RU2439749C1 - Superconducting device with josephson junction - Google Patents

Abstract

FIELD: electricity. ^ SUBSTANCE: superconducting device with Josephson junction includes area of weak link formed at substrate in the form of thin-film laminated structure containing layers of ferromagnetic material and normal metal and two electrodes made of superconducting material to connect weak-link area to power source; the device represents planar structure, which electrodes made of superconducting material with current leads are located on top of normal metal with option of superconductive correlations induction from the area of normal metal under electrode to weak-link area, at that layer of ferromagnetic material is connected to normal metal to induce superconducting correlations in it directly from area of normal metal. ^ EFFECT: increase of critical current amplitude at junction, more effective control of critical current value in case of availability of two ferromagnetic sublayers with different directions of magnetisation at simpler design of planar multilayer structures. ^ 15 cl, 11 dwg

Description

The invention relates to cryoelectronic devices and can be used in measuring equipment, radio engineering and information systems operating at low temperatures.

A large number of designs of superconducting devices based on the Josephson effect (hereinafter - SPD) that are promising for use in various low-current superconducting devices (fast single-quantum logic devices, transmitting and receiving devices, magnetometric devices) are described. Traditionally known SPDs are a multilayer thin-film structure formed on a dielectric substrate, including a superconductor, insulating, barrier, and functional layers. Depending on the purpose and design, the choice of substrate materials and the active media themselves is carried out.

For example, an SPD is described, which is formed on a single-crystal dielectric substrate and has three layers: two layers of YBA ₂ Cu ₃ O _7-x superconductor (YBCO), one of which is the lower one, placed directly on the substrate, separated by a barrier layer (US 6541789, Sato , et al. 04/01/2003). A weak bond is formed at the end of one of the superconductors. Also known SPD (JP 3190175, YUZURIHARA et al., 08/20/1991), which is a device with four current leads, in which the current, set through one of the pairs of current leads, transfers to the ferromagnetic state the film from the antiferromagnetic substance inside the device that is not in areas of Josephson contact. The magnetic moment arising in this case creates a magnetic field, which leads to the suppression of the critical current of the Josephson element located between two other current leads of the device and to the generation of a voltage pulse on it.

Known SPD, designed to control the flow of electrons and having a multilayer structure "superconductor S - normal metal N - superconductor S" and not using dielectric barrier layers (US 6995390, Tsukui, 02/07/2006). Another invention describes an SPD designed to control the critical current of five-layer double-barrier Josephson junctions in which the material inside the barriers contains a ferromagnetic film. Its purpose is to provide Zeeman splitting of the resonant levels of electrons in the intra-barrier region. This is necessary to control the critical current of the structure by controlling the position of the split levels relative to the Fermi energy of the electrodes by the voltage applied to the additional control contacts of the structure (US 6344659, Ivanov et al., 02/05/2002).

A SPD device at the Josephson junction is known (US 20090233798 A1, Maeda, September 17, 2009), which consists of a superconducting layer placed on a substrate over which a layer of ferromagnetic material is deposited, which can be electrically conductive and applied to the insulating layer. However, this device has a small scale of penetration of superconducting correlations and it does not provide the ability to control the critical current through an external magnetic field.

Known SPD (RU 2343591 C1, Karminskaya et al., January 10, 2009), in which a weak-bonding region is formed in the form of a multilayer thin-film FNF structure bonded to electrodes from superconductor S. The thin-film structure is made of layers F of ferromagnetic material with directions magnetizations lying in the plane of the structure, between which a layer N of a normal metal is placed. The layers of the ferromagnetic material are arranged to rotate the magnetization vectors relative to each other in the plane of the layered structure from the antiferromagnetic to the ferromagnetic state. Electrodes from the superconductor are attached to the opposite side faces of the FNF structure, which provides more efficient control of the critical current of the SPD through an external magnetic field due to the organization of a number of independent channels of its flow.

In another invention (RU 2373610 C1, Karminskaya et al., November 20, 2009 — closest analogue), the SPD includes a weak-bonding region in the form of a thin-film “FNF” layered structure formed on a substrate and superconductor electrodes S attached to opposite side faces of the layered structure. The layers F are made with the possibility of reversal of the indicated directions of magnetization relative to each other with the provision of generation of the triplet type of superconducting pairing in the field of weak coupling. The above allows controlling the critical current of the SPD through an external magnetic field due to the organization of a number of independent channels of its flow, as well as due to the provision of conditions for the generation of a weakly damped triplet type of superconducting pairing.

