JPH06177443A - Superconducting junction device - Google Patents

Abstract

PURPOSE:To control a critical current value easily by a method wherein a visible light beam is applied to a superconducting thin film weak coupling part from the open end of an optical fiber to elevate a superconductor transition temperature. CONSTITUTION:A resist pattern is formed on an SrTiO3 (100) substrate 1. A step 7 is formed by Ar ion milling with the resist pattern as a mask. After the resist pattern is removed, a YBa2Cu3Oy thin film 2 is built up on the substrate and processed by photolithography and ion milling to form a YBa2Cu3Oy thin film weak coupling part 3. A multimode optical fiber 5 is provided closely to a weak coupling type superconducting junction device and a He-Ne laser beam is applied to the weak coupling type superconducting junction device to realize a laser beam application with a very high power density. With this constitution, the rise of the temperature T deg.C of the superconducting thin film and the rise of the critical current value of the superconducting thin film weak coupling part which are caused by the He-Ne laser beam application can be controlled in a short time, so that the critical current value can be controlled more accurately.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a superconducting junction device,
More specifically, the present invention relates to a weakly coupled superconducting junction element capable of controlling a critical current value.

[0002]

Since the discovery of high-temperature oxide superconducting materials having a superconducting transition temperature exceeding the liquid nitrogen temperature, development of superconducting electronic devices using these materials has been vigorously pursued. In general, a superconducting electronic device has two main types of elements: a tunnel-type Josephson junction that exhibits hysteresis on a current-voltage characteristic and a weak-coupling Josephson junction that does not exhibit hysteresis. The former is 3n between two stacked superconducting thin film electrodes.
It has a structure in which a very thin insulator barrier layer of m or less is inserted. In particular, this type of junction is suitable for forming a digital arithmetic circuit such as a so-called Josephson computer. However, a high temperature superconducting material represented by YBa ₂ Cu ₃ Oy (y = 6 to 7) has a coherence length of 0.1.
It has a very short length of ~ 2.5 nm, and has a large electrical anisotropy in the CuO plane and in the direction perpendicular to the plane, so high-quality tunnel junctions have not been realized to date (high-quality junctions In order to obtain it, it is necessary to suppress the thickness of the interface deterioration layer at the superconducting layer-barrier layer interface to the coherence length or less). On the other hand, the weak-coupling type Josephson junction is formed by, for example, narrowing between the two superconducting thin film electrodes.
It has a structure in which the superconductivity is weakened by reducing the critical current. Since the high-temperature superconducting material has a low carrier concentration and a large electrical anisotropy, it has a property that the critical current density becomes smaller by a few orders of magnitude in the direction crossing the crystal grain boundaries than in the grain. For example, on a SrTiO ₃ single crystal substrate, good quality YBa ₂ Cu ₃ Oy having a high critical current density is formed.
Epitaxial thin film is obtained, but two SrTiO ₃
A weak bond junction can be easily formed on a twin grain boundary by using a twin crystal substrate in which single crystals are bonded with their crystal axes inclined. Further, when a thin film is deposited after forming a step on the SrTiO ₃ single crystal substrate, a weak bond bond is formed along the step. Electronic devices such as a DC-SQUID magnetometer (superconducting quantum interferometer) and a magnetic flux flow transistor that operate at a temperature near the liquid nitrogen temperature have been developed by utilizing the high temperature superconducting thin film weak bond junction formed by such a method. Is coming. In the superconducting junction device using these weakly coupled junctions, it is very important to control the critical current density to a certain value. For example, 2
The DC-SQUID using two weak coupling type junctions has the same critical current value Ic and the loop inductance L.
The sensitivity is highest when the product of I and Ic is about 1/2 of the quantum magnetic flux Φ ₀ . Control of Ic is important because L is determined by the shape of the element. However, in the weakly-bonded junction utilizing the grain boundaries described above, not only the shape and area cannot be accurately determined, but also the resistance value is very sensitive to the oxygen concentration in the grain boundaries, so that the critical current There is a problem that it is difficult to precisely control the value Ic. On the other hand, recently, Kudinov and Nieva et al. Have reported that high power Ar ions are added to a LnBa ₂ Cu ₃ Oy (Ln = Y, Gd) thin film showing a semiconductor-like electric resistance change due to oxygen deficiency and a very low superconducting transition temperature Tc. It has been reported that irradiation with laser light causes a decrease in electrical resistance and an increase in Tc, and that the change is maintained at low temperatures [Phys. Lett. A151 (1990), p.358 and Appl. . Phy
s. Lett. 60 (1992), p. 2159]. Since the critical current of a weak bond type junction generally increases with an increase in Tc of the thin film, it is considered possible to control the critical current using this effect.
However, there is a problem that the application to electronic devices is hindered, such as the need for a high power laser of about several W and the need to prevent the temperature rise by directly immersing the thin film in liquid nitrogen because of the high power. there were.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional superconducting weak bond type junction using the high temperature superconducting thin film, and to easily control the critical current value. The object is to provide a weakly coupled superconducting junction element.

