We have developed an extremely sensitive radio frequency amplifier based on the dc superconducting quantum interference device (dc SQUID). Unlike a conventional semiconductor amplifier, a SQUID can be cooled to ultra-low temperatures (100 mK or less) and thus potentially achieve a much lower noise temperature. In a conventional SQUID amplifier, where the integrated input coil is operated as a lumped element, parasitic capacitance between the coil and the SQUID washer limits the frequency up to which a substantial gain can be achieved to a few hundred MHz. This problem can be circumvented. by operating the input coil of the SQUID as a microstrip resonator: instead of connecting the input signal between the two ends of the coil, it is connected between the SQUID washer and one end of the coil; the other end is left open. Such amplifiers have gains of 15 dB or more at frequencies up to 3 GHz. If required, the resonant frequency of the microstrip can be tuned by means of a varactor diode connected across the otherwise open end of the resonator. The noise temperature of microstrip SQUID amplifiers was measured to be between 0.5 K $\pm$ 0.3 K at a frequency of 80 MHz and 1.5 K $\pm$: 1.2 K at 1.7 GHz, when the SQUID was cooled to 4.2 K. An even lower noise temperature can be achieved by cooling the SQUID to about 0.4 K. In this case, a noise temperature of 100 mK $\pm$ 20 mK was achieved at 90 MHz, and of about 120 $\pm$ 100 mK at 440 MHz.

New cuprates with the nominal composition $(Pb_{0.75}Sn_{0.25})Sr_2(Ca_{1-x}Lu_x)Cu_2O_z(x=0.4{\sim}1.0)$ have been synthesized. These materials exhibit superconducting properties for 0.4 $\leq$ x $\leq$ 0.7. It is also observed that the superconductivity of the as-prepared sample is enhanced by post-annealing at high temperature followed by quenching. X-ray diffraction analysis reveals that the c lattice parameter decreases as x increases whereas the a lattice parameter is nearly independent of the value of x for both the as-prepared and the post-treated samples. The thermoelectric power measurements indicate that the post-heat-treatments result in an increase in the hole carrier density which account for the observed increase of $T_c$. Bulk superconductivity with a $T_c$ value of 60 K is observed in this system.

The notion of the finite pairing interaction energy range Td is shown to result in a linear temperature dependence of the London magnetic penetration depth length, ${\Delta}{\lambda}{/\lambda}(0)=(T/Td)2/\pi)ln2$ at low T in the case of the s-wave pairing state, accounting for data of high Tc superconductor by Hardy et al.

[ $YBa_2Cu_3O_{7-\delta}$ ] thin films were deposited by laser ablation using modified melt-textured grown targets. As the laser energy density was increased, the films showed an increasing c-axis orientation and an improvement of superconducting properties. However, at 4 $J/cm^2$, the degree of c-axis alignment and $J_c$ of the film were considerably reduced. These were attributed to the enhanced a-axis outgrowths. It was shown that the increased laser energy density resulted in the formation of $Y_2O_3$ inclusions during the c-axis film growth, and that these inclusions nucleated the a-axis outgrowths.

A 40-channel superconducting quantum interference device (SQUID) system was constructed for measuring neuromagnetic fields. Main features of the system are the use of double relaxation oscillation SQUIDs (DROSs), and planar gradiometers measuring magnetic field components tangential to the head surface. The DROSs with high flux-to-voltage transfers enabled direct readout of the SQUID output by room-temperature dc preamplifiers and simple flux-locked loop circuits could be used for SQUID operation. The pickup coil is an integrated first-order planar gradiometer with a baseline of 40 mm. Average noise level of the 40 channels is around 1.2 $fT/cm/{\surd}Hz$ at 100 Hz, corresponding to a field noise of 5 $fT/{\surd}Hz$, operated inside a magnetically shielded room. The SQUID insert was designed to have low thermal load, minimizing the loss of liquid helium. The constructed system was applied to measure auditory-evoked neuromagnetic fields.