However, in this device, the end geometry of the Josephson junctions does not allow reaching substantially large critical currents and does not lead to a single 0-pi transition with a change in the length of the injection region of Cooper pairs.

A superconducting device with a Josephson junction includes a weak-bonding region formed on a substrate in the form of a thin-film layered structure containing layers of a ferromagnetic material and a normal metal, and two electrodes of a superconducting material with current leads to connect the weak-bonding region to a current source and is a planar structure in which electrodes made of a superconducting material with current leads are placed on top of a normal metal layer with the possibility of pointing superconducting correlations of normal metal region tions under the electrode in the weak communication area, wherein the layer of ferromagnetic material attached to a normal metal, with guidance therein correlations directly from the superconducting normal metal layers.

The device can be characterized in that a layer of ferromagnetic material is deposited on a substrate, and a layer of normal metal is placed on top of it, while the areas of said layers coincide.

The device can also be characterized by the fact that a layer of normal metal is deposited on a substrate, and a layer of ferromagnetic material is deposited on the surface of said layer of normal metal between electrodes of a superconducting material.

The device can also be characterized in that a layer of ferromagnetic material is placed between the normal metal layer and the substrate in the regions under the electrodes of the superconducting material.

The device can also be characterized by the fact that the layer of ferromagnetic material is formed by the first and second sublayers of the ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure to ensure the generation of the triplet type of superconducting pairing in the weak coupling region, while the first sublayer a layer of ferromagnetic material is deposited on a substrate, a layer of normal metal is placed on top of it, and a second sublayer of a ferromagnetic material Ala is placed on the surface of a normal metal layer between electrodes of a superconducting material.

The device can be characterized, in addition, by the fact that the layer of ferromagnetic material is formed by the first and second sublayers of the ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure with the generation of a triplet type of superconducting pairing in the weak coupling region, while the first sublayer of the ferromagnetic material is placed on the surface of the normal metal layer between the electrodes of the superconducting material, and the second second sublayer of ferromagnetic material layer disposed on the substrate at a first sublayer on the other side of the normal metal layer, wherein the area of the sublayers of ferromagnetic material coincide.

The device can also be characterized by the fact that the layer of ferromagnetic material is formed by the first and second sublayers of ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure, ensuring the generation of a triplet type of superconducting pairing in the weak coupling region, while the normal metal layer placed on a substrate, the first and second sublayers are deposited one on top of the other and placed between the electrodes of the superconducting material n the surface layer of normal metal.

The device can also be characterized by the fact that the angle of rotation of the magnetization vectors is in the range of values at which the maximum magnitude of the superconducting current is achieved, as well as the fact that the sublayers of the ferromagnetic material have different values of the coercive field.

The device can be characterized by the fact that niobium or an alloy based on it is used as a superconductor, and also by the fact that a rare-earth cuprate compound of the general formula ReBa ₂ Cu ₃ O _{7-x is} used as a superconductor, where Re is a rare-earth metal, and as The ferromagnetic material used is Ni, Co, Fe or metal alloys based on them.

The device can also be characterized by the fact that an element from the group Cu, Au, Al, Pt is used as a normal metal, the layer thickness of the ferromagnetic material is 10-100 nm, and the layer thickness of the normal metal is 10-100 nm.

The technical result of the invention consists in increasing the amplitude of the critical current of the transition compared to previous geometries and for certain values of the contact length of the superconductor – normal metal, as well as the occurrence of a single 0-pi transition with a change in the length of the superconductor – normal metal region. An additional technical result consists in the possibility of more efficient control of the critical current in the case of the presence of two ferromagnetic layers in the structure with different directions of magnetization. In addition, it is proposed to use a simpler technology of planar multilayer structures in comparison with the end transition described in the prototype.

The invention is illustrated in the figures, where:

1-3 show the structure of the SPD with a planar arrangement of layers of normal metal and ferromagnetic material;

4-6 - the same, but with two layers of ferromagnetic material;

7 is a dependence of the absolute value of the normalized values of the critical currents of patented structures as a function of the distance between the electrodes;

Fig.8 is a phase diagram for the structure shown in Fig.3, with several values of the thickness of the ferromagnetic layer;

Fig.9 is a phase diagram for SN-FN-NS structure shown in Fig.2;

figure 10 is a phase diagram for SNF-FN-FNS structure shown in figure 1;

11 - dependence of the amplitude of the critical current on the normalized length d SN border for patentable structures.

