Tsunehiro Hato

Development of Integrated HTS SQUIDs With a Multilayer Structure and Ramp-Edge Josephson Junctions

We have fabricated high-temperature superconductor (HTS) dc superconducting quantum interference devices (SQUIDs) with a multilayer structure and ramp-edge Josephson junctions. For stable operation at 77 K, SmBa₂Cu₃O_y (SmBCO) and La_0.1-Er_0.95Ba_1.95Cu₃O_y (La-ErBCO) films with a <i>T</i> _c above 90 K were employed as materials for the base-electrode and counter-electrode layers, respectively. Junctions were fabricated by employing a Cu-poor La-ErBCO layer as a precursor of the barrier. The fabricated parallel gradiometers with a baseline of 1-3 mm exhibited the field-induced modulation voltage of 20-50 muV at 77 K, and a clear modulation up to 86.8 K. Noise measurements at 77 K revealed a white flux noise of 4.5-10 muPhi₀/Hz^1/2 at 1 kHz. The magnetometer with an integrated 20-turn flux transformer and the effective area of about 1.8 mm² exhibited a field noise of 25 fT/Hz^1/2 or less at 1 kHz.

Development of HTS-SQUID magnetometer system with high slew rate for exploration of mineral resources

For the transient electromagnetic (TEM) method using a high-temperature superconducting interference device (HTS-SQUID), we have developed a magnetometer system with a wide dynamic range, a high slew rate, and superior transportability. To achieve high tolerance to a higher excitation magnetic field, we utilized a SQUID magnetometer containing ramp-edge junctions with La0.1Er0.95Ba1.95Cu3Oy and SmBa2Cu3Oy electrode layers, which was fabricated by using an HTS multi-layer fabrication technique. To operate the magnetometer stably in a rapidly changing magnetic field, we chose the proper materials for the RF shield of liquid nitrogen (LN2) glass Dewar and cables. The white noise level and the slew rate of the system were measured to be 30 fT Hz−1/2 and 10.5 mT s−1, respectively. The resultant signal-to-noise ratio was higher than that of the previous system and improved the exploration depth, which was successfully demonstrated in field tests. The weight of the Dewar, which retains the LN2 for 17 h, is 2.5 kg. The total weight of our system including the LN2 Dewar, a probe with a flux-locked loop (FLL) circuit, a battery, a receiver, and a 30 m-long cable between the FLL and the receiver is as low as 25.6 kg.

Advances in High-Tc Single Flux Quantum Device Technologies

We have developed the fabrication process, the circuit design technology, and the cryopackaging technology for high-T c single flux quantum (SFQ) devices with the aim of application to an analog-to-digital (A/D) converter circuit for future wireless communication and a sampler system for high-speed measurements. Reproducibility of fabricating rampedge Josephson junctions with I c R n products above 1 mV at 40 K and small I c spreads on a superconducting groundplane was much improved by employing smooth multilayer structures and optimizing the junction fabrication process. The separated base-electrode layout (SBL) method that suppresses the J c spread for interface-modified junctions in circuits was developed. This method enabled low-frequency logic operations of various elementary SFQ circuits with relatively wide bias current margins and operation of a toggle-flip-flop (T-FF) above 200 GHz at 40 K. Operation of a 1:2 demultiplexer, one of main elements of a hybrid-type Σ-△ A/D converter circuit, was also demonstrated. We developed a sampler system in which a sampler circuit with a potential bandwidth over 100 GHz was cooled by a compact stirling cooler, and waveform observation experiments confirmed the actual system bandwidth well over 50 GHz.

Dielectric-base transistors with doped channel

The dielectric-base transistor (DBT) is expected to be coupled with various functional oxides such as high-temperature superconductors and ferroelectrics. We experimented with lowering the conduction band of the channel to reduce the operating voltage. LaTiO3 deposited on SrTiO3 supplies carriers in the SrTiO3 substrate by displacing Sr2+ and La3+. With this technique, we fabricated a YBa2Cu3O7−x/In2O3/SrTiO3/LaTiO3/SrTiO3 transistor with a partially doped channel. The transistor operates at under 1 V while maintaining a voltage amplification factor of 2, which is one order smaller than the 15 V operating voltage of a transistor with an undoped channel. The base potential relative to the emitter conduction band has been reduced to 0.3 eV.

