This is a review of our high-Tc superconductor (HTS) sampler development. The design and experimental demonstration of a Josephson sampler circuit based on YBa₂ Cu₃O_x(YBCO)/PrBa₂Cu₃O_x/YBCO ramp-edge junctions is described. The sampler circuit contains five edge junctions with a stacked YBCO groundplane and is based on single-flux quantum (SFQ) operations. Computer simulation results show that the time resolution of the sampler circuit depends strongly on the IcRn product of the junction and can be reduced to a few picoseconds with realistic parameter values. The edge junctions were fabricated using an in-situ process in which a barrier and a counter-electrode layer are deposited immediately after the edge etching without breaking the vacuum. The in-situ process improved the critical current uniformity of the junctions to 1σ20% in twelve 4-µm-width junctions. An YBCO groundplane was placed on the junctions in a multilayer structure we call the HUG (HTS cricuit with an upper-layer groundplane) structure. The inductance of YBCO lines was reduced to 1 pH per square without junction-quality degradation in the HUG structure. SFQ current-pulse generation, SFQ storage, and SFQ readout in the circuit have been confirmed by function tests using 3-kHz pulse currents. The successful operation of the sampler circuit has been demonstrated by measuring a signal-current waveform at 50K.

Logic operations in principle have been demonstrated based on the planar high-T_c Superconducting QUantum Interference Device (SQUID). Two kinds of logic gates were produced by using the focused ion beam (FIB) superconducting weak links fabricated in NdBa₂Cu₃O_7-δ (NBCO) thin films. Logic gates investigated in this paper are (1) an rf-SQUID based logic gate which utilizes threshold characteristics, and (2) a dc-SQUID based logic gate which is an elementary gate of RSFQ circuits. Elementary logic operation such as (1) AND/OR logic and (2) SET-RESET flip-flop operation were successfully obtained in the logic gates. In addition to the present experimental results, some problems and future prospects are also discussed.

Recent progress of highly sensitive magnetmeter utilizing high T_c dc superconducting quantum interference device (SQUID) is reviewed briefly. Performance parameters of the SQUID magnetometer, such as field resolution, dynamic response and usability in unshielded environment, are focused on. Relationship between these performance parameters and SQUID characteristics are discussed quantitatively, and key factors which dominate each performance are clarified. With this result, design principle to obtain high performance SQUID magnetometer operating at T77K is shown. Present status on the performance of the magnetometer is discussed by comparing experimental results with theoretical predictions. Issues to much improve the performance of the high T_c SQUID magnetometer are also discussed.

High-T_c SQUID was applied to the detection of magnetized fine particle moving at high speed. Two types of SQUIDs were used. One was a large washer type and the other was a flux transformer type. Their Josephson junctions were step edge type. The iron particle was attached on a nylon wire and scanned under the SQUID. High-T_c SQUID detected an iron particle of 50 µm diameter running at 800 m/min. It was shown that the magnetic field measured by the SQUID was proportional to the volumer of the particle and is inversely proportional to the distance between the SQUID and the particle. This technique using high-T_c SQUID is hopeful not only to wire production line but also for the processing of food and medicine, etc.

This study shows the results of evaluating the flux noises at low frequency when the alternating current(AC) bias direct offset integrated technique(DOIT) with additional positive feedback (APF) is used in a high-T_c dc superconducting quantum interference device (SQUID). The AC-bias DOIT can reduce low-frequency noise without increasing the level of white noise because each operating point in the two voltage-flux characteristics with AC bias can always be optimum on the magnetometer in the high-T_c dc-SQUID. APF can improve the effective flux-to-voltage transfer function so that it can reduce the equivalent flux noise due to the voltage noise of the preamplifier in the magnetometer. The use of APF combined with the AC-bias DOIT reduced the noise of the magnetometer by factors of 1.5 (33µΦ₀/Hz vs. 50 µΦ₀/Hz) at100 Hz, 3.5 (43 µΦ₀/Hz vs. 150 µΦ₀/Hz) at 10 Hz, and 5.2 (67 µΦ₀/Hz vs. 351 µΦ₀/Hz) at 1 Hz as compared with the noise levels that were obtained with the static-current-bias DOIT. The contribution of the factors at 1 Hz is about 2 by APF and 2.6 by AC bias. The performance of improving the flux noise in the AC -bias DOIT with APF is almost equal to that of the flux locked loop (FLL) circuits in which the flux modulation uses a coupling system with a transformer and with the AC bias.

