Sadamu Kita

Grain Boundary Josephson Junctions Using Y-Ba-Cu-O Films Operative at 77 K

Grain boundary Josephson junctions were fabricated from superconducting oxide Y-Ba-Cu-O films. The junctions were made by a chemical wet etching technique with phosphoric acid (H3PO4) solution. Typical dimensions of the junctions were a width of 4 µm and a length of 20 µm. The junctions showed the so-called bridge-type Josephson effect. Shapiro steps were observed in the I-V curves at a temperature as high as 77 K.

Performances of Josephson Junction Harmonic Mixers with Harmonic Numbers 1–8 at 70 GHz

We have carried out a series of experiments at 70.03 GHz on Josephson junction harmonic mixers with harmonic numbers 1–8, using an identical Josephson junction. It was found that Josephson junction harmonic mixers operate differently depending on whether their harmonic numbers are odd or even. With odd-harmonic mixers, only finite-bias operation is possible, while with even-harmonic mixers, finite-bias and zero-bias operations are both possible. It is noted that a 2nd-harmonic mixer has a conversion efficiency equal to or higher than that of a fundamental mixer. The dependence of the conversion efficiency (η) on the harmonic number (N) was also studied, and the following approximate relations were determined: η~N-1.6 for finite-bias operation and η~N-1.4 for zero-bias operation.

Hall Coefficient of La–Sr–Cu Oxide Superconducting Compound

Hall Coefficient of a porous La-Sr-Cu oxide superconductor (Tc.zero=32 K) have been measured. The average concentratin n300 and mobility µ300 of electrons, the dominant carrier, are found to be 1.5×1021 cm-3 and 4.17 cm2/Vs, respectively, at room temperature. The observed n300 decreases with increasing temperature due probably to significant capture of thermally excited electrons at dense traps existing in the grain boundary surrounding the conductive channels.

High Tc Superconductivity of rf-sputtered Er-Ba-Cu-O Films

High Tc Er-Ba-Cu-O superconducting films were prepared by rf sputtering. The films revealed superconducting transition temperatures for onset and zero resistance of 95 K and 84 K, respectively. Although the X-ray diffraction measurement showed the film consists of an orthorhombic structure with the lattice constants a, b and c, respectively, of 3.883 A, 3.830 A and 11.70 A, most of the surface layer was found by electron spectroscopy for chemical analysis (ESCA) to be significantly Er-deficient.

Experimental Study of Josephson Junction 2nd-Harmonic Mixers at 70 GHz

Josephson junction (JJ) harmonic mixers with even-harmonic numbers can operate at zero bias voltage. Among them, low harmonic ones are expected to have a good conversion efficiency. Hence, we have done experiments on the 2nd-harmonic mixer at 70 GHz. It is found that mixing behaviors are classified into three types: zero-bias, finite-bias and intermediate operation. It is also found that the conversion efficiency of the 2nd-harmonic mixer is not so much less than that of the fundamental mixer. We obtained a conversion gain of +1.5 dB with the 2nd-harmonic mixer, and +2.0 dB with the fundamental mixer.

Electron-cyclotron-resonance microwave plasma oxidation of Er1Ba2Cu3Oy superconductive ceramics

An electron-cyclotron-resonance (ECR) microwave plasma was introduced to the forced oxidation of high-Tc Er1Ba2Cu3Oy superconducting ceramics in oxygen pressure as low as 0.9 Pa. The negative bias (-40 V) application to the specimen greatly helped the progress of the plasma oxidation as compared with the positive bias (+30 V) one. The accumulation of the oxygen atoms ionized positively is thought to be responsible for the observed bias effect. A higher substance temperature and input microwave power also enhanced the plasma oxidation. A 30-minute oxidation was sufficient to obtain the superconductivity with Tc92 K.

Annealing-Temperature Dependence of High Tc Superconductivity of Y1-xBaxCuO3

Y1-xBaxCuO3 ceramic superconductors were prepared under precise annealing-temperature control, and systematically measured in their crystalline and electrical characteristics. The critical temperature showed clear dependence on the annealing-temperature, the dependence itself being different from composition to composition. However, optimizing the annealing-temperature, the ceramics with an orthorhombic structure exhibited almost the same zero-resistance temperature (87-89 K) regardless of the Ba content (x=0.5-0.7).

Millimeter-Wave Mixing on NbN Superconducting Tunnel Junctions Integrated in a Linear-Array Configuration

NbN(base)/insulator/Pb(counter) tunnel junctions integrated in a linear-array configuration were prepared and used in a millimeter-wave mixing experiment at 35 and 70 GHz. Each junction worked well on the basis of the quantum theory of mixing, though the conversion efficiency was still low due to insufficiently developed mount design and fabrication technology. Owing to the allowance of an optimum local power as large as 10 dB, the linear-array SIS junctionsacted as a sensitive millimeter-wave image sensor: These junctions offered a field intensity distribution inside the waveguide at 35 GHz.

Low‐noise properties of unbiased even‐harmonic Josephson mixers

Noise properties of Josephson harmonic mixers were investigated both by simulation studies and by experiments. It was clarified that the unbiased mixing operation peculiar to even‐harmonic Josephson mixers has a lower noise temperature than the biased operation. A minimum noise temperature of 121 K was obained for an unbiased second‐harmonic Josephson mixer at 70 GHz. The cause of this low‐noise property of the unbiased mixing was attributed mainly to the nonexistence of noise down‐conversions by the odd‐harmonics of LO and the Josephson oscillation.

Frequency dependence of AC Josephson effect in Point-Contact (Y, Er)BaCuO/Nb

Microwave and millimeterwave dependences of ac Josephson effect were investigated in YBaCuO ceramic superconductors. We observed very clear constant-voltage steps (Shapiro steps) on the current-voltage curve of YBaCuO/Nb junctions at signals of 14.1,25.5 and 35.0 GHz. The observed step voltages were frequency dependent in accordance with the (h/2e)f rule. This strongly suggests that pair-electrons serve as the superconducting particle even in ceramic materials. In addition, the ac Josephson effect was also observed in point-contact ErBaCuO ceramics containing a magnetic element Er.