APPLICATION of HIGH-Tc SUPERCONDUCTING JOSEPHSON JUNCTION DEVICES

APPLICATION OF HIGH-Tc SUPERCONDUCTING JOSEPHSON JUNCTION DEVICES

Shailaj Kumar Shrivastava1 and Girijesh Kumar2 1 Professor (Associate), P.G. Dept of Physics A.N.S. College, Barh, Patliputra University, Patna, Bihar 2 Professor (Assistant), Department of Physics, M.S.Y. College, Mirzapur, Karpi, Jahanabad, Bihar

Abstract: The application of superconducting devices based on Josephson junction has been investigated for many years. Josephson junction is based on quantum mechanical tunneling of electrons between weakly coupled two superconducting regions. Its unique properties make it a building block for future superconducting electronic circuits. In this paper, attempt has been made to highlight the wide range of application of Josephson junction including Josephson voltage standard, SQUIDs, Quantum Computer, analog to digital converter, RSFQ digital electronics, terahertz emitter and detector etc.

Index Terms: Josephson voltage Standard, RSFQ logic, Quantum Computers, SQUIDs, A/D converter.

I. INTRODUCTION

The practical applications of superconductivity are steadily improving every year. However, the actual use of superconducting devices is limited by the fact that they must be cooled to low temperatures to become superconducting. The discovery of high-Tc superconductors extends the feasible application of superconductors [1]. The Josephson effect [2] has enabled the development of unique electrical devices and systems including SQUIDs [3], quantum computers [4], analog to digital (A/D) converters [5], Josephson voltage standard [6], THz emitters and detectors [7], single flux quantum devices[8] etc. A single Josephson junction has memory and can therefore be used for information storage. Josephson junction is used in implementation of superconducting qubits which is essential building block of quantum information processing devices. Superconductive electronics based on Josephson junctions offer several advantages over conventional superconducting devices, including higher switching speed, lower power dissipation, extreme detection sensitivity and minimal signal distortion. By measurement of the magnetic field in the human body, it is possible to make non-invasive diagnosis of diseases by magneto encephalography (MEG) and magneto-cardiography (MCG) and magnetic Resonance Imaging (MRI). Josephson junctions are considered as possible candidates for fast nuclear particle detection. Josephson junctions are extremely attractive for very sensitive detection of high frequency radiation.

II. JOSEPHSON EFFECT

The Josephson effect [2] is a macroscopic quantum phenomenon of super current. To observe the Josephson effect, the superconductor is cooled until the electrons are bound to one another in pairs, called cooper pairs, due to interactions with phonons. These cooper pairs are very weakly bound fermions. Two bound fermions are a system with integer spin, so a cooper pair is a boson, and bosons can flow into the same quantum mechanical state and become a superconductor. Josephson predicted that these cooper pairs could flow across an insulating layer of Josephson junction, effectively causing a dc super current, up to a maximum value called the current Ic, without developing a voltage drop across the weak link. This is called the dc Josephson effect. The supercurrent flowing through the junction is related to the difference in the phase of two wave functions as I=IcSinϕ. Josephson also predicted that when a current greater than Ic is forced through the weak link, a dc voltage appears across the weak links. In the presence of a dc voltage V across

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© 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162) the junction, in addition to normal conducting current, an ac super current also flows across the weak links at a frequency fJ given by 2eV=hfJ, where V is the dc voltage drop. This ac super current can be frequency modulated by an applied ac voltage of frequency, f, and the current then has Fourier components at frequencies (2eV/h= nf), where n is an integer. If for a particular value of n, the super current has a dc component depending on the magnitude and phase of the ac voltage. In the current–voltage characteristics, a voltage step with a differential resistance equal to zero, therefore, occurs. The current width of the steps depends on the magnitude of the ac voltage. The current width of the nth step varies with the magnitude of ac voltage like the nth order Bessel function. This is called the ac Josephson effect. If an unbiased junction is irradiated with rf radiation, a dc voltage is generated across the junction. In case of hysteretic tunnel junction this dc voltage is quantized. This effect is called the inverse ac Josephson effect.

