JP2853242B2 - Josephson junction element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a device having a superconducting Josephson junction device, for example, utilizing the Josephson effect of a SQUID, an infrared detector, a mixer, a three-terminal device, a computer, etc. The present invention relates to an element that can be applied to any device.

<Prior art> Many proposals and reports have been made on Josephson junction devices using high-temperature superconductor thin films such as YBCO, but at present, only those using grain boundaries have been realized. Absent.

The reason is that the coherent length of high-temperature superconductors is as short as Å10 to a dozen or so, so it is impossible to artificially obtain the Josephson effect regardless of tunnel type or weak coupling type. .

<SUMMARY invention> Now, the Josephson junctions utilizing grain boundary, it is not possible to control the grain boundary, to suppress the variation in the presence or absence of the critical current value I _c and Josephson effect occurs Is not done. That is, at present, a high-performance Josephson junction device with good reproducibility has not been manufactured.

An object of the present invention is to provide a high-performance grain boundary Josephson junction device with good reproducibility.

<Means for Solving the Problems> In order to achieve the above object, according to the present invention, as shown in FIG. 1 corresponding to the embodiment, the upper surface of the substrate 1 is inclined at a predetermined angle θ. 1a and a lower plane 1b, and a superconductor epitaxially grown thin film is formed on both upper and lower planes 1a and 1b with a wedge-shaped vertical step 1c formed at a boundary portion between the upper plane 1a and the lower plane 1b. Are formed, and the upper and lower superconductor thin films 2a and 2b are vertically
The bridge is narrowed in the vicinity of c to form a bridge.

<Effect> In a high-temperature superconductor thin film such as YBCO, generally, the critical superconducting current has crystal anisotropy, and the critical current density _Jc is particularly large in the direction perpendicular to the c-axis.

Then, the mutual coupling state of the crystal, the critical current density J _c is known to vary (PHY-SICAL REVIE
W LETTERS, Vol 61, No. 2, pp 219-222).

In addition, using a YBCO thin film, the naturally occurring c
Good SQUI using Josephson junction with axis shifted several degrees
There is also a report that obtained D (Cur-rent in YBCO Bridge by MO
-CVD Thin Film, by Tsutomo Yamashita et al.).

 The present invention utilizes such a fact.

In other words, a superconducting thin film having a crystal anisotropy of critical superconducting current, such as a high-temperature superconducting thin film, is epitaxially grown on a substrate having an inclined step. manner as shown, shifting forces by the inclination angle θ of the substrate, the by forming a bridge in the vertical stepped portion, reproducibly high performance Josephson junction by reducing the critical current density J _c of the bridge Have gained.

<Example> FIG. 1 is an external view of a main part of an example of the present invention.

Mg0 (100), or SrTi0 ₃ (100), the substrate 1 such as a (110) is formed sloped step of about theta ≒ 7 ° is at its upper and lower planes 1a and 1b, the vertical stepped portion 1c The c-axis oriented YBCO epitaxial growth films 2a and 2
b is formed.

In the vicinity of the vertical step portion 1c, the width of the YBCO epitaxially grown films 2a and 2b is reduced, and the YBCO epitaxial growth films 2a and 2b serve as bridges 3 to join the upper and lower flat films 2a and 2b to each other.

 The above structure can be manufactured by the following procedure.

First, the surface of the substrate 1 made of the above-described material is formed into an upper plane 1a and a lower plane 1b that are inclined at a predetermined angle θ, and a wedge-shaped boundary is formed between the upper plane 1a and the lower plane 1b. The vertical step 1c is formed. As a method,
When shaving the substrate 1 surface with an argon ion milling machine,
A method of giving a gradient of the current density can be adopted.

Next, YBCO is applied to the surface of the upper and lower step planes 1a and 1b of the substrate 1 having the above-described inclined steps, and c is applied to each surface.
Epitaxial growth is performed so that the axis is vertical. Here, if the substrate 1 is heated to 650 ° C. or more, in the case of YBCO, even if a certain degree of misfit with the lattice constant of the substrate occurs, epitaxial growth occurs in the c-axis orientation.

Then, the YBCO thin films 2a and 2b are vertically processed by the photolithography technology and ion milling technology
, The width in the vicinity is narrowed to 1 μm or less.

According to the structure of the embodiment of the present invention as described above, the c-axis direction of the YBCO thin film 2a on the upper plane 1a as shown in FIG. 2 is a schematic view seen from the direction facing the vertical step 1c. (The solid arrow in the figure) and the direction of the c-axis of the YBCO thin film 2b on the lower plane 1b (dotted arrow in the figure) are shifted from each other substantially by the inclination angle θ between the planes 1a and 1b.

This means that in the bridge 3 of the small cross-sectional area means that the portion crystal c axis is joined shifted by θ from each other occurs, decreases the critical current density J _c at that portion,
Means that the weak links with very small junction critical current I _c is obtained.

Here Notably, the deviation of the c-axis, forced by the inclined step formed on a substrate (vertical stepped portion 1c), and be a point that occurs reproducibly, small critical current I _c The point is that a high-performance Josephson junction device can be obtained with good reproducibility. Another point to be noted is that since the surfaces on which the YBCO thin films 2a and 2b are formed are flat, the orientation of the films is improved, and variations are suppressed.

The magnitude of the inclination angle θ is preferably in the range of about 0 ° <θ <30 °, but is not particularly limited to 30 °.

Further, the crystal axis of the superconductor thin film is not particularly limited to the c-axis, but at the present time it is desirable to use a c-axis oriented film in order to increase the critical current at the electrode portion.

Further, the type of the superconductor thin film is not limited to YBCO, and any superconductor such as a Bi-based or Tl-based superconductor can be used as long as the critical superconducting current density has crystal axis anisotropy.

Further, as the material of the substrate, any other material can be used as long as the superconductor to be used can epitaxially grow.

<Effects of the Invention> As described above, according to the present invention, the surface of the substrate is formed into an upper plane and a lower plane that are inclined at a predetermined angle, and the surface of the substrate is formed at the boundary between the upper plane and the lower plane. A wedge-shaped vertical step is formed, and an epitaxially grown superconductor film is formed on the upper and lower planes so as to straddle the vertical step, and a narrower portion is formed in the vertical step to form a bridge. As a result, the crystal axes of the upper and lower superconducting thin films are shifted from each other by almost the same angle as the inclination angle of the plane on both the upper and lower stages. Well obtained. Also,
Because both upper and lower steps forming the superconductor thin film are flat,
Since the orientation of the film is improved and the variation is suppressed, a junction having an extremely small critical current density can be obtained with higher reproducibility,
As a result, a Josephson junction element using a grain boundary, which has conventionally been regarded as having no reproducibility, has high performance and good reproducibility.

[Brief description of the drawings]

FIG. 1 is an external view of a main part of an embodiment of the present invention, and FIG. 2 is a schematic operation explanatory view thereof. 1 Substrate 1a Upper plane 1b Lower plane 1c Vertical step 2a, 2b YBCO epitaxial growth film

Claims (1)

(57) [Claims] An upper surface and a lower surface which are inclined at a predetermined angle from each other are formed on an upper surface and a lower surface, and the upper and lower surfaces of the upper surface and the lower surface are sandwiched by a vertical step which is a boundary between the upper surface and the lower surface. A Josephson junction device in which a superconductor epitaxially grown thin film is formed, and the superconductor thin films of the upper and lower stages are reduced in width in the vicinity of the vertical step.