JP2861265B2 - Josephson junction element - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a device having a Josephson junction, and more particularly, to a superconductor having crystal anisotropy in critical current density and coherent length such as a high-temperature superconductor. The present invention relates to a used Josephson junction device.

Note that the present invention is applicable to any device using the Josephson effect, such as a SQUID, an infrared detector, a mixer, a three-terminal device, and a computer.

<Prior art> Many proposals and reports have been made on Josephson junction devices using a high-temperature superconductor thin film such as YBCO (for example, Akinobu Irie et.al., Japanese J
ounal of Applied Physics Vol.28, No.10, October, 198
9, pp. L1816-1819), most of those realized at the present time use grain boundary bonding.

The reason is that the coherent length of the high-temperature superconductor is as short as Å order to more than ten Å, so it is impossible to artificially obtain a Josephson effect regardless of tunnel type or weak coupling type .

<Problems to be Solved by the Invention> By the way, in the Josephson junction using the grain boundary, a grain boundary, which cannot be controlled artificially, is used.
An important factor such as a critical current density or a critical current varies widely, and an element having good reproducibility and good characteristics cannot be obtained.

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

<Means for Solving the Problems> A configuration for achieving the above object will be described with reference to FIG. 1 corresponding to the embodiment. A first surface 11 that is a plane and a second surface 12 that is a curved surface that intersects the first surface 11 at an obtuse angle θ and that gradually approaches parallel to the bottom surface as the distance from the intersection increases. On the surface of the substrate 1, a thin wire 2 made of a superconducting thin film having crystal anisotropy in critical current density and coherent length is formed on the first and second surfaces 11.
It is characterized by being formed so as to cross 12 intersection lines 3.

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

It is also known that the critical current density changes depending on the mutual bonding state of crystals (PHYSICAL REVIEW LETT
ERS, Vol 61, No. 2, pp. 219-222). Furthermore, there has been a report that a good SQUID has been obtained by using a YBCO thin film and a Josephson junction in which the spontaneously formed c-axis is shifted by several degrees (Cur
rent in YBCO Bridge by MO-CVD Thin Film, by Tsutom
u Yamashita et al.).

 The present invention utilizes such a fact.

That is, the first and second thin wires 2 made of a superconductor thin film having crystal anisotropy in critical current density and coherent length such as a high-temperature superconductor thin film intersect each other at an obtuse angle θ on the substrate 1.
2 is formed so as to cross the intersection line 3 between the surfaces 11 and 12, so that the crystal axis direction of the thin film thin line 2 on the first and second surfaces 11 and 12 is schematically shown in FIG. Can be forcibly shifted by the intersection angle θ, and a so-called sub-grain boundary can be artificially formed at this portion. In other words, by forcibly reducing the critical current density of this part,
It is possible to artificially obtain a weak bond. In addition, since the second surface 12 is a curved surface approaching parallel to the bottom surface, the weak coupling is formed at the intersection at the intersection angle θ, that is, at the intersection of the upper stage of the stepped substrate and the second surface 12. Surface only,
No interface is formed between the lower portion of the step substrate and the second surface 12 as a discontinuous crystal surface.

<Embodiment> FIGS. 1A and 1B are views showing a configuration of an embodiment of the present invention, wherein FIG. 1A is a plan view of a main part, and FIG.

The surface of the MgO (100) substrate 1 has a first surface 11 which is a plane parallel to the bottom surface and a curved surface which intersects the first surface 11 at an obtuse angle θ and gradually approaches the bottom surface as the distance from the intersection increases. Is formed.

The c-axis-oriented YBCO high-temperature superconductor thin film 2 having a minute width is formed so as to cross the intersection 3 of the first and second surfaces 11 and 12. Both ends of the fine wire 2 are connected to electrode portions 4 and 5 made of the same YBCO high-temperature superconductor thin film.

The above structure can be manufactured by the following procedure.

