The phenomenon of superconductivity, in which the electrical resistance of Imagine that both the ideal conductor and superconductor are above their critical. Chapter 1 Room Temperature Superconductivity 1. Adir Moysés Luiz. Chapter 2 Unconventional Superconductivity Realized Near. Magnetism. A free online edition of this book is available at riastanufulthep.gq Additional hard copies can be obtained from [email protected] Superconductor, Edited by .
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Low-temperature superconductivity is treated at an introductory level. The There is an intimate relation between superconductivity and magnetic fields. PH Introduction to superconductors. 3. Progress in Tc of superconductor materials with time. High temperature superconductors discovered in. PDF | After getting a Ph.D. in physics from the University of Tokyo, and doing a research at the in order to catch up with the ongoing research on the high T c superconductors such as YBa 2 What type of file do you want?.
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Mondal, P. Adams, V. B 80, CrossRef Google Scholar Haldar, M. Laad, S. Hassan, M. Chand, P. Raychaudhuri, Phys Rev. Kamlapure, G. Ganguli, V. Bagwe, P. Raychaudhuri, S. Pai, Rev. Baturina, D. Islamov, J. Bentner, C. Strunk, M. Baklanov, A. Gantmakher, M. Golubkov, V. In that case the superconductor is of Type It is conventional if it can be explained by the BCS theory or its derivatives, or unconventional , otherwise. Superconductor material classes include chemical elements e. Most of the physical properties of superconductors vary from material to material, such as the heat capacity and the critical temperature, critical field, and critical current density at which superconductivity is destroyed.
On the other hand, there is a class of properties that are independent of the underlying material. For instance, all superconductors have exactly zero resistivity to low applied currents when there is no magnetic field present or if the applied field does not exceed a critical value. The existence of these "universal" properties implies that superconductivity is a thermodynamic phase , and thus possesses certain distinguishing properties which are largely independent of microscopic details.
The simplest method to measure the electrical resistance of a sample of some material is to place it in an electrical circuit in series with a current source I and measure the resulting voltage V across the sample. If the voltage is zero, this means that the resistance is zero. Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found in MRI machines.
Experiments have demonstrated that currents in superconducting coils can persist for years without any measurable degradation. Experimental evidence points to a current lifetime of at least , years. Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universe , depending on the wire geometry and the temperature.
In a normal conductor, an electric current may be visualized as a fluid of electrons moving across a heavy ionic lattice.
The electrons are constantly colliding with the ions in the lattice, and during each collision some of the energy carried by the current is absorbed by the lattice and converted into heat , which is essentially the vibrational kinetic energy of the lattice ions. As a result, the energy carried by the current is constantly being dissipated. This is the phenomenon of electrical resistance and Joule heating.
The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. Instead, it consists of bound pairs of electrons known as Cooper pairs. This pairing is caused by an attractive force between electrons from the exchange of phonons. The Cooper pair fluid is thus a superfluid , meaning it can flow without energy dissipation. In a class of superconductors known as type II superconductors , including all known high-temperature superconductors , an extremely low but nonzero resistivity appears at temperatures not too far below the nominal superconducting transition when an electric current is applied in conjunction with a strong magnetic field, which may be caused by the electric current.
This is due to the motion of magnetic vortices in the electronic superfluid, which dissipates some of the energy carried by the current. If the current is sufficiently small, the vortices are stationary, and the resistivity vanishes.
The resistance due to this effect is tiny compared with that of non-superconducting materials, but must be taken into account in sensitive experiments. However, as the temperature decreases far enough below the nominal superconducting transition, these vortices can become frozen into a disordered but stationary phase known as a "vortex glass".
Below this vortex glass transition temperature, the resistance of the material becomes truly zero. In superconducting materials, the characteristics of superconductivity appear when the temperature T is lowered below a critical temperature T c. The value of this critical temperature varies from material to material. Solid mercury , for example, has a critical temperature of 4. As of , the highest critical temperature found for a conventional superconductor is K for H 2 S, although high pressures of approximately 90 gigapascals were required.
The explanation for these high critical temperatures remains unknown.
Electron pairing due to phonon exchanges explains superconductivity in conventional superconductors, but it does not explain superconductivity in the newer superconductors that have a very high critical temperature. Similarly, at a fixed temperature below the critical temperature, superconducting materials cease to superconduct when an external magnetic field is applied which is greater than the critical magnetic field.
This is because the Gibbs free energy of the superconducting phase increases quadratically with the magnetic field while the free energy of the normal phase is roughly independent of the magnetic field.
If the material superconducts in the absence of a field, then the superconducting phase free energy is lower than that of the normal phase and so for some finite value of the magnetic field proportional to the square root of the difference of the free energies at zero magnetic field the two free energies will be equal and a phase transition to the normal phase will occur.
More generally, a higher temperature and a stronger magnetic field lead to a smaller fraction of electrons that are superconducting and consequently to a longer London penetration depth of external magnetic fields and currents.
The penetration depth becomes infinite at the phase transition. The onset of superconductivity is accompanied by abrupt changes in various physical properties, which is the hallmark of a phase transition.
For example, the electronic heat capacity is proportional to the temperature in the normal non-superconducting regime. At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to be linear. This exponential behavior is one of the pieces of evidence for the existence of the energy gap. The order of the superconducting phase transition was long a matter of debate. Experiments indicate that the transition is second-order, meaning there is no latent heat.