[0004]

In order to achieve the above-mentioned object of the present invention, superconductivity of a first and second superconducting thin film electrodes formed on a substrate and a superconducting current having a smaller critical current connecting the two electrodes. In a weak-coupling type superconducting junction element composed of a thin-film weak-coupling portion, visible light is irradiated to the superconducting thin-film weak-coupling portion from an open end of an optical fiber which is arranged close to the superconducting thin-film weak-coupling portion. By increasing the superconducting transition temperature, the critical current value of the weakly coupled portion of the superconducting thin film is controlled. The invention further superconducting thin weak coupling portion of the superconducting junction device has the general formula LnBa ₂ Cu ₃ Oy
(Ln is Y, La, Nd, Sm, Eu, Gd, Tb, D
At least one element selected from y, Ho, Er, Tm, Yb, and Lu is shown, and y is 6.35 ≦ y ≦ 6.
Represents a range of 9. ) Is a superconducting junction element having the composition shown in FIG. Quartz optical fiber has a wavelength of 1.3μ
It is widely used for optical communication using infrared light of m or 1.55 μm, but it can also transmit visible light over a short distance, and the loss increases so much even at extremely low temperature such as liquid helium temperature. do not do. For example, GI (Graded Ind)
ex) When a multimode optical fiber is used, its core diameter is 50 μm. Therefore, if the fiber open end is placed close to the weak coupling type junction, it is possible to directly irradiate a laser beam with a diameter of 1 mm by spatial propagation. In comparison, irradiation with a power density about 400 times higher can be effectively obtained. In a single mode optical fiber with a core diameter of 8 μm, it is even larger, about 15,000.
You can easily create double the power density,
A large effect can be expected even if a low-power laser light source is used. In addition, since the size of the junction that is usually used for the superconducting electronic device is 1 to 10 μm, it is possible to effectively irradiate only the weak coupling portion of the superconducting thin film and suppress the temperature rise of the entire superconducting junction device. be able to.

[0005]

Embodiments of the present invention will be described below in more detail with reference to the drawings. <Example 1> SrTiO ₃ was prepared using the reactive co-evaporation method.
About 100 nm thick YBa on a (100) single crystal substrate
_{A 2} Cu ₃ Oy thin film was deposited. Y was evaporated by an electron beam and K-cell was used as a source of Ba and Cu.
Substrate temperature during deposition is about 700 ° C, oxygen pressure is 1 × 10 ~ ⁴
Oxygen was activated by applying RF (high frequency) power of about 100 W in mmHg (Torr).
The thin film shows c-axis orientation, and the lattice constant in the c-axis direction is 11.6.
8Å, Tc (zero resistance) was 90K. Oxygen-deficient thin films were prepared by reducing these in a rapid annealing furnace. The atmosphere is Ar + 0.2-
2vol% O ₂ mixed gas, temperature 400-550 ° C.
After annealing for 30 minutes in, the thin film was rapidly cooled to a temperature of about 150 ° C. Thin films with different oxygen concentrations were prepared by changing the oxygen concentration and temperature during annealing.
Its lattice constant [c ₀ (Å)] and superconducting transition temperature [T
c (K)], the oxygen composition [y] estimated from these values, and the amount of increase in Tc due to light irradiation [δTc]
The value of (K)] is shown in Table 1.

[0006]

[Table 1]