Fabrication of Low-Noise HTS-SQUID Gradiometers and Magnetometers With Ramp-Edge Josephson Junctions

We have studied the fabrication process of HTS-SQUIDs with ramp-edge Josephson junctions (JJs). SmBa₂Cu₃O_y (SmBCO) and Er_0.95La_0.1Ba_1.95Cu₃O_y (L1ErBCO) having <i>T</i>_cs around 90 K were used for base- and counter-electrodes, respectively. SrSnO₃ (SSO) was used for insulating layers. A black-colored Pr_1.4Ba_1.6Cu_2.6Ga_0.4O_y (P4G4) layer was first grown on MgO substrate via a thin BaZrO₃ buffer layer. It was expected to work as a temperature homogenizer over the whole substrate area during deposition of the upper layers. We fabricated planar gradiometers having pickup coils with sizes of 0.5 × 1.0, 1.0 × 1.0 and 3.0 × 3.0 mm². Their baselines are 1.0, 1.0 and 3.0 mm, respectively. The gradiometers exhibited flux noise at 77 K as low as 3.3 ~ 4.5 μΦ₀/Hz^1/2 at 1 kHz. Directly and inductively coupled magnetometers having a 13.5 mm square pickup coil were also fabricated. The latter has an integrated 20-turn input coil. They showed field noise levels of 55 and 10 fT/Hz^1/2 at 1 kHz, respectively.

Non-Destructive Testing of YBCO Coated-Conductor by Multi-Channel HTS SQUID Gradiometers

We developed a non-destructive testing (NDT) system utilizing multi-channel high-temperature superconductor (HTS) SQUID gradiometers in order to examine rare earth (RE)-123 coated conductors striated into multi-filamentary lines. The 5-channel HTS SQUID gradiometer array was composed of ramp-edge junctions with LaErBaCuO and SmBaCuO electrode layers, and fabricated by using an HTS multilayer fabrication technique. The parallel gradiometers detected the vertical element of magnetic field gradient induced around defects by an eddy current. It was demonstrated that we can detect macroscopic defects in each filamentary superconducting line at high speed up to 30 m/h.

Simulation study of high-T/sub c/ superconducting sampler circuit with bandwidth over 100 GHz

We investigated high-T/sub c/ superconducting sampler circuits by computer simulation and proved that our latest version of sampler circuit can operate with bandwidth over 100 GHz. In our conventional sampler circuit, we found several problems in its circuit design. An improved sampler circuit, which has additional reset junctions in the dc/SFQ and SFQ/dc parts of the circuit and has a Josephson transmission line with an optimized dc bias in front of the comparator, accurately reconstructed signal waveforms with bandwidth above 160 GHz for a typical I/sub c/R/sub n/ product of 1 mV in the simulation.

Non-Destructive Evaluation of Deep-Lying Defects in Multilayer Conductors Using HTS SQUID Gradiometer

We examined the ability to detect deep-lying defects in multilayer stacks of aluminum plates using an eddy-current testing technique with HTS planar-type SQUID gradiometers. In addition to a previous gradiometer with a baseline of 1 mm, a new gradiometer with a baseline of 8.5 mm and a larger effective area fabricated by HTS multilayer and ramp-edge junction technologies was used. The frequency and depth dependences of the SQUID signal were compared between the two gradiometers. By employing the longer-baseline gradiometer and an excitation frequency of 35 Hz, a slit-like defect located at about 38 mm in depth could be clearly observed with a signal-to-noise ratio larger than 10.

Fabrication of Integrated Two-Axis High-

We have fabricated a two-axis high-Tc planar gradiometer. Two gradiometers with planar gradiometric pickup coils lie diagonally on a square substrate. Therefore, they can measure field gradients, dBz/dx and dBz/dy at the same site. The two-axis gradiometer was fabricated on a 15 × 15 mm MgO (100) substrate using an HTS multilayer process including crossovers, superconductive contacts and ramp-edge junctions. The width of the pickup coil and the baseline length are 6 mm and 8.5 mm, respectively. Voltage modulations in V-Φ characteristics more than 20 μV at 77 K were obtained for both the gradiometers. Parasitic effective areas measured by applying a uniform magnetic field were about 0.0001 mm-2. The gradiometer balance was estimated to be about 10-2. The parasitic effective area was not due to the imbalance of the gradiometric pickup coil, but due to a sensitivity of SQUID itself. Flux noise level as low as 10 μΦ0/Hz1/2 at frequencies down to 3 Hz and a cross-talk ratio less than 1/230 were obtained.

T_{\rm c}

We have fabricated a large high-temperature superconducting gradiometer for geophysical application such as a Transient Electro-Magnetic (TEM) method. The seamless gradiometric pickup coil was made of a commercially available GdBCO coated conductor formed on a Hastelloy tape. Two pickup loops with a diameter of 100 mm and an integrated input coil on a SQUID chip were connected with low resistance contacts. The input coil was magnetically coupled with a directly-coupled gradiometer which was made by a RE123-based multi-layer process including two superconducting layers with crossover, contact structure and ramp edge Josephson junctions. The baseline length defined as the distance of two pickup loops is about 130 mm. The performances of the gradiometer such as flux noise, gradiometer balance and frequency dependence of sensitivity were investigated.