- Superconductor Definition: A superconductor is defined as a material that exhibits zero electrical resistance and expels magnetic fields when cooled below a critical temperature
- Zero Electric Resistance: Superconductors have zero electric resistance below their critical temperature, allowing for infinite conductivity.
- Meissner Effect: When cooled below their critical temperature, superconductors expel magnetic fields, a phenomenon known as the Meissner Effect.
- Critical Temperature: This is the temperature at which a material abruptly changes from a normal conductor to a superconductor.
- Properties of Superconductors: Key properties include zero electric resistance, Meissner effect, critical temperature, critical magnetic field, persistent currents, Josephson currents, and critical current.
Superconducting materials have extraordinary properties that are crucial for modern technology. Research continues to explore and use these properties in various technological fields. Here are the key properties of superconductors:
- Zero Electric Resistance (Infinite Conductivity)
- Meissner Effect: Expulsion of magnetic field
- Critical Temperature/Transition Temperature
- Critical Magnetic Field
- Persistent Currents
- Josephson Currents
- Critical Current
Zero Electric Resistance or Infinite Conductivity
In a superconducting state, a material exhibits zero electric resistance (infinite conductivity). When cooled below its critical temperature, its resistance drops suddenly to zero. For example, mercury shows zero resistance below 4K.
Meissner Effect (Expulsion of Magnetic Field)
When a superconductor is cooled below its critical temperature (Tc), it expels magnetic fields and prevents them from penetrating inside. This phenomenon is known as the Meissner effect.
Critical Temperature/Transition Temperature
Critical temperature of a superconducting material is the temperature at which the materials changes from normal conducting state to superconducting state. This transition from normal conducting state (phase) to superconducting state (phase) is sudden / sharp and complete. The transition of mercury from normal conducting state to superconducting state is shown in figure below.
Critical Magnetic Field
The superconducting state of a material breaks when the magnetic field (either external or from the current in the superconductor) exceeds a certain value, causing it to behave like a regular conductor. This threshold is called the critical magnetic field. The critical magnetic field value depends on temperature—lower temperatures increase its value.
Persistent Current
When a superconducting ring is placed in a magnetic field above its critical temperature and then cooled below it, removing the magnetic field induces a current in the ring due to self-inductance. According to Lenz law, this induced current opposes the change in flux. In a superconducting state with zero resistance, this current continues to flow and is called persistent current. This current maintains a constant magnetic flux through the ring.
Josephson Current
If two superconductors are separated by a thin film of insulating material, which forms a low resistance junction, it is found that the cooper pairs (formed by phonon interaction) of electrons, can tunnel from one side of junction to the other side. The current, due to flow of such cooper pairs, is called Josephson Current.
Critical Current
When a current is passed through a conductor under superconducting state, a magnetic field is developed. If the current increase beyond certain value the magnetic field increased up to critical value at which conductor returns to its normal state. This value of current is called critical current.
