This volume consists of lectures highlighting fundamentals of advances in superconducting materials, related technologies and applications. Theory. An introduction to and overview of the contents of this Special Issue are given. 32 classes of superconducting materials are discussed, grouped under the three. Find the latest research, reviews and news about Superconducting properties and materials from across all of the Nature journals.
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Superconducting materials - IOPscience
Several superconducting materials have been synthesized. Crucial progress was made in with the discovery of high temperature superconductivity in copper-based compounds cuprates which have revealed new fascinating properties. Innovative theoretical tools have been developed to understand the striking features of cuprates which have remained for three decades the 'blue-eyed boy' for superconducting materials in superconductor physics.
The history of superconducting materials has been notably marked by the discovery of other compounds, particularly organic superconductors which despite their low critical temperature continue to attract great interest regarding their exotic properties.
Last but not least, the recent observation of superconductivity in iron-based materials pnictides has renewed hope in reaching room temperature superconducting materials. However, despite intense worldwide studies, several features related to this phenomenon remain unveiled.
One of the fundamental key questions is the mechanism by which superconductivity takes place. Superconductors are also able to maintain a current with no applied voltage whatsoever, a property exploited in superconducting electromagnets such as those found superconducting materials MRI machines.
Experiments have demonstrated that currents superconducting materials superconducting coils can persist for superconducting materials without any measurable degradation. Experimental evidence points to a current lifetime of at leastyears.
Theoretical estimates for the lifetime of a persistent current can exceed the estimated lifetime of the universedepending 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 heatwhich 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 superconducting materials Cooper pairs.
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This pairing is caused by an superconducting materials force between electrons from the exchange of phonons. The Cooper pair superconducting materials is thus a superfluidmeaning it can flow without energy dissipation.
In a class of superconductors known as type II superconductorsincluding all known high-temperature superconductorsan extremely low but nonzero resistivity appears at temperatures not too superconducting materials 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 superconducting materials. If the superconducting materials 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. The value of this critical temperature varies from material superconducting materials material. Solid mercuryfor example, has a critical temperature of 4.
As ofthe highest critical temperature found for a conventional superconductor is K for H2S, 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 superconducting materials 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 superconducting materials 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.