUID arrays to achieve higher signal to noise ratio compared to that for a single SQUID. In our directly coupled scheme, ten SQUIDs are connected in parallel with the same pickup coil composed of four parallel rectangular loops. Each SQUID having inductance of 70 pH and junctions of 1.5 /spl mu/m width was fabricated from an Au/YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on a SrTiO/sub 3/ bicrystal substrate with 30/spl deg/ misorientation angle. To avoid forming superconducting loops between junctions of adjoining SQUIDs, a Au wiring layer interrupts a current bias line. The obtained critical current of 20I/sub c/ and normal resistance of R/sub n//20 including contact resistance for the Au wiring layer are 350 /spl mu/A and 0.4 /spl Omega/, respectively. The modulation voltage of 25 /spl mu/V for multiple-SQUIDs is the same level as for a single SQUID. On the other hand, the modulation current of 40 /spl mu/A is several times larger than that for a single SQUID, but rather smaller than the expected value when all SQUIDs have equal parameters. A flux noise level of 15 /spl mu//spl Phi//sub 0//Hz/sup 1/2/ at 100 Hz, including preamplifier noise, was observed by using flux-locked loop electronics in a magnetically shielded room.

Effects of flux dam on low-frequency noise in high-T/sub c/ SQUID magnetometers

We demonstrated that the low-frequency noise in a high-T/sub c/ superconducting quantum interference device (SQUID) magnetometer when an external magnetic field is changed could be reduced by forming slots in a flux dam. We designed and fabricated directly coupled dc SQUID magnetometers having a mesh structure and flux dams. In order to suppress the vortex motion in the flux dams, we formed 5-/spl mu/m-wide strip lines and slots across the grain boundary of the flux dams. The output of the magnetometer in a flux-locked loop (FLL) operation became stable and low-frequency noise was suppressed up to an applied field of 83 /spl mu/T in field cooling and 40 /spl mu/T for field change after zero field cooling. The importance of the structure of the flux dam in controlling the vortex motion is discussed.

Characteristics of YBCO step-edge weak links and SQUIDs

The authors have fabricated step-junction and bicrystal-junction superconducting quantum interference devices (SQUIDs) using YBCO films that were deposited respectively on etched step on

Study of YBCO/Au Surface Using Low-Temperature Scanning Tunneling Microscopy/Scanning Tunneling Spectroscopy

A low-temperature scanning tunneling microscope (LTSTM) has been used to study the surface electronic state of a 30-nm-thick Au film deposited on (103)-oriented YBCO thin film. Conductance spectra with the BCS-like dip induced by the superconductivity proximity effect were observed in a small area. Also, spectra with zero-bias conductance peaks were observed in another limited area. Most parts of the areas studied showed constant conductance spectra. Since this fact suggests the existence of a degraded layer on most surfaces of the YBCO film, the Josephson coupling in YBCO/Au/Nb is assumed to occur primarily in this degraded layer.

Fabrication of NbN Josephson Tunnel Junctions and Their Application to DC-SQUIDs

We have fabricated all hard metal NbN/NbN tunnel junctions. The low dielectic constant of a plasma oxidized a-Si barrier leads to low-capacitance junctions (3 µF/cm2) which are suitable for SQUID applications. Then, planar dc-SQUIDs have been fabricated with NbN junctions. Higher modulation voltages, as high as 90 µV, with an applied flux were obtained for the SQUIDs with larger values of the product of the critical current and the shunting resistance. Effects of a damping resistance across the loop inductance have also been examined.

Effect of a static magnetic field on a slotted high-T/sub c/ SQUID magnetometer without a flux dam

A direct-coupling-type high-critical-temperature (high-T/sub c/) SQUID magnetometer with two pickup loops per SQUID on a bicrystal substrate has been fabricated. To avoid flux trapping, the superconducting films of the pickup loops have slits with a separation of 5 /spl mu/m. To suppress flux entry into the pickup loops, there are no weak links on the pickup loops, i.e., no flux dams. In field cooling, noise did not substantially increase in the low frequency region unless the field reached around 100 /spl mu/T. Regardless of whether the magnetometer was previously zero-field cooled or field-cooled, the flux did not enter the pickup loop until the changing field exceeded about 10 /spl mu/T. The magnetometer cooled under geomagnetic- and environmental-field noise could work in an unshielded environment. A gradiometer was made by connecting the feedback coils of two magnetometers in series. The environmental field noise was reduced by the gradiometer by about 35 dB.

Vortex dynamics of Nb variable-thickness bridges measured by direct flux detection method

In order to study the dynamical behavior of vortices moving through superconducting thin film devices, we constructed a probe having a concentric thin-film planar gradiometer coupled to a direct current SQUID. The probe was designed to directly detect the flux generated by a single vortex, based on the monopole approximation. Measurements of static current–voltage (I–V) and flux noise characteristics were made for variable-thickness bridges (VTB) made on epitaxial Nb films. The critical current was unambiguously determined from the onset of vortex motion. Multiple step noise of the flux with a minimum step height of 0.28Φ0 was observed above the critical current in the time traces. This step height is in agreement with the numerical calculation for single vortex motion, confirming the validity of the monopole approximation. It is suggested that the multiple step noise is generated by the successive entry of single vortices into the bridge. VTBs made on polycrystalline Nb films were also measured. The I–V ...

High-Tc SQUID magnetometers for use in moderate magnetically-shielded room

We have fabricated and characterized high-T/sub c/ planar SQUID magnetometers and first derivative gradiometers with directly-coupled pickup loops. The devices were made from single layer of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film on LSAT bicrystal substrate with 30/spl deg/ or 24/spl deg/ misorientation angle. Magnetic properties were investigated by applying a magnetic field B/sub 0/ for the SQUID magnetometer patterned with holes to reduce the maximum structural width. We found an increasing low frequency noise with cooling fields B/sub 0/ larger than 1.5 /spl mu/T. This value consists with the threshold field estimated from the maximum structural width. A magnetic field noise level of 500 fT/Hz/sup 1/2/ at 10 Hz was observed by using FLL electronics with a bias current reversal in a moderate magnetically-shielded room consisting of only two 1 mm thick layers of permalloy. Measurements of magnetocardiograms demonstrate the suitability of this magnetometer for biomagnetic applications. On the other hand, the gradiometer with two symmetric pickup loops was operated without any shielding. The performance obtained was a field gradient resolution of about 1 pT/cmHz/sup 1/2/ at 1 kHz and 10 pT/cmHz/sup 1/2/ at 1 Hz, with a baseline of 4 mm. The imbalance of this gradiometer was around 0.7%, limited by the sensitivity to homogeneous field of the SQUID itself.

Direct detection of vortex motion in high-T/sub c/ grain boundary junctions

Using a superconducting thin-film coil and Nb-based SQUID, the flux generated by vortices in a field-cooled (10 /spl mu/T) wide bicrystal YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// grain boundary junction (GBJ) was measured directly at 77 K. At and above a small threshold field change, random switching noise was observed, with switching heights corresponding to long-distance movement of vortices over 20 to 500 /spl mu/m within the GBJ, driven by the shielding current. The vortex movement was suppressed to lengths of less than 1 /spl mu/m by making slots in the GBJ. From these results, we estimated the flux noise of a directly coupled dc SQUID magnetometer with a weak link in its pick-up coil.