This current effectively what is superconductivity pdf an electromagnet that repels the magnet. In a superconductor, the resistance drops abruptly to zero when the material is cooled below its critical temperature. 77 K, and thus superconduction at higher temperatures than this facilitates many experiments and applications that are less practical at lower temperatures.
There are many criteria by which superconductors are classified. Furthermore, in multicomponent superconductors it is possible to have combination of the two behaviours. On the other hand, there is a class of properties that are independent of the underlying material. If the voltage is zero, this means that the resistance is zero. 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 100,000 years.
In such instruments, the measurement principle is based on the monitoring of the levitation of a superconducting niobium sphere of mass 4 grams. As a result, the energy carried by the current is constantly being dissipated. The situation is different in a superconductor. In a conventional superconductor, the electronic fluid cannot be resolved into individual electrons. 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. The value of this critical temperature varies from material to material. 92 K, and mercury-based cuprates have been found with critical temperatures in excess of 130 K. The explanation for these high critical temperatures remains unknown.
The penetration depth becomes infinite at the phase transition. At the superconducting transition, it suffers a discontinuous jump and thereafter ceases to be linear. 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. Calculations in the 1970s suggested that it may actually be weakly first-order due to the effect of long-range fluctuations in the electromagnetic field. The results were strongly supported by Monte Carlo computer simulations.