The superconducting device (see Figs. 1-3) is formed on the substrate 1 and contains a multilayer thin-film structure consisting of layer 2 of ferromagnetic material F, layer 3 of normal metal N, electrodes 4 of superconducting material S of size d with current leads 5 for connecting a weak region connection to the current source. The thicknesses of layers 2 of a ferromagnetic material (Ni, Co, Fe or metal alloys based on them) and a normal metal, which can be used as an element from the group of Cu, Au, Al, Pt, are 10-100 nm. As electrodes 4 of a superconducting material, for example, niobium, vanadium, indium, tin, lead, or an alloy based on these metals, a compound of rare-earth cuprates of the general formula ReBa ₂ Cu ₃ O _7-x , where Re is a rare-earth metal, can be used.

Figure 2-3 shows the placement of a layer of ferromagnetic material. The layer 21 of the ferromagnetic material can be placed between the electrodes 4 of the superconducting material on the free surface of the normal metal layer 3. The layers 22 and 23 of the ferromagnetic material can be placed between the normal metal layer 3 and the substrate 1 in the regions under the electrodes 4 of the superconducting material.

Figure 4-6 shows embodiments of a ferromagnetic material from two sublayers F1, F2 of a ferromagnetic material with thicknesses d _F1 and d _F2 , respectively. The sublayers are made interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure, with the provision of the generation of the triplet type of superconducting pairing in the weak coupling region.

Sublayers of ferromagnetic material are single-domain, their manufacturing technology is known. The sublayers 24, 25 (FIG. 4), 24, 26 (FIG. 5) and 24, 27 (FIG. 6) of the ferromagnetic material must have different coercive fields, which allows the magnetization directions in these layers to be reversed relative to each other. This can be achieved, for example, by performing these layers with slightly different thicknesses (~ 30%) or film width, as well as by choosing the substrate material 1 or normal metal in layer 3.

The first sublayer 24 of the ferromagnetic material (Fig. 4) is placed on the free surface of the normal metal layer 3 between the electrodes 4 of the superconducting material, and the second sublayer 25 is between the normal metal layer 3 of thickness d _N and the substrate 1.

In another embodiment (FIG. 5), the second sublayer 26 of the ferromagnetic material has a size (area) of the first sublayer 24 and is placed below it.

Figure 6 shows the design when the first 24 and second sublayers 27 are deposited one on top of the other and placed on the free surface of the normal metal layer 3 between the electrodes 4 of the superconducting material S. The thickness of the upper ferromagnetic film F can be arbitrary, the thickness of the lower should not exceed the coherence lengths in a normal metal in order to ensure a continuous low value of triplet superconducting correlations at the boundary of ferromagnetic sublayers 24, 27.

The device operates as follows. When current is supplied through current leads 5 to electrodes 4 of a superconducting material, superconducting current is induced in a normal metal layer located directly below the superconducting electrode. Then, from the normal metal region located under the superconducting electrode, superconducting correlations (superconducting states) are induced in the complex multilayer weak-coupling region.

If the layer of ferromagnetic material is formed by the first and second sublayers of ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure to ensure the generation of the triplet type of superconducting pairing in the weak coupling region, the proposed geometry, in which the superconducting electrode is on top of the normal layer metal, allows to increase the efficiency of control of the critical current value by turning the vecto s magnetization and generate a triplet structure correlations by increasing its amplitude.

Figure 7 shows the dependence of the absolute value of the normalized values of the critical currents of the structures: SNF-FN-FNS (figure 1), SN-FN-NS (figure 2), SNF-N-FNS (figure 3), as a function of distance between the electrodes. By the value of critical currents, far from the points of 0-pi junctions, the structures are arranged in the following order: SN-FN-NS, SNF-N-FNS and SNF-FN-FNS. This order is due to a sequential increase in the suppression of superconductivity from the F side of the film. In SN-FN-NS structures, superconducting correlations are suppressed by the F film only in the weak-binding region, thus providing large critical currents. In SNF-N-FNS structures (Fig. 3), the critical current is less than in SN-FN-NS structures due to suppression of superconductivity in the region under the electrodes. In SNF-FN-FNS structures, there is suppression of superconductivity in all parts of the structure. The physical reason for this fact is that a small thickness of 10-100 nm of a normal metal film leads to an effective increase in superconductivity at the boundary of the complex SN electrode and the weak coupling region compared with the magnitude of superconducting correlations at the SN boundary for the end geometry of superconducting structures described in the nearest analogue (RU 2373610). In the SNF-N-FNS structure, the critical current for these parameters has a negative value and decays with increasing distance between the electrodes without oscillations.

