RU2544275C2 - Shf-amplifier based on high-temperature squid with four josephson contacts - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: method includes identical and paralleled first and second Josephson contacts formed in a layer of high-temperature superconductor (HTSC) and placed along bicrystal boundary of the layer and an input inductive element coupled between adjoining current leads of Josephson contacts. There are auxiliary third and fourth Josephson contacts, at that critical current values of the first and second Josephson contacts coincide, the same value of the third Josephson contact is less and the same value of the fourth Josephson contact is more. HTSC layer is shaped as a path that crosses the bicrystal boundary twice and forms a closed circuit with the above inductive element placed at one side of the bicrystal boundary. The third and fourth Josephson contacts are placed at cross points of the above path with bicrystal boundary, and width of the path at the site of the fourth contact placement exceeds the width for placement of the third one.

EFFECT: increasing amplification linearity in gigahertz band of frequencies without use of feedback circuits SHF-amplifier based on high-temperature SQUID.

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Description

Technical field

The invention relates to cryogenic electronics and can be used in measurement technology, radio engineering and information systems operating at low temperatures.

State of the art

Radio frequency amplifiers based on superconducting quantum interferometers (SQUIDs) have high sensitivity and low noise temperature, are compatible with other superconducting devices. SQUID is a superconducting ring containing one or two Josephson junctions; it has matching and measuring devices.

Various designs and circuitry solutions for microwave superconductors are described. In particular, a schematic of a matched SQUID amplifier with a load for the range up to 0.1 GHz is described (JPS60247311, Noguchi, December 7, 1985). However, both the operating frequency range and the linearity of conversion for amplifiers on a single low-temperature SQUID are insufficient for modern applications.

There are two problems that make it difficult to switch to higher frequencies of 1–10 GHz signals while maintaining high gain and noise temperature characteristics characteristic of superconducting devices. Firstly, SQUID amplifiers are a special type of parametric amplifiers in which the signal power at its frequency F _S is amplified by converting the signal to the frequency F _S + F _J , where F _J is the frequency of the Josephson generation, and then converting down again to signal frequency. Based on the Manly-Rowe ratios, the power gain, G, of such an amplifier does not exceed:

$$G\approx F_J/F_S.(\mathrm{one})$$

Therefore, the Josephson SQUID junctions must have a high characteristic voltage - V _C , so that the characteristic frequency of the Josephson generation

$$F_C=V_C/{\mathrm{<figure-callout\; id="0"\; label="F"\; filenames="00000006.png"\; state="\{\{state\}\}">F</figure-callout>}}_0,(2)$$

where Ф ₀ - magnetic flux quantum, equal to 2.07 × 10 ^-15 VB, was several orders of magnitude higher than the signal frequency.

For SQUIDs based on Nb Josephson junctions, typical V _C values do not exceed 100-200 μV, respectively, F _C values do not exceed 50-100 GHz. Typical values of F _J at the operating point are approximately an order of magnitude smaller than F _C and, as can be seen from formula (1), the gain disappears for signals with a frequency of the order of 10 GHz. The noise characteristics of the amplifier naturally worsen.

The second problem for microwave amplifiers based on classical SQUIDs with a multi-turn input coil in the frequency range above 0.01 GHz is the nonlinear response to an external magnetic field and the inability to linearize the response using traditional feedback systems at such high frequencies, which leads to a low gain linearity level.

The solution to the first problem - increasing the characteristic voltage V _C , lies in the use of high temperature superconducting (HTSC) Josephson junctions in SQUIDs, which demonstrate V _C values in the millivolt range and can provide significant amplification at frequencies up to 10 GHz. Also, to solve the first problem, multi-element Josephson structures can be used, including those consisting of N series SQUIDs connected in series or in parallel. The addition of the responses from the SQUIDs that make up such a chain makes it possible to achieve the required gain level.

The invention (US 7095227, Tarutani et. Al., 08/22/2006) describes an amplifier based on a sequential SQUID chain, which makes it possible to use a direct current source and occupying a small area as a power source. However, such a device allows to achieve only a relatively small linear gain of the signal.

Amplifiers-drivers for the GHz range (up to tens of GHz) based on chains of SQUIDs separated into pairs isolated from the ground were proposed to reduce spurious capacitances in the system (US 6486756, Tarutani, 11.26.2002), however, such amplifiers are suitable only for digital applications and cannot be used to amplify an analog signal. An example of an amplifier of signals of the BOC logic, which converts them at the output into voltage pulses with a value sufficient to use semiconductor electronics, is an invention (US 6917216, Herr Quentin, 10/14/2004), which uses the separation and re-reflection of the output of the BOC pulse to obtain sufficient output values voltage pulses. But this offer is again designed exclusively for use in digital devices.