First, in order to form the first and second surfaces 11 and 12 which intersect at an angle θ on the surface of the MgO (100) substrate 1, a resist is applied to the substrate on a flat plate so as to cover the first surface 11 portion. After patterning, the substrate surface is milled by Ar ion milling. At this time, by adjusting the milling conditions, it is possible to obtain a curved second surface 12 in which the angle of the intersection is relatively steep and gradually approaches parallel to the bottom surface as the distance from the intersection increases. .

Next, while the substrate 1 is heated to 550 to 650 ° C., a YBCO thin film is formed so as to cross the intersection 3 of the first and second surfaces 11 and 12.

As a result, a YBCO thin film having a c-axis orientation in a direction perpendicular to the substrate surface grows in each portion.

According to the structure of the embodiment of the present invention as described above, as shown in FIG. 2 which is a schematic diagram viewed from a direction perpendicular to the axial direction of the fine wire 2, the first surface 11 of the YBCO high-temperature superconductor thin film 2
The c-axis direction of the upper portion and the c-axis direction of the portion near the intersection line 3 on the second surface 12 of the YBCO high-temperature superconductor thin film 2 are substantially at angles (as indicated by arrows in the figure), respectively. π−
θ). Therefore, a discontinuous portion of the crystal is formed here, and a so-called sub-grain boundary 20 is obtained. This part of the sub-grain boundary 20 has a significantly lower critical current density compared to the other parts, and therefore, in this part,
The YBCO high-temperature superconductor thin film wire 2 is bonded in a weak manner, and a Josephson junction is obtained.

The sub-grain boundary 20 formed in this portion can be artificially controlled by the intersection angle θ of the two surfaces, the width and the film thickness of the YBCO high-temperature superconductor thin film 2, and has a desired critical current with good reproducibility. A Josephson junction can be obtained.

The material of the substrate is, in addition to the above-mentioned MgO, SrT
Any material that does not damage the high-temperature superconductor thin film used, such as iO ₃ (100), (110), YSZ, or GGG, may be used.

Also, any superconductor thin film may be used as long as it has strong crystallinity.

Further, the crystal axis direction of the superconductor thin film is not particularly limited to the c-axis, but may be another direction.

<Effects of the Invention> As described above, according to the present invention,
A first surface that is parallel to the bottom surface intersects the first surface at an obtuse angle θ and forms a second surface that forms a curved surface, and crosses the intersection of these two surfaces. Since a superconducting thin film with crystal anisotropy was formed, a discontinuous surface where the crystal axis direction changes sharply occurs in the superconducting thin film on this intersection, and a sub-grain boundary is formed artificially. , the use of high-temperature superconducting thin film or the like, can create stable reproducibility good SQUID like which can artificially by controlling the I _c and dynamic resistance R. Further, since the weak coupling in the stepped substrate in the present invention is formed only in the upper stage, for example, the weak link length is too long compared to the configuration in which the weak coupling is formed in both the upper stage and the lower stage of the stepped substrate. To obtain the desired Josephson junction,
There is no problem that the operation becomes unstable due to the different weak couplings in the upper and lower stages.

[Brief description of the drawings]

FIG. 1 is a diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a schematic operation explanatory diagram thereof. DESCRIPTION OF SYMBOLS 1 ... Substrate 11 ... 1st surface 12 ... 2nd surface 2 ... YBCO high temperature superconductor thin film thin wire 20 ... Sub-grain boundary 3 ... Intersection

Claims (1)

(57) [Claims] 1. A first surface which is a plane parallel to a bottom surface thereof intersects at an obtuse angle with the first surface and gradually becomes parallel to the bottom surface as the distance from the intersection increases. A second surface, which is a curved surface approaching, is formed, and a thin line made of a superconducting thin film having crystal anisotropy in critical current density and coherent length is formed on the surface of the substrate by crossing the first and second surfaces. Josephson junction element formed so as to cross.