The coherence length FN of the SN-FN-NS and SNF-FN-NFS structures is complex, which leads to decaying oscillations of the critical current as a function of the distance L between the superconducting electrodes 4. The period of these oscillations and their attenuation scale are the same for all these structures and are caused only by properties FN areas of weak coupling. However, the initial conditions for these oscillations at the boundary with the weak coupling region are different, which causes a shift in these oscillations along the axis of the structure.

On phase (d, L) diagrams, the lines indicate the zero position of the critical current. These lines separate regions with a negative and a positive sign of the critical current (0 and pi states).

Fig. 8 shows a phase diagram for the SNF-N-FNS structure (Fig. 3) at several thicknesses d _{F of the} layer 22, 23 of the ferromagnetic material. For such a geometry of the structure, in the region of weak coupling between the complex SNF electrodes 4, there is only a normal metal film 3. The coherence length in the film 3 of a normal metal is a real value; therefore, for such a structure, the critical current does not undergo oscillations. However, calculations show that a single 0-pi transition is possible in such a structure. The existence of only a single transition can be explained as follows. Oscillations of the condensate function exist only under the complex SNF electrode; therefore, the sign of the critical current is due to the value of the condensate function at the interface between the normal film and the complex electrode. Since the value of the condensate function is determined by two complex coefficients, they can lead only to two possible regions of the sign of the critical current, i.e. to one line in the phase diagram. Depending on the structure parameters, with increasing distance L between the superconducting electrodes and with increasing length d SN of the boundary, the critical current may change its sign once or not at all.

As can be seen from Fig. 8, the position of the contour line nonmonotonically depends on the thickness of the ferromagnetic film. At zero film thickness d _F , only the state with a positive sign of the critical current for any other parameters is realized in the structure. As the thickness d _{F of the} ferromagnetic film increases, the curve first shifts to the lower left corner of the phase diagram, and then turns back and, at a certain critical value of the thickness of the ferromagnetic film, goes to infinity. Thus, only the state with a positive value of the critical current begins to be realized in the structure with a further increase in the thickness of the ferromagnetic layer.

FIG. 9 is a phase diagram for the SN-FN-NS structure shown in FIG. 2. Since there is no ferromagnetic film in the complex electrode (items 4 and 3), therefore, 0-pi transitions are realized due to the complex coherence length in the weak-binding region NF, which is attached to complex electrodes. Therefore, multiple 0-pi transitions are observed in the structure with increasing distance L between electrodes 4 and a single 0-pi transition with increasing length d of the SN boundary. Depending on the length of the SN boundary, the contour lines look almost vertical, thus demonstrating a weak dependence of the position of the 0-pi transition on the length d of the SN boundary.

FIG. 10 is a phase diagram for an SNF-FN-FNS structure (FIG. 1). In these structures, the coherence length is complex both in the region of complex electrodes of size d and in the region of weak coupling on the length L. The manifestation of 0-pi transitions in this case strongly depends on the crosslinking of the oscillations at the SNF and NF boundaries. As a result of this, the contour lines are not as vertical as in the previous case, but exhibit a strong dependence on the length d SN of the boundary.

11 shows the dependence of the critical current amplitude on the normalized length d / ξ _N (where ξ _N is the penetration length of the superconducting state from the superconductor into the normal metal) SN boundaries for patented structures: (a) for the SNF-FN-FNS structure shown in figure 1; (b) for the structure of SN-FN-NS - figure 2; (c) for the structure of SNF-N-FNS - figure 3. The graph shows that with an increase in the length d SN of the boundary, the critical current can change sign. The magnitude of the critical current has a maximum optionally at infinitely large d. So, for SNF-N-NFS structures with d / ξ _N = 0.5, the critical current is 50 times larger than at infinity. The above makes it possible in the patented structure to obtain a larger critical current value with a smaller area of the SN boundary.

From the above data it can be seen that the invention allows to achieve significantly higher critical current values, which, however, does not lead to a single 0-pi transition with a change in the length of the injection region of Cooper pairs.

