Engineering Physics
Semiconductors & Superconductivity
Unit IV: Superconductivity. This unit covers the essential physics of superconductors, key properties, the Meissner effect, critical parameters and the classification and applications of superconducting materials used in modern technology.
Phenomenon of Superconductivity
- Superconductivity is the state of certain materials in which electrical resistance drops to zero when cooled below a characteristic transition temperature Tc.
- Perfect conductivity results in persistent currents that can flow indefinitely without power loss.
- Transition to superconducting state is a phase change involving electron pairing (Cooper pairs).
- Occurs in metals, alloys and certain ceramics depending on material structure.
Diagram to paste: superconductivity-transition.png
Properties of Superconductors
- Zero electrical resistance below Tc.
- Perfect diamagnetism indicating expulsion of magnetic flux from the interior.
- Persistent currents in closed loops with extremely long lifetimes.
- Energy gap in electronic density of states due to Cooper pairing.
- Isotopic effect showing dependence of Tc on atomic mass.
Diagram to paste: superconductor-properties.png
Meissner Effect and Critical Parameters
- Meissner effect is the complete expulsion of magnetic flux from a superconductor below Tc, demonstrating perfect diamagnetism.
- Critical magnetic field Hc is the maximum field below which superconductivity can exist. Above Hc the material returns to the normal state.
- Temperature dependence:
Hc(T) = Hc(0) [1 - (T/Tc)^2]. - Critical current density Jc is the highest current per unit area the material can carry in superconducting state.
Diagram to paste: meissner-effect.png, critical-field.png
Types of Superconductors
- Type I Superconductors
- Show perfect Meissner effect.
- Single critical field Hc.
- Found mainly in pure metals like Pb, Hg, Al.
- Type II Superconductors
- Characterized by two critical fields Hc1 and Hc2.
- Allow partial flux penetration in mixed state.
- Found in alloys and high Tc superconductors.
Diagram to paste: type1-type2.png
Applications of Superconductors
- Magnetic levitation and high speed maglev trains.
- Superconducting magnets for MRI and NMR systems.
- Josephson junctions and SQUIDs for precision measurement.
- Cryogenic power cables and lossless energy transmission.
- Quantum computing components based on superconducting qubits.
Diagram to paste: superconductor-applications.png
This unit provides the essential concepts behind superconducting materials and their technological importance. If you need a one page revision sheet, ask for the Unit IV cheat sheet.