We report on progress in the development of high-current-density all-NbN tunnel junctions for application as submillimeter wave SIS mixers. A very high current density up to 54 kA/cm2, roughly an order of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions with a thin aluminum nitride (AIN) tunnel barrier. Even though the junctions have a very high current density, they showed high-quality junction characteristics with a large gap voltage, sharp quasipartical current rise, and small subgap leakage current. The junctions also exhibited good Josephson tunneling behavior, excellent terahertz response, and sensitive heterodyne mixing properties. NbN/AIN/NbN tunnel junctions were integrated with a NbN thin-film antenna to investigate the terahertz responses and the heterodyne mixing properties in a quasioptical mixer testing system. Photon-assisted tunneling steps were clearly observed on the I-V curve with irradiation up to 1 THz, and low-noise heterodyne mixing was demonstrated in the 300-GHz band.

A 90 GHz band planar-type superconducting mixer using Ba_1-xK_xBiO₃ (BKBO) bicrystal junctions was fabricated on a MgO bicrystal substrate. The mixer is integrated with microwave circuits and two junctions, but we could not operate the mixer in image rejection mode because of process damage to the junction properties. However we confirmed the mixing operation; the intermediate frequency (IF) signal was observed up to 17K (LO87 GHz, RF92 GHz).

It is shown that a three-fluid model, which was successfully introduced to explain microwave characteristics of high-T_c superconductors phenomenologically, is suit also to explain those of low-T_c superconductors. In this model, the two contributions of a residual normal electron, in addition to a super and a normal electron in the two-fluid model, and of the temperature (T) dependence of momentum relaxation time τ for the two normal electrons are taken into account. Measured results of the T dependence of surface resistance R_s for a Nb film with critical temperature T_c9.2K agree very well with an R_s curve calculated using the present model, where a residual surface resistance at T0K, R_so, and the T dependence of τ were determined using the surface reactance at 0K X_so37.6mΩ calculated using the BCS theory to fit a calculated R_s curve with the measured values as a function of T. Furthermore, microwave characteristics predicted from the BCS theory cannot be explained phenomenologically using the conventional two-fluid model. This difficulty can be solved by using an improved two-fluid model, called the two-fluid (τ) model, where the T dependence of τ is taken into account. Finally the frequency dependence of R_s calculated for the Nb film is f^1.9 for the BCS theory and f^2.0 for the three-fluid (τ) model on the assumption of the frequency independence of τ.

We report the strong microwave Josephson radiation from an array of high-T_c junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. For a five-junctions array, we observed the coherent output power of about 13 pW at receiving frequency f_REC22GHz without an external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the selfradiation in coherent state less than 1 GHz by the adjustment of the external flux. The phase differences between adjacent junctions with different I_cR_n products could be controlled by an external small magnetic field. Submillimeter-wave detector response of the five-junction array was also studied experimentally at frequency f478 GHz.

New THz radiation devices made of high-T_c superconductors are fabricated and their characteristics are studied in detail. Ultrashort electromagnetic pulses with 0.5 ps width have been radiated into free space from current biased devices made of superconducting YBa₂Cu₃O₇ (YBCO) films by exciting with femtosecond laser pulses. The Fourier spectrum of them extends up to 3 THz. The radiation mechanism is ascribed to the ultrafast supercurrent modulation by the optical pulses. The THz waveform is analyzed using rate equations describing the relaxation of photoexcited quasiparticles. By the improvement of the device structure and the collecting optics, the radiation power can be increased up to 0.5 µW. A new type THz radiation from YBCO films under an external magnetic field without a transport current is also reported.