After forming an Au thin film pattern for electrode extraction on the thin film produced by the above method by vacuum deposition using a metal mask, a resist pattern is formed by a photo process and Ar ion milling using this as a mask. A superconducting junction element having a weakly coupled structure with a width of 10 μm and a length of 30 μm was produced. Next, a He-Ne laser (λ
= 632.8 nm) and the irradiation effect was investigated using a light source. The laser light was focused by a lens system and introduced into a GI multimode optical fiber having a core system of 50 μm. The optical fiber is passed through a sample holder for low temperature measurement, and the other open end is made of YBa ₂ Cu ₃ O using an alumina guide 6 as shown in FIG.
The y thin film weakly bonded portion 3 was placed close to the thin film weakly bonded portion 3 so that the entire thin film weakly bonded portion 3 was vertically irradiated with light. After completing the alignment using the microscope, the alumina guide 6 is set to SrTiO.
_{3 It} was adhered and fixed to the (100) substrate 1. The optical power at the substrate position was about 2.5 mW. Insert the sample holder into the liquid helium dewar and maintain the temperature of the thin film at about 100K by adjusting the height from the liquid surface to 2.5m.
Table 1 shows the amount of increase in Tc [δTc (K)] after irradiating the weakly coupled portion for 8 hours with W light. A rise in Tc was also observed in the original thin film having a Tc of 88K, and a tendency was shown that the lower the Tc, the greater the increase in Tc. In addition, the thin film irradiated with light had an increased Tc as long as it was kept at a temperature of 200 K or lower, and no relaxation to the original low Tc was observed. On the other hand, when heated to a temperature of 325 K or higher and held for several hours, the same Tc as at the beginning was exhibited. Furthermore, the sample No. shown in Table 1 in which Tc was relaxed at high temperature as described above. The same irradiation experiment was performed using the thin film of No. 4 and this time using light sources having different wavelengths. The light source used was an AlGaAs LED (λ = 850 nm), AlG
aInP laser diode (λ = 670 nm), He
There are four types: -Ne laser (? = 543.5 nm) and Ar ion laser (? = 488 nm). Adjustment of diode current or ND (Neutral Density) in gas laser
The optical power was attenuated by using a filter so that the power at the sample position was 1.5 mW. 5 hours when using a visible light source by irradiation for 8 hours at 100K
An increase in Tc of ~ 6.5K was observed. On the other hand, infrared light L
ED did not cause any increase in Tc.

Example 2 Using the same vapor deposition apparatus as in Example 1, LnBa ₂ Cu ₃ Oy having a thickness of about 100 nm was used.
A (Ln = Eu, Gd, Dy, Ho, Er, Tm) thin film was deposited on a SrTiO ₃ (100) substrate under almost the same conditions. Each of the deposited thin films exhibits c-axis orientation, and its T
c was 85 to 90K. Ar + 0.
3 at 430 ° C. in a _{2 to 2} vol% O ₂ gas flow
After annealing for 0 min, it was rapidly cooled to form a thin film lacking oxygen. Table 2 shows the reduced LnBa ₂ Cu ₃ Oy.
(Ln = Eu, Gd, Dy, Ho, Er, Tm) The superconducting transition temperature Tc (K) of the thin film and the increase amount Tc (K) of Tc due to light irradiation are shown. Although there is some variation in Tc, the oxygen composition y is y = 0.4-
It was estimated to be 0.5. Next, a weakly coupled structure having a width of 10 μm and a length of 30 μm was produced by the same method as in Example 1,
The multimode fibers were arranged close to each other, and the same light irradiation experiment as above was performed. An AlGaInP laser diode (λ = 670 nm) was used as the light source, and the optical power at the sample position was 3 mW. Table 2 shows the amount δTc (K) of increase in Tc of the thin film due to irradiation for 8 hours at 100K.

[0009]

[Table 2]

As shown in Table 2, YBa ₂ C of Example 1
It can be seen that the Tc rises to the same extent as in the case of the u ₃ Oy thin film. Note that LnBa ₂ Cu ₃ Oy (Ln = Y, La,
Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, T
(m, Yb, Lu) thin film has the same structure and has a superconducting transition temperature of about 90 K, so that similar effects can be expected for other lanthanoids.