Industrial applicability. As elements suitable for implementing the patented device, materials used in the technology of cryoelectronic materials and known to specialists can be used. For example, any standard substrates (for example, silicon, sapphire, etc.) can be used as substrate 1. As the ferromagnetic materials of the layers, pure ferromagnets Ni, Co, Fe or ferromagnetic alloys based on them: Pt _x Fe _1-x , Pt _x Ni _1-x , Pt _x Co _1-x , Pd _x Fe _1-x , Pd _x Ni _1-x , Pd _x Co _1-x , Cu _x Ni _1-x , Nd _x Ni _1-x ; as normal metal - Cu, Au, Ag, Al, Pt. As a material for superconducting electrodes 4 — niobium, niobium nitride, vanadium, indium, tin, lead or MgB ₂ and compounds based on it, or high-temperature superconductors based on rare-earth cuprates of the general formula ReBa ₂ Cu ₃ O _7-x , where Re is rare earth metal, or other oxides (see, for example, US 6011981, Alvarez et al., 04.01.2000), the technology of applying layers on substrates is known. Typical layer thicknesses of ferromagnetic material and normal metal for the patented topology are 10-100 nm and are in the range technologically feasible for thin-film electronics.

Claims (15)

1. A superconducting device with a Josephson junction, including a weak-bonding region formed on a substrate in the form of a thin-film layered structure containing layers of a ferromagnetic material and a normal metal, and two electrodes of a superconducting material with current leads for connecting the weak-bonding region to a current source, characterized in that
represents a planar structure in which electrodes of a superconducting material with current leads are placed on top of a normal metal layer with the possibility of inducing superconducting correlations from the normal metal region under the electrode to a weakly bonded region, while the layer of ferromagnetic material is attached to the normal metal with the possibility of superconducting correlations in it directly from a normal metal layer. 2. The device according to claim 1, characterized in that the layer of ferromagnetic material is deposited on the substrate, and a layer of normal metal is placed on top of it, while the areas of said layers coincide. 3. The device according to claim 1, characterized in that the layer of normal metal is deposited on the substrate, and the layer of ferromagnetic material is deposited on the surface of said layer of normal metal between the electrodes of the superconducting material. 4. The device according to claim 1, characterized in that the layer of ferromagnetic material is placed between the normal metal layer and the substrate in the areas under the electrodes of the superconducting material. 5. The device according to claim 1, characterized in that the layer of ferromagnetic material is formed by the first and second sublayers of ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure with the generation of a triplet type of superconducting pairing in the field of weak coupling, the first sublayer of the ferromagnetic material is deposited on the substrate, a layer of normal metal is placed on top of it, and the second sublayer of the ferromagnetic material is placed on overhnosti normal metal layer of a superconducting material between the electrodes. 6. The device according to claim 1, characterized in that the layer of ferromagnetic material is formed by the first and second sublayers of ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure with the generation of a triplet type of superconducting pairing in the field of weak coupling, the first sublayer of the ferromagnetic material is placed on the surface of the normal metal layer between the electrodes of the superconducting material, and the second sublayer is fer of the magnetic material is placed on the substrate under the first layer on the other side of the normal metal layer, while the areas of the sublayers of the ferromagnetic material coincide. 7. The device according to claim 1, characterized in that the layer of ferromagnetic material is formed by the first and second sublayers of the ferromagnetic material, interacting with each other with the possibility of rotation of the magnetization vectors relative to each other in the plane of the layered structure with the generation of a triplet type of superconducting pairing in the field of weak coupling, the normal metal layer is placed on the substrate, the first and second sublayers of the ferromagnetic material are deposited one on top of the other and placed between the electrodes from superconducting nicks material on the surface layer of normal metal. 8. The device according to claim 1, characterized in that the angle of rotation of the magnetization vectors is in the range of values at which the maximum magnitude of the superconducting current module is achieved. 9. The device according to any one of paragraphs.5-7, characterized in that the sublayers of the ferromagnetic material have different values of the coercive field. 10. The device according to claim 1, characterized in that niobium or an alloy based on it is used as a superconductor. 11. The device according to claim 1, characterized in that the compound of rare-earth cuprates of the general formula ReBa ₂ Cu ₃ O _7-x , where Re is a rare-earth metal, is used as a superconductor. 12. The device according to claim 1, characterized in that Ni, Co, Fe or metal alloys based on them are used as the ferromagnetic material. 13. The device according to claim 1, characterized in that an element from the group Cu, Au, Al, Pt is used as a normal metal. 14. The device according to claim 6, characterized in that the thickness of the layer of ferromagnetic material is 10-100 nm. 15. The device according to claim 7, characterized in that the thickness of the normal metal layer is 10-100 nm.

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