However, in the presence of an external magnetic field there is latent heat, because the superconducting phase has a lower entropy below the critical temperature than the normal phase. It has been experimentally demonstrated  that, as a consequence, when the magnetic field is increased beyond the critical field, the resulting phase transition leads to a decrease in the temperature of the superconducting material.
Calculations in the s suggested that it may actually be weakly first-order due to the effect of long-range fluctuations in the electromagnetic field. In the s it was shown theoretically with the help of a disorder field theory , in which the vortex lines of the superconductor play a major role, that the transition is of second order within the type II regime and of first order i.
When a superconductor is placed in a weak external magnetic field H , and cooled below its transition temperature, the magnetic field is ejected. The Meissner effect is a defining characteristic of superconductivity. The Meissner effect is sometimes confused with the kind of diamagnetism one would expect in a perfect electrical conductor: In a perfect conductor, an arbitrarily large current can be induced, and the resulting magnetic field exactly cancels the applied field.
The Meissner effect is distinct from this—it is the spontaneous expulsion which occurs during transition to superconductivity. Suppose we have a material in its normal state, containing a constant internal magnetic field. When the material is cooled below the critical temperature, we would observe the abrupt expulsion of the internal magnetic field, which we would not expect based on Lenz's law.
The Meissner effect was given a phenomenological explanation by the brothers Fritz and Heinz London , who showed that the electromagnetic free energy in a superconductor is minimized provided. This equation, which is known as the London equation , predicts that the magnetic field in a superconductor decays exponentially from whatever value it possesses at the surface.
A superconductor with little or no magnetic field within it is said to be in the Meissner state. The Meissner state breaks down when the applied magnetic field is too large. Superconductors can be divided into two classes according to how this breakdown occurs.
In Type I superconductors , superconductivity is abruptly destroyed when the strength of the applied field rises above a critical value H c. Depending on the geometry of the sample, one may obtain an intermediate state  consisting of a baroque pattern  of regions of normal material carrying a magnetic field mixed with regions of superconducting material containing no field. In Type II superconductors , raising the applied field past a critical value H c 1 leads to a mixed state also known as the vortex state in which an increasing amount of magnetic flux penetrates the material, but there remains no resistance to the flow of electric current as long as the current is not too large.
At a second critical field strength H c 2 , superconductivity is destroyed. The mixed state is actually caused by vortices in the electronic superfluid, sometimes called fluxons because the flux carried by these vortices is quantized.
Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis. This experiment measured the magnetic fields of four superconducting gyroscopes to determine their spin axes.
This was critical to the experiment since it is one of the few ways to accurately determine the spin axis of an otherwise featureless sphere. Superconductivity was discovered on April 8, by Heike Kamerlingh Onnes , who was studying the resistance of solid mercury at cryogenic temperatures using the recently produced liquid helium as a refrigerant. At the temperature of 4. The precise date and circumstances of the discovery were only reconstructed a century later, when Onnes's notebook was found.
Great efforts have been devoted to finding out how and why superconductivity works; the important step occurred in , when Meissner and Ochsenfeld discovered that superconductors expelled applied magnetic fields, a phenomenon which has come to be known as the Meissner effect. The first phenomenological theory of superconductivity was London theory. It was put forward by the brothers Fritz and Heinz London in , shortly after the discovery that magnetic fields are expelled from superconductors.
A major triumph of the equations of this theory is their ability to explain the Meissner effect ,  wherein a material exponentially expels all internal magnetic fields as it crosses the superconducting threshold. By using the London equation, one can obtain the dependence of the magnetic field inside the superconductor on the distance to the surface.
The first equation follows from Newton's second law for superconducting electrons. During the s, theoretical condensed matter physicists arrived at an understanding of "conventional" superconductivity, through a pair of remarkable and important theories: In , the phenomenological Ginzburg-Landau theory of superconductivity was devised by Landau and Ginzburg.
Abrikosov and Ginzburg were awarded the Nobel Prize for their work Landau had received the Nobel Prize for other work, and died in The four-dimensional extension of the Ginzburg-Landau theory, the Coleman-Weinberg model , is important in quantum field theory and cosmology.
Also in , Maxwell and Reynolds et al. The recent discovery of materials with unexpectedly high superconducting transition temperatures has brought renewed interest in the science and technology of superconductors throughout the world. The purpose of this paper is to briefly review recent activities at NIST in this field and to discuss current usage and future possibilities for both conventional and high temperature superconductors.
Some references will be made to historical superconductivity contributions at NIST but a complete review is not attempted. Background The field of superconductivity began with the discovery by H. Kamerlingh-Onnes in that mercury wire at 4. Zero resistance implied transmission of current at any distance with no losses, the production of large magnetic fields, or—because a superconducting loop could carry current indefinitely—storage of energy.
These applications were not realized because, as was quickly discovered, the superconductors reverted to normal conductors at a relatively low current density, called the critical current density, Jc, or in a relatively low magnetic field, called the critical field, Hc. In , Silsbee, at the National Bureau of Standards, hypothesized [ 1 ] that the critical current for a superconducting wire was equal to that current which gave the critical field at the surface of the wire.