To create a current amplifier with large output currents, it was proposed to use a set of parallel-connected DC SQUIDs (JP 2003209299, MOROOKA et al., July 25, 2003). However, this proposal is not designed to operate in the gigahertz range.

It was suggested that a chain of SQUIDs of variable area be used as a highly sensitive magnetometer (WO 01/25805, Schopohl et al., April 12, 2001; US 7369093, Oppenlander et al., May 6, 2008), however, the described structures do not provide the necessary linear signal amplification. They do not provide sufficient linearity in the conversion of the input magnetic signal to the output voltage and heterogeneous parallel contact chains (US 7369093), which do not allow anything else to achieve significant values of the output signal.

A known superconducting broadband microwave amplifier containing a series of two-contact SQUIDs connected to input and output superconducting lines is associated with means for setting the operating current and magnetic field modes (JPH10028021, Takeda et al., January 27, 1998). The design of a microwave amplifier on linear DC SQUID chains is described, which has an increased gain and high linearity in the frequency band 1-10 GHz (RU 2353051, Kornev et al., 06.06.2007). However, the implementation of the required SQUID chains based on HTSC has not been technologically solved, which limits the scope of promising applications of the amplifier.

A structure with three Josephson contacts is described (US 8179133, Kornev et al, 05.15.2012; S. Berggren, G. Prokopenko et al, "Development of 2D Bi-SQUID Arrays with High Linearity", IEEE Transactions on Applied Superconductivity, Vol.23 , Iss. 3, p. 1400208, 2013). In this element, it is proposed to include a third Josephson junction, which is always in the superconducting state and plays the role of nonlinear inductance, in parallel with the main inductance of the two-contact squid. Connecting the Josephson junction parallel to the inductance of SQUID forms a double SQUID, hereinafter referred to as bi-SQUID. Such a cell makes it possible to achieve a sufficiently high linearity of the conversion of the input signal to the output voltage of the amplifier, however, in this case, it is not possible to implement the proposed structure on the basis of high-temperature bicrystalline Josephson contacts.

Closest to the patented amplifier is a superconducting microwave amplifier at high temperature SQUID (RU 2325004, Kalabukhov, Snigirev, 05.20.2008 - prototype). The microwave amplifier contains a two-contact quantum interferometer based on a layered structure (HTSC) / insulator / normal metal formed on an insulating bicrystalline substrate, connected to the input and output transmission lines, electric matching means. The transmission lines are made in the form of coplanar lines installed coaxially with the interferometer and having an extended central electrode formed in the HTSC layer on the substrate surface and placed with a gap on both sides of the central electrode at least one pair of external electrodes formed in the normal metal layer, performing function of capacitor plates of means of electrical matching. However, the conversion of the magnetic signal into voltage in the interferometer of such an amplifier is still non-linear.

Disclosure of invention

The subject of the present invention is the construction of a microwave amplifier based on a high-temperature direct current SQUID having an increased linearity in converting the applied magnetic signal to a voltage response.

A patented microwave amplifier based on a high-temperature SQUID includes parallel-connected first and second Josephson junctions formed in a loop in the form of a high-temperature superconductor (HTSC) and placed along the bicrystal interface of the substrate, and an input inductive element connected between adjacent current leads of the Josephson junctions.

The difference is that the third and fourth Josephson contacts are additionally introduced, with the critical current of the first and second Josephson contacts coinciding, the third being less than the critical current of the first and second transitions, and the fourth contact exceed this value.

The loop in the HTSC layer is closed by a path that crosses the bicrystal boundary twice and forms a closed loop with the mentioned inductive element located on one side of the bicrystal boundary.

The third and fourth Josephson contacts are located at the intersections of the aforementioned path with a bicrystal boundary, and the width of the path at the location of the fourth Josephson contact exceeds that of the third Josephson contact.

The amplifier can be characterized in that the ratio of the critical current of the fourth Josephson contact to the critical current of the remaining Josephson contacts is more than 4.

The amplifier can be characterized by the fact that the bicrystal substrate is made of sapphire with a misorientation angle of 24 °, and also because the HTSC layer is a compound of the general formula YBa ₂ Cu ₃ O _7-x and is formed on a CeO ₂ sublayer.

