Akiyoshi Nakayama

Y–Ba–Cu–O/Nb Tunnel Type Josephson Junctions

Y-Ba-Cu-O/Au/AlOx/Nb tunnel type Josephson junctions were fabricated and their current-voltage relation exhibited typical characteristics of Josephson tunnel junctions such as supercurrent, hysteresis and rf-induced voltage steps at 4.2 K.

Modulation of Josephson current of NB junctions by two-dimensional scan of external magnetic field

Josephson current of superconducting junctions can be modulated by an external field. This modulation of the Josephson current I/sub c/ is usually observed by one-dimensional scan of the applied magnetic field. In this paper, using two pairs of the Helmholz coils, the external magnetic field H is scanned in two dimensions and the two-dimensional surfaces of the I/sub c/-H dependence are first measured. Modulations of Fiske steps of the current-voltage characteristics are also studied.

Y-Ba-Cu-O/AlOx/Nb Josephson Tunnel Junctions

We have fabricated Y-Ba-Cu-O/AlOx/Nb Josephson tunnel junctions using electron-beam evaporation of Al and Nb films and natural oxidation. Superconducting Josephson current, hysteresis, and rf-induced voltage steps at a voltage greater than 0.4 mV have been clearly observed at 4.2 K. Moreover, rf-induced subharmonic steps have appeared. Magnetic field dependence of the superconducting Josephson current has also been studied.

Nb/AlOx/Nb Josephson tunnel junctions using electron beam evaporation

Niobium/aluminum-oxide/niobium Josephson tunnel junctions have been fabricated using electron beam evaporation to form all-metal films in a high vacuum system. A quality factor, Vm (the product of the maximum dc Josephson current and the subgap resistance defined at 2 mV) between 36–37 mV, has been obtained for the best junctions of a 6 nm-thick Al layer by cooling down the surface of the base electrode for 100 minutes under a high background pressure, before the deposition of aluminum, after the formation of the base niobium electrode. An array of 80 series-connected junctions has also been fabricated using the selective niobium anodization process and the planarization process. The current-voltage characteristics of the array showed a standard deviation of the maximum dc Josephson current of 3.3%.

Plasma-Oxidized Tunnel Barrier of Nb/Nb Josephson Junctions with Copper Interlayer

Niobium/niobium Josephson tunnel junctions were fabricated using electron-beam evaporation of Nb films and rf-plasma oxidation. Niobium reacts easily with oxygen, resulting in a decrease in its transition temperature. In order to suppress this reaction between the tunnel barrier and the Nb counter-electrode, a thin layer of Cu was evaporated on the tunnel oxide before the Nb counter-electrode was deposited. The sub-gap current of Nb/Nb junctions with a Cu interlayer was found to be smaller than without the interlayer. Moreover, it was found that using a clean and stabilized plasma process in forming the tunnel barrier is useful in decreasing the sub-gap current of the Nb/Nb junctions.

The shape dependency of two-dimensional magnetic field dependence of a Josephson junction

Modulation characteristics of a Josephson current are usually measured by applying the external magnetic field parallel to the junction plane from one direction, and uniformity in tunnel barrier is discussed. So far, we have measured two-dimensional magnetic field dependence of a square Josephson junction by independently scanning the magnetic field (Hx,Hy) parallel to the junction plane from two directions. We can get a lot of information about spatial critical current distribution in a Josephson junction by observing the magnetic field dependence of a Josephson junction in two dimensions. This time, we have fabricated the different-shaped Josephson junctions and investigated the shape dependency of two-dimensional magnetic field dependence of a Josephson junction. We observed the Ic‐(Hx,Hy) characteristics of triangular, hexagonal, and circular Josephson junctions quite different from the Ic‐(Hx,Hy), characteristics of a square Josephson junction. Furthermore, we simulated two-dimensional magnetic field...

Magnetic field dependence of Josephson current by applying the external magnetic field perpendicular to the Josephson junction

The hysteresis of the Josephson current to the external magnetic field Hz perpendicular to the Josephson junction is reported. We have measured the magnetic field Hz dependence of the Josephson current of the Nb∕Al-AlOx∕Nb junction. The Josephson current decreases with oscillating as the magnetic field Hz was increasing. When the magnetic field Hz was applied more than 2800[A∕m], the Josephson current became 0. Even if the magnetic field Hz was decreased from 2800[A∕m] to 0, no Josephson current was observed. The Josephson current exhibited hysteresis to the external magnetic field Hz.

Modulation of I–V curves of Nb single and double junctions by 2D scan of magnetic field

Abstract Modulation of current–voltage characteristics of niobium junctions by two-dimensional scan of the external magnetic field has been measured. Josephson current of superconducting junction can be modulated by applying an external magnetic field. The modulation of the Josephson current Ic of the superconducting junctions is usually observed by one-dimensional scan of the applied magnetic field. In this article, by using two pairs of the Helmholtz coils, the external magnetic field H to the junction is scanned in two dimensions, and the dependence on magnetic field of the DC Josephson currents of Nb/Al-oxide /Nb single and double junction structures have been observed.

Wave functions of Andreev bound states in superconductor/normal-metal/superconductor junctions

In this article the wave functions of Andreev bound states in superconductor/normal-metal/superconductor (SNS) junctions have been studied. Discrete energy levels of the Andreev bound states have been obtained as a function of the phase difference and as a function of the normal-metal thickness of a SNS junction. Concrete form of the two-component wave functions of Andreev bound states has been described. Using special orthogonal group SO(3), new representation of the two-component wave functions has also been proposed.

Analysis and application to superconducting quantum interference devices of double barrier superconducting junctions

We have calculated the dc Josephson current through double barrier superconducting junctions. Using a formula derived by Furusaki and Tsukada [Solid State Commun. 78, 299 (1991)], we have numerically estimated the current–phase relationship from the probability amplitude of the Andreev reflections in the junctions, and also discussed the resonant phenomena of quasiparticles. Moreover, using the obtained current–phase relationship, we have analyzed the characteristics of superconducting quantum interference devices containing double barrier superconducting junctions.