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Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. It was discovered by Dutch physicist Heike Kamerlingh Onnes on April 8, 1911 in Leiden. Like ferromagnetism and atomic spectral lines, superconductivity is a quantum mechanical phenomenon. It is characterized by the Meissner effect, the complete ejection of magnetic field lines
from the interior of the superconductor as it transitions into the
superconducting state. The occurrence of the Meissner effect indicates
that superconductivity cannot be understood simply as the idealization
of perfect conductivity in classical physics.

Elementary properties of superconductors

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.

Zero electrical DC resistance

Electric cables for accelerators at CERN. Both the massive and slim cables are rated for 12,500 A. Top: conventional cables for LEP; bottom: superconductor-based cables for the LHC

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. The resistance of the sample is given by Ohm’s law as R = V / I. 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 100,000 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.[1]
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

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. Due to quantum mechanics, the energy spectrum of this Cooper pair fluid possesses an energy gap, meaning there is a minimum amount of energy ΔE that must be supplied in order to excite the fluid. Therefore, if ΔE is larger than the thermal energy of the lattice, given by kT, where k is Boltzmann’s constant and T is the temperature, the fluid will not be scattered by the lattice. 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 small amount of 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.

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