A prototype for a highly sensitive, automated measurement system for the microwave surface resistance of high-$T_c$ superconductor films was set up, and tested by measuring the microwave surface resistances of high-$T_c$ $YBa_2Cu_3O_{7-\delta}$ (YBCO) films at the frequency of about 19.6 GHz and the temperature of 30 K $\sim$ 90 K. An open-ended $TE_{011}$ mode sapphire-loaded cylindrical cavity resonator was used as the measurement probe, where YBCO films were used as the endplates of the cylindrical cavity. The characteristics of the measurement system include functions to display the unloaded Q and the resonant frequency of the $TE_{011}$ mode resonator as well as the microwave surface resistance of the YBCO films, all simultaneously as a function of temperature. Applicability of the measurement system for investigating the homogeneity in the microwave properties of large high-$T_c$ superconductor films is discussed.

Ni tapes were textured by taking advantage of their secondary recrystallization. The obtained 18cm long tapes had textures of single crystalline qualities with their [001] axes tilted with respect to the surface and their [010] axes parallel to the rolling direction. The reproducibility of this texturing was checked by taking statistics on the crystal orientations of 24 Ni tapes. $YBCO/CeO_2/YSZ/CeO_2$ films grown on the Ni tape had the same crystalline orientations. Magnetic field dependent I-V relations were measured on a 5cm section of the tape. $J_c$ defined by $20{\mu}V/mm$ criterion was $1.5\times10^5A/cm^2$ at 77K under zero field and was reduced by $\sim$50% under the applied magnetic field of 5T.

The mixed-state Hall angle has been measured in Hg-based superconducting thin films as functions of magnetic fields (H) up to 18 T. The temperature dependence of the Hall angle shows a peak (T*) at low temperature, which is consistent with a crossover point from the thermally activated flux flow (TAFF) to a critical region (CR). At low fields below 10 T, T* shifts to low temperature with increasing fields. Interestingly, however, we found that T* is independent of fields above 10 T, suggesting unusual vortex state. A physical implication of H-T* line will be discussed.

We report on the tunneling spectroscopy of tunnel junctions using d-wave superconductor in relation to Andreev reflection. The zero bias conductance peak (ZBCP) which has maximum on [110] direction of ab-plane is observed on metal $Au/YBa_2Cu_3O_y$ tunnel junctions while it is suppressed on the ferromagnetic $Co/Au/YBa_2Cu_3O_y$ tunnel junctions. The effects of Andreev reflection on the differential conductance of each junction are dependent on the tunnel direction. For the $Co/Au/YBa_2Cu_3O_y$ junction, the suppression of Andreev reflection takes place by spin-polarized quasiparticles tunneling from a ferromagnetic material to a d-wave superconductor. By comparing these experimental results with recent theoretical works on Andreev reflection, the existence of Andreev bound state due to the d-wave symmetry of the pair potential is verified in high-$T_c$ superconductor.

We have constructed a multi-channel SQUID magnetometer system. The system is designed to operate normally with 10 high temperature direct coupled SQUIDs. The main features of the system include a remote control by serial communication, low noise with wide bandwidth and high slew rate by several MHz modulation, signal conditioning and calibration by digital signal processing.

A $Y_1Ba_2Cu_3O_{7-\delta}$ square spiral microstrip antenna (YBCO antenna) was epitaxially grown on a $LaAlO_3$ substrate by laser ablation. Also fabricated was a gold square spiral microstrip antenna (gold antenna) having the same structure as that of the YBCO antenna in order to compare the properties of both antennas. Both the YBCO antenna and the gold antenna were operated in Ku (12-18 GHz) band, and their properties such as the return loss, SWR, power gain, and radiation patterns were investigated at 77 K. The return loss below -10 dB was obtained in two frequency ranges, i.e., 14.05-14.90 GHz, and 16-18 GHz for the YBCO antenna at 77 K (YBCO superconducting antenna), and in the frequency range of 15.05-17.60 GHz for the gold antenna at 77 K. The SWR bandwidths are 0.85 GHz and 2 GHz for the YBCO superconducting antenna, and 2.55 GHz for the gold antenna at 77 K. The gain improvement of the superconducting YBCO antenna over the gold antenna at 77 K was about 10 dB in the frequency range of 16 GHz to 18 GHz. The radiation patterns show the YBCO superconducting antenna has the omni-directional property of a spiral antenna.