III. JOSEPHSON JUNCTIONS

A Josephson junction consists of two superconductors coupled by a weak link. There are two types of Josephson junctions (i) Superconductor-Insulator-Superconductor (SIS) junction and (ii) Superconductor- Normal-Superconductor (SNS) junction. SIS junctions are also known as tunneling junctions because tunneling of cooper pairs takes place from one superconductor to the other through the insulator barrier. In the case of SNS junctions there is no insulator barrier, there are only two SN interfaces. The current voltage characteristic curve of a SNS junction exhibits a negative resistance region. Taking advantage of this negative resistance region, two terminal devices based on SNS junctions may be projected for a great number of applications in superconducting electronics [9]. Combining a SNS junction with appropriate resonant circuits, it is possible to project many types of generators [10]. Two terminal devices based on SNS junctions may also be used to design electronic switches [11], mixers and detectors [12]. Signal amplification and harmonic generation may be obtained using SNS junctions with appropriate circuits [13]. Tetra hertz oscillations have also been obtained using HTS Josephson junction [14]. The I-V characteristic of a Josephson junction is extremely non-linear. This non-linear behavior has been used to fabricate very sensitive mixer and detectors of microwave and terahertz radiation. The non-linear response of the Josephson device to radiation has been used to construct the internationally accepted voltage standard which has demonstrated a precision of better than 1 part per billion.

IV. JOSEPHSON VOLTAGE STANDARD

Josephson junction standards (JVS) are employed for high precision dc voltage calibrations. When a dc voltage is applied to a Josephson junction, an oscillation of frequency fJ=2eV/h occurs at the junction. This relationship of voltage to frequency involves only fundamental constants. Since frequency can be measured with extreme accuracy, the Josephson junction has become the standard voltage measurement. These standards can reach a voltage of 10V with an uncertainty that is typically smaller than 1 part in 109 [6]. The standard volt is now defined in terms of a Josephson junction oscillation. For one microvolt applied to the junction the frequency is fJ =483.6 MHz. The standard volt is the voltage required to produce a frequency of 483579.9 GHz. Earlier the voltage standard consists of single junctions, which provided only small voltages, typically 5mV to 10mV. The attempts were made to increase the Josephson voltage output by connecting several junctions in series to form a Josephson junction arrays. Programmable Josephson voltage standards are also in operation for dc calibrations are currently being implemented in low frequency (<400Hz) ac applications, in particular for the calibration of ac power instruments [15].

V. SUPERCONDUCTING QUANTUM INTERFERENCE DEVICE (SQUID)

A Superconducting Quantum Interference Device (SQUID) uses the properties of electron-pair wave coherence and Josephson Junctions to detect very small magnetic fields. The central element of a SQUID is a ring of superconducting material with one or more weak links. Superconducting quantum interference devices (SQUID) are the most sensitive detectors of magnetic flux based on superconducting loops containing Josephson junctions. They are amazingly versatile, being able to measure any physical quantity that can be converted to a flux. Therefore, SQUID are used for the detection of tiny magnetic fields

