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Concepts and Principal Provisions of Fundamental and Applied Superconductivity
Published in V.R. Romanovskii, Basic Macroscopic Principles of Applied Superconductivity, 2021
In nonideal type-II superconductors, the vortexes are pinned by the impurities or defects under the so-called pinning force Fp. The sites, in which the vortices stay at a fixed position, are called pinning centers. There are various types of pinning centers (both naturals and artificially created): zero-dimensional pinning centers like a crystal lattice point defects, the one-dimensional pinning centers like displacement lines, the two-dimensional pinning centers like grain and domain boundaries or surface defects and the three-dimensional pinning centers like artificially created precipitations and columnar defect channels. For the specific current, the Lorentz force overcomes the pinning force and the vortexes start the movement. As a result, a dissipation phenomenon occurs. They will continually move through the superconductor until the vortexes are pinned by other pinning centers. The higher the pinning forces Fp, the higher the current density, which can be passed through the superconductor before the transition to a normal state. In other words, the flux pinning phenomenon determines the critical current of nonideal type-II superconductors. Therefore, the critical current density of nonideal type-II superconductors corresponds to the current when the Lorentz force equals the average pinning force, that is, it is equal to |Jc| = |Fp|/|B|. Besides, the pinning force, the pinning center density also influences the critical current.
Introduction to Superconducting Devices
Published in Raja Sekhar Dondapati, High-Temperature Superconducting Devices for Energy Applications, 2020
Moreover, for many years, it was assumed that only one type of superconductor existed; however, it was later realized that two distinct types of superconductors exist. The two superconductors shared many properties; however, the most distinguishing feature is the characteristics they exhibit under an external magnetic field. The first kind of superconductor, the Type I superconductors (also known as “soft” superconductors, usually elements), loses its superconducting properties in a relatively weak magnetic field, whereas, Type II superconductors (also known as “hard” superconductors, usually alloys) can withstand very strong magnetic fields before losing superconducting properties. However, few elements are notable exceptions, which include Niobium, Vanadium, and Technetium, which are Type II superconductors. Further discussion on the properties of superconductors is presented in subsequent sections.
Formulation and Classification of Electronic Devices
Published in Michael Olorunfunmi Kolawole, Electronics, 2020
Semiconductors are unique materials, usually a solid chemical element—such as silicon (Si), or compound—such as gallium arsenide (GaAs) or indium antimonide (InSb), which can conduct electricity or not under certain conditions. Semiconductor behavior is not restricted to solids: there are liquid conductors, for instance, mercury (Hg) and those regarded as “Type II alloys” superconductors. It should be noted that the resistivity of most metals increases with an increase in temperature and vice versa. As such, there are some metals and chemical compounds whose resistivity becomes zero when their temperature is brought near zero-degree Kelvin (0°K) (i.e. −273°C). {Note that absolute temperature, T, in degree Kelvin (°K), is expressed by = 273 + t, where t =measured temperature in degree Celsius.} At this stage such metals or compounds are said to have attained superconductivity. The transition from normal conductivity to superconductivity takes place almost suddenly over a very narrow range of temperature. Mercury, for example, becomes superconducting at approximately 4.5°K. Type-II superconductors usually exist in a mixed state of normal and superconducting regions [1]. However, because of atomic diffusion, regions with different dopings will mix rapidly and a stable device with an inhomogeneous structure is not possible [2].
Vibrating reed magnetometer studies of superconducting and magnetic materials
Published in Philosophical Magazine, 2020
L. E. De Long, A. P. Kaphle, B. Farmer
When a type II superconductor is cooled into the mixed state in a magnetic field, FL form and are pinned by sample imperfections, leading to anisotropic screening currents. If the sample is configured as a VR, the screening currents can generate strong restoring forces on the VR (e.g. ‘line tension’ or ‘tilt’ effects, in addition to ever-present elastic restoring forces; see Figure 1). Consequently, a strong increase in the VR resonance frequency fo, and a peak in dissipation (measured by 1/Q) are observed as the temperature crosses below the upper critical magnetic field HC2(T) (see Figure 2) [33]. Brandt, Esquinazi and co-workers showed [12–15,17,33–42] detailed models of FL pinning can provide quantitative explanations of the strong resonant frequency shifts and dissipation peaks observed in SC VR in applied magnetic fields. Brandt’s initial calculations applied to VR in a parallel geometry, assuming an extreme-type-II SC material (κ >> 1) with VR length l >> width w >> thickness t. The University of Kentucky Group (UKG) made technical improvements in the RF cavity design of Xiang et al [21] for VRM measurements at low temperatures and in high magnetic fields. This permitted sensitive measurements of FL motion in SC Nb foils and single-crystal NbSe2 [16], and a variety of other complex SC materials to be discussed herein [18–20,44].
Effective field study of the magnetism and superconductivity in idealised Ising-type X@Y60 endohedral fullerene system
Published in Philosophical Magazine, 2019
On the other hand, the system exhibits the superconducting-like phenomena with antiferromagnetic C–S interactions. In Figure 7, the centre and surface hysteresis loops show different dependencies with antiferromagnetic C–S interactions. In here, superconducting-like phenomena are the consequence of the existence of a centre magnetic atom. In Figure 7(a), the hysteresis loop of the centre displays type II superconductivity behaviour. According to this behaviour, the hysteresis curves exhibit four coercive field points (±HC1 and ±HC2). Therefore, susceptibilities make two peaks at ±HC1 and two broad maximum at ±HC2. We should also mention that the type II superconductors behaviour was obtained for the first time by Abrikosov [72]. Moreover, type II superconductors were presented by the various theoretical models such as Ginzburg–Landau theory [73], Monte Carlo simulations [74, 75], SO(5) theory [76], effective-field theory [66], Emery model within the composite operator method [77] and renormalised mean-field theory [78, 79]. As seen in Figure 7, with the increase of temperature, the HC1 and HC2 values decrease and when the temperature approaches its critical value (TC = 0.60), the centre coercive field illustrates a single point. In T > TC, the susceptibility shows a broad maximum, as seen in Figure 7(b) and (c) [80].
Study of the critical current density and the thermodynamic critical field in deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br organic superconductor
Published in Phase Transitions, 2018
Youssef Ait Ahmed, Ahmed Tirbiyine, Ahmed Taoufik, Hassan El Ouaddi, Habiba El Hamidi, Abdelhalim Hafid, Abdelaziz Labrag, Hassan Chaib
Type-II superconductors present one capacity to transport a high current density in the presence of a magnetic field. The limiting value of the critical current density is given by the equality of the two opposing forces acting on the flux lines (vortex). These forces are the pinning force due to the spatial evolution of the condensation energy and the Lorentz force exerted by the transport current. The energy is dissipated when the flow lines move. At low temperatures, the quantum fluctuations of the Josephson vortices cause peculiar energy dissipation, and provide strong indications showing that the pancake vortices and pancake vortices melt separately [22].