<Embodiment 3> As shown in FIG. 2, SrTi
After forming a resist pattern on the O ₃ (100) substrate 1 by a photo process, using this as a mask, a step 7 having a height of 100 nm was formed by Ar ion milling. After removing the resist, a YBa ₂ Cu ₃ Oy thin film 2 having a thickness of about 150 nm was deposited on the above substrate by a DC magnetron sputtering method using a sintered body target having a stoichiometric composition.
At this time, the substrate temperature was set to 650 ° C. using Ar + 5 vol% O ₂ gas of 10 Pa. This thin film is processed by a photo process and ion milling, and the width is 5 μm as shown in FIG.
m, length 30 μm, cross-stepped YBa ₂ Cu ₃ O
y The thin film weakly bonded portion 3 was produced. This weak-coupling type superconducting junction element exhibits a superconducting (zero voltage) current at a temperature of 72 K or lower, and at lower temperatures, RSJ (Resist) as shown in FIG.
The current-voltage characteristics of a typical weakly coupled Josephson junction that closely follows the shunted junction model were observed. To the weakly coupled superconducting junction element manufactured by the above procedure,
The same multimode optical fiber 5 as that of the first embodiment is arranged close to each other, and the He—Ne laser light (λ = 63) at 65K is arranged.
The effect of irradiation (2.8 nm) was investigated. The maximum optical power coupled into the fiber is 2.5 mW, and by inserting an ND (Neutral Density) filter in front of the introduction part to the fiber, the power is -10 dB (decibel).
Alternatively, light irradiation with -20 dB attenuation was also performed. The light irradiation time dependence of the normalized critical (zero resistance) current value is shown. The critical current increases proportionally to the logarithm of time for any optical power. Also, −10 dB and −
It can be seen that it takes 10 times and 100 times, respectively, to obtain the same increase amount as 0 dB with the power of 20 dB. These mean that the critical current value can be controlled by the product of power and time, that is, the dose amount of photons, and that the time required for current value control can be significantly shortened by light irradiation with high power density. Next, the same weak-bonding type Josephson junction as that described above was attached to a copper block head in a gas flow type cryostat using low temperature grease and cooled to 65K by heat conduction from the block. In this case, the YBa ₂ Cu ₃ Oy thin film is placed in a vacuum. 1
W power Ar ion laser light (λ = 514.5 nm,
The beam diameter (1 mm) was attenuated to 1/10, and the vicinity of the YBa ₂ Cu ₃ Oy thin film weakly coupled portion was irradiated through a quartz window of a cryostat, and the time change of the critical current value was examined. As shown in FIG. 5, during light irradiation, He − with a power of 0.25 mW.
The increase in the critical current value is small as compared with the case where the Ne laser light is irradiated through the optical fiber. However, 100 mi
When the light irradiation was stopped after n, the critical current value suddenly increased to a level comparable to that through the fiber (while the change in the critical current with the fiber was hardly observed). It is considered that this is because the temperature of the YBa ₂ Cu ₃ Oy thin film remarkably increased during light irradiation without passing through the fiber.

[0012]

As described in detail above, according to the superconducting junction element of the present invention, a structure in which an optical fiber is disposed close to the superconducting thin film weakly coupled portion allows the visible light of small power to be emitted. Irradiation with a very high power density can be realized even by using a laser. Therefore, the increase in Tc of the superconducting thin film and the increase in the critical current value of the weak junction of the superconducting thin film due to light irradiation can be reduced in a short time. Can be controlled. Further, since it is possible to irradiate light only on the weakly coupled portion of the superconducting thin film, it is possible to suppress an increase in temperature of the superconducting thin film joint portion and to control the critical current value more accurately.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the configuration of a weakly coupled superconducting junction element exemplified in Example 1 of the present invention.

FIG. 2 is a schematic diagram showing a configuration of a weakly coupled superconducting junction element exemplified in Example 3 of the present invention.

FIG. 3 is a diagram showing current-voltage characteristics of the weak coupling type superconducting junction element exemplified in Example 3 of the present invention.

FIG. 4 is a diagram showing a change with time of a critical current value of a weakly coupled superconducting junction element illustrated in Example 3 of the present invention with respect to He—Ne laser light irradiation.

FIG. 5 is a case of irradiating a He—Ne laser beam through an optical fiber to the weakly coupled superconducting junction element illustrated in Example 3 of the present invention, and a case of directly irradiating an Ar ion laser beam without passing through the optical fiber. The figure which shows the time change of the critical current value at the time of doing.

[Explanation of symbols]

1 ... SrTiO ₃ (100) substrate 2 ... YBa ₂ Cu ₃ Oy thin film 3 ... YBa ₂ Cu ₃ Oy thin film weakly-bonded portion 4 Au electrode pattern 5 multimode optical fiber 6 alumina guide 7 step

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Suzuki 1-16, Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. A structure comprising first and second superconducting thin-film electrodes formed on a substrate, and a superconducting thin-film weak-coupling part connecting between the first and second superconducting thin-film electrodes having a smaller critical current. In the weak coupling type superconducting junction element, the superconducting transition temperature is raised by irradiating the weak coupling portion of the superconducting thin film with visible light from the open end of the optical fiber arranged close to the weak coupling portion of the superconducting thin film. A superconducting junction device, characterized in that it has means for controlling a critical current value by controlling. 2. The superconducting junction element according to claim 1, wherein
The superconducting thin film weakly-bonded part has the general formula LnBa ₂ Cu ₃ Oy (wherein Ln is Y, La, Nd, Sm, Eu, Gd, T
at least one element selected from b, Dy, Ho, Er, Tm, Yb, and Lu, and y is 6.35 ≦ y
It represents a range of ≦ 6.9. ) A superconducting junction element characterized by comprising a composition shown in FIG.