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© 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162) produced by the human brain and for the measurement of fluctuating geomagnetic fields in remote areas and also for the observation of spin noise in an ensemble of magnetic nuclei. SQUIDs are sensitive enough to measure fields as low as 5×10−18 T. Their noise levels are as low as 3fT·Hz-½. Because they measure changes in a magnetic field with such sensitivity, they do not have to come in contact with a system that they are testing. Magneto encephalography (MEG) is a completely non-invasive, non-hazardous technology for functional brain mapping. It provides a spatial resolution of about 2mm and excellent temporal resolution on the order of 1ms, during the localization and characterization of the electrical activity of the central nervous system by measuring the associated magnetic fields emanating from the brain. MEG uses measurements from an array of SQUIDs to make inferences about the intercellular currents of the neurons in the brain giving a direct information on the spontaneous or stimulated brain activity. Another area where SQUIDs are used is magnetogastrography, which is concerned with recording the weak magnetic fields of the stomach. A novel application of SQUIDs is the magnetic marker monitoring method, which is used to trace the path of orally applied drugs. In the clinical environment SQUIDs are used in cardiology for magnetic field imaging (MFI), which detects the magnetic field of the heart for diagnosis and risk stratification. As a new clinical examination apparatus for cardiac diseases, attention is focused on the development of high-temperature superconducting SQUID system for magnetocardiography [16]. The various body tissues e.g. blood, organs, vessels and bone exhibits a slightly different chemical environment for the hydrogen atoms contained in its constituent molecules. When a strong magnetic field is applied to the body, these chemical environments can be readily distinguished. SQUID-detected MRI uses measurement fields that lie in the microtesla regime. Since the MRI signal drops off as the square of the magnetic field, a SQUID is used as the detector because of its extreme sensitivity and its future application may include tumor screening [17]. The use of SQUIDs in oil prospecting, mineral exploration, earthquake prediction and geothermal energy surveying is becoming more widespread as superconductor technology develops; they are also used as precision movement sensors in a variety of scientific applications, such as the detection of gravitational waves [18]. Probably the most common commercial use of SQUIDs is in magnetic property measurement systems (MPMS). These are turn-key systems, made by several manufacturers that measure the magnetic properties of a material sample. This is typically done over a temperature range from that of 4 K to roughly 190 K, though higher temperatures mean less precision [19]. SQUID system plays an important role in determining the magnetic properties of earth. High-Tc SQUID magnetometers are promising for geophysical surveying such as magnetotellurics. In magnetotellurics, the fluctuating horizontal component of the electric and magnetic field at earth surface are measured simultaneously. These fluctuating fields originate in magnetosphere and ionosphere. Application of magnetotellurics includes surveying for oil and gas, mineral and geothermal sources and locating subsurface fault lines. Another application is the scanning SQUID microscope, which uses a SQUID immersed in liquid helium as the probe. Scanning SQUID microscopy is a modern technique capable of imaging the magnetic field distribution in close proximity across the surface of a sample and devices such as superconducting and magnetic films and electronic circuits with a high sensitivity and spatial resolution of few µm. SQUIDs based on high-Tc superconductors are used in the imaging of surface and subsurface cracks or pits due to corrosion or fatigue in aging aircraft and reinforcing rods in concrete structure. The advantage of the SQUID sensor is that the flux changes can be measured with unchanged sensitivity in rather high background fields. This method of detection is also called remote magnetometry. SQUIDs can be used as gradiometer of different order for suppression of perturbing magnetic field fluctuations caused by the environment. Gravity gradiometers can be used for mapping the earth’s gravity gradient and have the potential for testing the inverse square law.

VI RAPID SINGLE FLUX QUANTUM (RSFQ) ELECTRONICS

The Rapid Single flux quantum (RSFQ) electronics is based on shunted Josephson junction and presents an efficient way to achieve ultrafast digital operations. In this case, the superconducting RSFQ logic based on the transfer of very fast voltage pulses are related to the junction switching event associated with the emission of single flux quantum (SFQ) that carries the digital information. The exchange of information between the logic cells are mediated by the presence or absence of the SFQ voltage pulses. Josephson transmission line allows the transfer of SFQ pulses over long distances. In RSFQ logic, information is stored

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© 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162) in the form of magnetic flux quanta and transferred in the form of single flux quantum (SFQ) voltage pulses [20]. Pico second duration’s voltage pulses produced by Josephson junctions are used to encode, process and transport digital information. The generation of the SFQ pulse is conditioned by the Josephson junction phase slip 2π, the phase slip duration is in order of picoseconds allowing the RSFQ circuits to operate with clock frequencies in the order of 100GHz [21]. In RSFQ circuit, each Josephson junction has an external shunt resister so as to satisfy the critical damping condition. As a result, the switching time becomes shortest and stable operation of RSFQ circuit is guaranteed. The Josephson junctions are needed to be driven by current sources because bias currents provided to the Josephson junctions are the driving forces for single flux quanta. High speed operation of RSFQ circuits enables high frequency sampling and utilizing flux quantization brings high precision in digitization. Several types of microprocessors based on RSFQ circuits have been demonstrated so far [8]. A reconfigurable data path (RDP) processor is a compact, high performance computation engine. An RSFQ-RDP processor made up of 11000 Josephson junctions has been demonstrated up to 45GHz [22].