We successfully observed curent-voltage (I-V) curves which showed the behavior of intrinsic Josephson junctions using Tl₂Ba₂CaCu₂O_x (Tl-2212) thin films on MgO substrates by structuring mesas and measuring the electrical transport properties along the c-axis. For a 55 µm² mesa, a hysteretic I-V curve was observed up to 80 K, which showed that series-connected SIS-type junctions are formed. Compared with the critical current density (J_c) of more than 106 A/cm2 parallel to the ab-plane, an anisotropic J_c of 1.410² A/cm² along the c-axis was observed at 4.9 K. By focusing on the I-V curve at lower bias current, the constant voltage jumps measured at the first seven branches were estimated to be 26 mV. The normal resistance (R_nk) of a unit SIS junction was estimated to be 580 Ω by substituting the measured voltage jump in the Ambegaokar and Baratoff relation. Using the calculation for McCumber parameter (β_c), the capacitance (C_k) of the unit SIS junction was estimated to be 3.610^-10 F/cm² at 77 K. The I_ckR_nk product was estimated to be 6.4 mV and the cut-off frequency (f_c1/2πR_nkC_k) was calculated to be 3.1 THz at 77 K. The J_c and the hysteresis decreased with an increase in the mesa area, and finally, for a 300300µm² mesa, a resistively shunted junction (RSI) like curve without hysteresis was observed up to 98 K. A J_c of 5.610¹ A/cm² along the c-axis was observed at 6.4 K. This may be explained by the higher content of conductive grain boundaries for a larger mesa area.

The electric field intensity of the THz radiation from YBCO thin films excited by ultrashort laser pulses has been enhanced by a factor of 3 using a-axis oriented films instead of c-axis oriented ones used previously under the same excitation conditions. This corresponds to the enhancement of a factor of 10 for the radiation power. From the transmittance measurements of the millimeter wave for a-and c-axis oriented films, the origin of the enhancement is attributed to the increased fraction of the THz electromagnetic wave power transmitted from the YBCO film to free space. This result indicates that the utilization of the anisotropic properties of high-T_c superconductors is effective to enhance the radiation power.

Ferroelectric / superconducting heterostructures of Pb(Zr_0.52Ti_0.48)O₃ [PZT] / (Y_0.6Pr_0.4)Ba₂Cu₃O_y [YPBCO] have been formed on SrTiO₃(100) substrate using an ArF pulsed laser deposition. The crystallinity and surface morphology of heterostructures were investigated by X-ray diffraction measurements and atomic force microscopy. We also measured dielectric and ferroelectric properties of PZT film in the Au/PZT/YPBCO structure. Furthermore, we fabricated a three-terminal devices having the structure described above using an in-situ metal mask exchange system, and investigated the ferroelectric field effect. As a result, we observed a modulation of channel resistance approximately equal to that estimated from the induced carrier and memory effect due to remanent polarization of PZT.

This paper describes a new reduction mechanism of the stress induced leakage current that is induced by step tunneling of electrons through the step tunneling sites. The concept of this mechanism is based on the deactivation of step tunneling sites for thin oxide. It is verified that the deactivation is electrically realized by the injected electrons int the sites. It is because the step tunneling probability of electrons though the deactivated sites is suppressed, since the electron capture cross section of the neutralized deactivation sites becomes extremely low. The deactivation scheme is as follows: (1) The deactivation of tunneling sites can be realized that the tunneling sites trapped holes change to neutralized tunneling sites due to electrons injection. (2) The injected electron can deactivate the activation tunneling sites only under energy level than the energy level of the injected electrons. It is shown that the above reduction phenomenon can be quantifiably with formulation. These results are very important for high reliable thin oxide films and for high performance ULSI.

This paper describes the new write/erase operation methods in order to improve the read disturb characteristics for Flash EEPROM cells which are written by channel hot electron injection and erased by F-N tunneling emission from the floating gate to the substrate. The new operation methods is either applying a reverse polarity pulse after each erase pulse or applying a series of shorter erase pulses instead of a long single erase pulse. It is confirmed that by using the above operation methods, the leakage current can be suppressed, and then the read disturb life time after 10⁵ cycles write/erase operation is more than 10 times longer in comparison with the conventional method. This memory cell by using the proposed write/erase operation method has superior potential for application to 256 Mbit Flash memories as beyond.

An analysis is carried out about the reflection influence on the microwave attenuation measurement for moisture content determination. A new method taking into account the reflection influence is proposed and it is proved valid by the experiment results. Using this method, the density dependence of the attenuation is measured and the measured data can be fitted well by a straight line passing through the origin. Therefore, the attenuation per unit density and propagation distance is a function which depends only on the moisture content and the function is useful to the determination of the moisture content.