Superconductivity is one of the most remarkable and fascinating phenomena in the materials science, which has been the subject of intensive research by several scientific community over years. It was discovered by Heike Kamerlingh Onnes, who observed that the resistance of all metals drops to zero at an ultra-low temperature of about 4 Kelvin, which is later referred as critical temperature Tc. From then, there has been a great progress in our understanding and manipulation of this phenomenon.
Superconductors are classified into type I and type II based on the behavior of their magnetic field when cooled to very low temperatures. Type I materials have a single critical field and magnetic field can penetrate the material abruptly up to that value when the temperature reaches Tc. As the superconductivity is destroyed above the critical temperature, in this type the material loses its ‘Superconductivity’ characteristics. In contrast type II superconductors allow penetration of magnetic field up to a lower critical field called lower critical field (Hc1). In this case, as the temperature approaches Tc, the magnetic field can penetrate the sample slowly and at a further lowering of the temperature, Hc2 (upper critical field) the material collapses into a normal non-superconductive material.
In addition to these two categories, superconductors are also classified according to their properties at room temperature such as their critical temperature, their critical currents and their critical magnetic fields. The classification of these three is based on the diamagnetic properties, namely, the amount of magnetic field penetration for a given magnetic intensity.
Type I superconductors have a low critical temperature and a low critical current. They are highly diamagnetic and can usually be magnetized only to a weak field. They can withstand very high magnetic fields, but the critical fields of type I materials are relatively low. Type II superconductors have a higher critical temperature and a higher critical current. They have higher diamagnetic properties and can withstand much higher magnetic fields than type I materials.
The fundamental mechanism for superconductivity lies in the interaction of electrons with lattice vibrations, also known as phonons. When these two interact, electrons become bound together forming Cooper pairs, which move through the lattice without any resistance. This is due to the fact that while the phonons scatter the electrons they also induce attractive forces between the electrons, forming a pair which move freely together.
In order to understand the various electromagnetic properties of superconductors, there are three important equations; the London equations, the equations of London-Landau theory, and the Ginzburg-Landau equation. The London equation describes the way in which the electrons form Cooper pairs and the way in which lattice vibrations cause the current to vary with temperature. The London-Landau equation describes the behavior of the superconductor in the presence of an alternating current. Finally, the Ginzburg-Landau equation is an approximation that gives a greater understanding of the behavior of the superconductor in the presence of a magnetic field.
These equations allow us to study and study the various electromagnetic properties of superconductors like resistivity, critical temperature, critical field, and critical current. The critical temperature is the temperature at which the material transitions from being a normal conductor to a superconductor. The critical field is the magnetic field at which the material causes superconducting current to suddenly decrease to zero. The critical current is the current which the material can carry without experiencing resistance.
Moreover, superconductors also display various interesting electromagnetic properties such as the Meissner effect, where a superconductor expels all external fields from its interior. This is explained in terms of the London equations and the Ginzburg-Landau equation. Other properties of superconductors include flux pinning and flux flow, which are related to the interaction of the material with an external magnetic field.
In conclusion, superconductivity is one of the most fascinating and captivating phenomena in condensed matter and it is continuously being studied and researched by scientists. The development of many new applications is expected to arise from our better understanding of this phenomenon. The electromagnetic properties of superconductors are very important since they determine how a material behaves in different environmental conditions, and can be derived from equations like the London equations, the London-Landau theory, and the Ginzburg-Landau equation.