The technical result of the invention is to provide increased linearity of the response without the use of feedback circuits in the gigahertz frequency range.

The invention is illustrated by drawings.

Brief Description of the Drawings

Figure 1 presents a block diagram of a microwave amplifier;

Figure 2 presents a schematic diagram of a two-contact SQUID with an additional two Josephson contacts;

Figure 3 shows a schematic topology of a HTSC layer on an insulating bicrystal substrate with two identical Josephson contacts and an additional nonlinear Josephson bypass of the inductive element;

Figure 4 presents the calculated form of the current-voltage dependence of bi-SQUID with four Josephson contacts in comparison with the voltage-current dependence of a two-contact SQUID;

Figure 5 presents the voltage responses of the bi-SQUIDs connected in series in the case of a nominal critical current I _{C of} Josephson junctions and in the case of a ± 2% deviation of I _C from the nominal value.

The implementation of the invention

The invention is based on the optimization of the design of a device for a superconducting high-temperature microwave amplifier at SQUID: two Josephson contacts in series, which are always in the superconducting state and play the role of nonlinear inductance, are connected in parallel with the main inductance of a two-contact SQUID. The introduction of an additional nonlinear shunting of the inductive element by means of Josephson junctions provides a nonlinear transformation of the input magnetic signal into the Josephson phase of the two-contact interferometer, which provides, with proper selection of parameters, a linear dependence of the output voltage on the magnitude of the applied flux.

A microwave amplifier based on a high-temperature SQUID (Fig. 1, 2) contains an input line 1, bi-SQUID 2 with four Josephson contacts 21, 22, 23 and 24. The input line 1 is characterized by an internal impedance Z _i (pos. 3) and inductive element 4, which sets the magnetic flux Ф through the inductive element of bi-SQUID 2. The circuit includes a line 5 for setting the operating point of bi-SQUID, and the magnitude of the current flowing through this line is equal to I _B , and the output line 6.

Bi-SQUID with four Josephson junctions represents (Fig. 3) a structure placed on a sapphire bicrystalline substrate 7 with a bicrystalline boundary 8. The crystallophysical parameters of the substrate and the technology for the formation of a layered structure (symbolically designated as pos. 9) are not given in this description and are known from techniques (see, e.g., E. Stepantsov et al., THz Josephson properties of grain boundary YBaCuO junctions on symmetric, tilted bicrystal sapphire substrates, J. Appl. Phys., v. 96, No. 6, pp.3357-3361, 2004).

A microwave amplifier based on high-temperature bi-SQUID 2 includes identical and parallel-connected first 21 and second 22 Josephson contacts formed in a loop-shaped layer 10 of a high-temperature superconductor (HTSC) and placed along the bicrystal boundary 8 of substrate 7.

High temperature bi-SQUID 2 contains an inductive element 11 connected between adjacent current leads of Josephson contacts 21 and 22, which is also formed in layer 10. The third 23 and fourth 24 Josephson contacts are introduced, and the critical current of the first and second contacts 21 and 22 coincides, the third contact - less than this value, and the fourth contact - exceeds this value, i.e. I _{C (24)} > I _{C (21.22)} > I _{C (23)} .

The loop-shaped HTSC layer 10 is connected to a track 12, which crosses the bicrystal boundary 8 twice and forms a closed loop with an inductive element 11 located on one side 13 of the bicrystalline boundary 8. On the other side 14 of the boundary 8, parts 15, 16 of the HTSC layer are located.

The third and fourth Josephson contacts 23, 24 are located at the intersections of the aforementioned track 12 with a bicrystal boundary 8. The width b of the track 12 at the location 17 of the fourth Josephson contact 24 is greater than the a of the same name for the third Josephson contact, so that a <0.2b ( below, the value of a is limited by the capabilities of modern technology). This is necessary in order to ensure the proper form of conversion of the input magnetic signal into the Josephson phase of the two-contact interferometer and the mentioned Josephson phase of the interferometer into the output voltage response.

