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APPLICATION of HIGH-Tc SUPERCONDUCTING JOSEPHSON JUNCTION DEVICES © 2019 JETIR January 2019, Volume 6, Issue 1 www.jetir.org (ISSN-2349-5162) 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 JETIR1901668 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 517 © 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 JETIR1901668 Journal of Emerging Technologies and Innovative Research (JETIR) www.jetir.org 518 © 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
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