VII QUANTUM COMPUTERS

In a quantum computer, the information is encoded in quantum bits or qubits. Superconducting qubits are micro fabricated electrical circuits that contain superconducting elements and they behave like artificial atoms with discrete energy levels. Two different electrical excitations in the circuit correspond to the two states of the qubit. A quantum bits can take values (quantum mechanical states) |0>, |1> in Dirac notation or both of them at the same time. The bits in a superposition of two states at the same time allow quantum computers to work. Qubits can be realized in a wide variety of physical systems displaying quantum mechanical properties. These include atoms, ions, electronics and nuclear magnetic moments, charges in quantum dots, charges and fluxes in superconducting circuits and many more. Josephson junction forms the heart of the superconducting qubit, a leading candidate for the creation of fault tolerant quantum computation [23]. The first evidence of quantum behavior in a Josephson junction came from experiments in which macroscopic quantum tunneling was found to occur and energy levels were shown to be quantized. The nonlinearity of the Josephson inductance breaks the degeneracy of the energy level spacing’s, allowing dynamics of the system to be restricted only the two qubit states. The Josephson junction is a remarkable non-linear element because it combines negligible dissipation with extremely large non-linearity. The non- linear inductance of the Josephson junction creates an anharmonicity in its energy level spectrum. This allows a quantum mechanical basis (1or 0) to be established between discrete energy levels, which is essential for forming a quantum bit or qubit. In principle, a computer based on Josephson junctions could be several times faster and 100 times smaller than present computers. A SQUID consists of tiny loops of superconductors employing Josephson junctions to achieve superposition: each electron moves simultaneously in both directions. Because the current is moving in two opposite directions, the electrons have the ability to perform as qubits. Their quantum state is manipulated by using electromagnetic pulses to control the magnetic flux, the electric charge or the phase difference across a Josephson junction ( a device with non-linear inductance and no energy dissipation). Josephson qubits are non-linear resonators whose critical element is the non-linear inductance of the Josephson junction. Josephson qubits are possible even when Ej >Ec provided that the junction is biased to take advantage of its strong non-linearity. There are three basic types of superconducting qubits: charge, flux and phase use this non-linearity differently. The major difference between the phase, flux and charge qubits is the shape of their non-linear potentials, which are respectively cubic, quartic, and cosine. The charge qubits omits the inductance. Qubits of this type [24] were proposed and developed in the regime of Ej /Ec<<1.where Ej = Josephson energy and Ec is the single electron charging energy. The flux qubits [25] also known as persistent current qubit.The inductance is substituted by an array of Josephson junction Ej /Ec >>1. A flux qubit consists of a superconducting loop interrupted by one or three Josephson junction .The state of the flux qubit is measured with a dc SQUID. The flux qubit has the largest non-linearity. The flux qubit allows operation time less than ~1 ns, where as for the phase qubit 10ns is more typical. In the phase qubit [26], the potential is biased at a different point and again Ej /Ec >>1. The Implementation of phase qubit is challenging because a current bias is required with large impedance. Superconducting qubits provide a wide variety of promising tools for quantum, state

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VIII. ANALOG TO DIGITAL CONVERTER (ADC) Analog to digital converters (ADCs) are electronic circuits that convert an electrical signal from the analog domain to digital domain of binary numbers which can then be processed by computers. Josephson analog to digital converters (ADCs) are medium–scale integrated circuit requiring hundreds or thousands of Josephson junctions for complete systems. Josephson ADC has advantages over semiconductor analog to digital converters in low power dissipation, wide signal bandwidth, high dynamic range or sensitivity to small signals. Superconductor ADC generally falls into two categories. Nyquist-sampling parallel ADCs and oversampling ADCs. The Nyquist ADCs are usually composed of a large number of separate quantizers arranged in parallel–each defining a single quantized level. The performance of such an ADC is limited by the precision of all quantization levels. These parallel types ADC are best for digitizing high bandwidth signals when a moderate resolution up to 8 bit is adequate. In the oversampling ADCs the signal is sampled using a single quantizer. A superconductive analog to digital converter uses a dc SQUID as a quantiser and a flip-flop counter as a digitizer. Analog to digital converters operating with clock frequencies in the 40 GHZ range have been demonstrated to date which can digitize signal well into the microwave frequency range. These circuits employ the movement of magnetic flux quanta around the circuit and therefore this technology inherently has extremely high precision as the “unit of information is quantized [28]. Quasi one junction SQUID A/D convertors have performed 5 bit accurate conversion of 2GHz sine wave signals. The Josephson flash Analog to digital converter architecture requires 2n-1 comparators for N-bits of precision [29]. This circuit uses a regular R-2R resistor network to reduce to input current by a factor of two for each successive bit, each of which is encoded using a SQUID comparator with a periodic threshold. The direct digital output is actually in ‘Gray code’ rather than standard binary, but code conversion is straight forward. A 6 bit flash ADC of this design has been demonstrated to operate at greater than 10GHz Nyquist badwidth, as part of transient digitizer instruments [30]. The tracking A/D convertor employs the flux quantization phenomena in SQUIDs to track an analog signal. The analog input signal is magnetically coupled to a SQUID; each time the signal rises or falls to inject or expel a magnetic flux quantum from the SQUID, a voltage pulse is produced across the Josephson junction. The pulses are typically 1-10 pico sec in duration and of millivolt amplitude. Therefore, circuit employing Josephson junction are necessary to detect and count them. A second counting type superconducting A/D converter is based upon the AC Josephson uses the microscopic frequency proportional to the applied voltage (483.6MHz/µV), the oscillations from the junction drive a counter. The Counter output is proportional to a time averaged Josephson frequency and consequently time-averaged voltage.

