Introduction to Superconductors: Properties, Classification, and High-Temperature Superconductors

What is a superconductor? 

Superconductors are materials whose resistivity becomes immeasurably small or actually becomes zero below a critical temperature, Tc. For example, (La, Sr)2CuSO4 has Tc = 36K.

Factors that control superconductivity: 

Superconductivity of metals depends on current, temperature, and magnetic field properties. Superconductivity is only present when temperature, magnetic field strength, and current remain within a critical space. The relationship among these critical values is such that an increase in the critical value of one of these parameters decreases the critical value of the remaining two. Eliminating superconductivity requires increasing one of these parameters above its critical value

Properties of superconductors:

• Zero resistance: The resistance of superconducting materials is zero below their critical temperature, resulting in highly efficient electrical conduction. 
• Absence of thermoelectric effects: No Seebeck voltage, no Peltier-heat, no Thomson-heat detectable.
• Ideal diamagnetism: Xm = -1. Superconducting materials have perfect diamagnetic properties, meaning they expel any magnetic field from their interior.
• Meissner-effect: If a superconductor is cooled down in the presence of a weak magnetic field, below Tc, the field is completely expelled from the bulk of the superconductor.
• Flux quantization: The magnetic flux through a superconducting ring is quantized and constant in time.
• Critical magnetic field: The magnetic field strength that can be applied to a superconducting material without destroying its superconductivity is limited by its critical magnetic field.
• Critical current density: The maximum current density that can be carried by a superconducting material without losing its superconductivity is known as the critical current density.
• Critical temperature: The temperature at which a material transitions from a normal conductor to a superconductor is called its critical temperature.
• Josephson effect: The ability of superconducting materials to produce a DC voltage across a thin insulating barrier between two superconductors, allowing for the creation of sensitive voltage sensors.

Classification of superconductors: 

According to transition period, superconductors are classified into the following two classes:

Type-1 (soft) superconductors: The superconductors which transition from a superconducting state to a normal state sharply due to any limiting parameters (especially magnetic field) are known as type-1 or soft superconductors.

Above figure shows the schematic representation of the resistivity of a soft superconductors when a magnetic field H is applied. These solids behave like normal conductors above Hc.

Type-II (hard)superconductors: The superconductors which transition from a superconducting state to a normal state gradually due to any limiting parameters (especially magnetic field) are known as type-II or hard superconductors.

Above figure shows the schematic representation of the resistivity of a hard superconductor when a magnetic field H is applied. The region between Hc1 and HC2 is called the vortex state in which superconducting and normal conducting areas are mixed. Above Hc2 the solid behaves like a normal conductor.

High-temperature superconductors: 

The temperature at which materials transition from a normal state to a superconducting state is called critical temperature or transition temperature. The materials having a critical temperature above 77K are known as high-temperature superconductors. They are technologically interesting because they do not require liquid helium or liquid hydrogen (-253°C) for cooling. Despite considerably higher transition temperatures, they have not yet revolutionized new technologies mainly because of their inherent brittleness, their incapability of carrying high current densities, and their environmental instability. These problems may be overcome using bismuth-based materials, composite materials, ductile substrates, silver, etc.