The ratio of the critical current of the Josephson junction 24 to the critical current of the Josephson junction 21, 22 is more than four. This condition is selected for reasons of optimizing the linearity of the voltage response and its amplitude. The ratio of the normalized inductance l of the inductive element 11 to the normalized inductance of the track 12 is more than 10. The principles for selecting the optimal value l are not given in this description and are known from the prior art (see, for example, Kornev V., Soloviev I., Klenov N., Mukhanov O ., "Bi-SQUID: a novel linearization method for dc SQUID voltage response", Supercond. Sci. Technol., Vol.22, p. 114141, 2009). The ratio of the critical current of the Josephson junction 23 to the critical current of the Josephson junction 21, 22 can be in the range 0.5-1, i.e. I _{C (23)} = (0.5-1) I _{C (21.22)} and is selected so as to ensure the proper type of conversion of the input magnetic signal into the Josephson phase of the two-contact interferometer and the mentioned Josephson phase of the interferometer into the output voltage response.

The structure of the highest-temperature bi-SQUID 2 itself has two layers: on the entire plane, the substrate 7 is coated with a sublayer of cerium oxide CeO ₂ , on top of which a HTSC film - YBa ₂ Cu ₃ O _7-x compound is deposited without breaking the vacuum. Then, if necessary, to form input and output lines, an insulator layer, SiO ₂ , is deposited on a substrate with lithography formed in CeO ₂ / YBCO layers, and part of the matching elements are formed in it by lithography. Then a layer of gold is applied in which the remaining elements of the microwave amplifier are formed. The film thicknesses are: CeO ₂ sublayer - 30 nm, HTSC layer (YBCO) - 250 nm, SiO ₂ layer - 400 nm, gold layer - 200 nm. Bicrystalline sapphire substrates with a misorientation angle of 24 ° provide a characteristic Josephson voltage V _{C of} 0.2 mV at 77 K (A. Kalabukhov, et al., "Design, fabrication and experimental investigation of high-Tc dc SQUID RF amplifier with a slot-line input circuit "- in Extended Abstract of International Superconductive Electronics Conference (ISEC'05), Noordwijkerhout, The Netherlands, September 5-9, 2005, Report PH 23).

Figure 4 presents the calculated form of the current-voltage dependence of bi-SQUID with four Josephson contacts in comparison with the voltage-current dependence of a two-contact SQUID. A comparison of the presented results shows that when the normalized inductance is l = 0.5 of a two-pin interferometer and the ratio of the critical current I _{C of} contact 24 to the critical current of I _{C of} contacts 21 (22) is 4, the response of voltage V bi-SQUID to the magnetic signal Ф significantly linearized, ensuring the achievement of a technical result. To achieve a technical result, it is also necessary that the ratio of the normalized inductance l of the inductive element 11 to the normalized inductance of the track 12 be more than 10.

Figure 5 presents the responses of the voltage V of the bi-SQUIDs connected in series in the case of the nominal value of the critical current I _{C of} Josephson contacts and in the case of a ± 2% deviation of the critical current from the nominal value. It can be seen that the deviation of the critical current from the nominal value leads mainly to a change in the amplitude of the responses, while practically not changing the linearity and shape of the responses.

Thus, the data presented indicate the achievement of a technical result in the gigahertz frequency range in terms of increasing the linearity of the response without the use of feedback circuits.

Claims (5)

1. A microwave amplifier based on a high-temperature SQUID, including parallel-connected first and second Josephson contacts formed in a layer of a high-temperature superconductor (HTSC) in the form of a loop and placed along the bicrystal boundary of the substrate, and an input inductive element connected between adjacent current leads of the said Josephson contacts
characterized in that
additionally introduced the third and fourth Josephson contacts, the critical current of the first and second Josephson contacts coincides, the third is less than the critical current of the first and second transitions, and the fourth contact exceeds this value,
the loop in the HTSC layer is closed by a path that crosses the bicrystal boundary twice and forms a closed loop with the mentioned inductive element located on one side of the bicrystal boundary, while
the third and fourth Josephson contacts are located at the intersections of the mentioned track with a bicrystal boundary on both sides of the first and second Josephson contacts, and the width of the track at the location of the fourth Josephson contact is greater than that of the third Josephson contact. 2. The microwave amplifier according to claim 1, characterized in that the width of the track at the location of the fourth Josephson junction exceeds that of the third Josephson junction. 3. The microwave amplifier according to claim 1, characterized in that the ratio of the critical current of said fourth Josephson contact to the critical current of the remaining Josephson contacts is more than 4. 4. The microwave amplifier according to claim 1, characterized in that the bicrystal substrate is made of sapphire with a misorientation angle of 24 °. 5. The microwave amplifier according to claim 1, characterized in that the HTSC layer is a compound of the general formula YBa ₂ Cu ₃ O _7-x and is formed on a CeO ₂ sublayer.

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