IX THz EMITTERS AND DETECTORS

THz frequencies (300GHz-3000GHz) lie above mm-wave and below infrared frequencies in the electromagnetic spectrum. The investigation of coherent terahertz (THz) emission from stacks of intrinsic Josephson junctions made of high-temperature superconductor has become a major field of research [31,32]. The strongly coupled intrinsic Josephson junctions should be capable to coherently radiate terahertz-range radiation. Josephson effect based devices allow generation , detection , mixing and parametric amplification of high frequency signals up into the terahertz region and exhibit high sensitivity, low energy consumption and small size. The most promising devices are based on intrinsically layered Bi2Sr2CaCu2O8 single crystals which consist of natural stacks of Josephson tunnel junctions. A superconducting integrated receiver(SIR) [33] was successfully implemented for the first spectral measurements of THz radiation emitted from intrinsic Josephson junction stack (BSCCO mersa) in the frequency range 585-735GHz; line width as low as 5 MHz has been recorded in the high bias regime. For coherent operation all junctions in the stack should oscillate in phase and the whole system should be impedance matched to the load. THz technology holds significant potential in the areas of security screening (remote detection of explosives through spectroscopic response of crystalline compounds, drugs, hidden weapons, and other objects), material characterization, nondestructive imaging of items concealed in packaging, medical imaging (imaging of skin cancer beneath

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© 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162) the skin due to increased water content in tumor cells), chemical sensing and food and agricultural product inspection. THz radiation is non-ionizing and is therefore safer for humans as compared to X-ray based imaging. The THz detectors consist of a HTS step-edge Josephson junction integrated across the slot of a ring-slot antenna which was designed with a centre frequency of 600 GHz and ~ 12% bandwidth. In operation, an incident THz wave excites current on the antenna which passes through the junction; the RF signal is detected via changes in the junction’s electrical characteristics. The high temperature superconducting Josephson junction detector has been developed and applied successfully to THz imaging at an operation temperature of 77K [7]. High-Tc superconducting YBCO thin films are prepared using various techniques [34]. Josephson junctions are patterned in a YBCO film on a bicrystal sapphire substrate and are voltage controlled to generate radiation in the frequency range of 0.05-1THz. Radiation emitted by HTS JJs has been the subject of intense research in recent years motivated by the need for a spectral source of terahertz radiation. Individual Josephson junctions coupled to planar antennas are the simplest realization of such terahertz sources [35].

X. CONCLUSIONS

The most relevant applications of Josephson junction were discussed among them are SQUIDs, Analog to digital converter (ADC), Josephson voltage standard (JVS), RSFQ logic circuits, etc. The voltage biased individual Josephson junction made of HTS can potentially serve as the tunable spectral source of terahertz radiation. Although application of Josephson junctions are very promising for future high speed computer based RSFQ logic and quantum processing devices, fabrication technology for HTS Josephson junction still need its attention. Well defined and integrable fabrication technology is needed for commercialization of these technologies. The superconductor need to have perfect crystallization and perfectly defined interface has to be formed at the level of single unit cell. The film defects to make good tunnel junctions and flux flow should be under control. To achieve the successful HTSC Josephson devices, one has to grow epitaxial films and should have control over the surface morphology and stoichiometry for multilayer.

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