SUPERCONDUCTORS

SUPERCONDUCTORS 

Basic Properties of Superconductors

• A superconductor shows zero electrical resistance below its critical temperature
• The Meissner effect means expulsion of magnetic field from the interior of a superconductor
• Superconductivity cannot exist at any temperature merely due to pressure, though high pressure may increase transition temperature in some materials

Types of Superconductors

Type I Superconductors

• Show almost complete expulsion of magnetic field below critical field

Type II Superconductors

• Allow partial magnetic field penetration through quantized flux vortices
• Used in:
  • MRI machines
  • Particle accelerators
  • High-field magnets

• Exist in mixed state (vortex state)
  • Magnetic flux enters in quantized vortices

Examples of Superconductors

• Mercury:
  • First discovered superconductor
  • Type I superconductor
• Niobium-Titanium (Nb-Ti):
  • Type II superconductor
  • Widely used in superconducting magnets
• YBCO (Yttrium Barium Copper Oxide):
  • Ceramic high-temperature superconductor

Theory of Superconductivity

• BCS Theoryexplains superconductivity through:
  • Formation of Cooper pairs
• Cooper pairs:
  • Paired electrons with opposite spin and momentum

Critical Parameters

• Superconductivity breaks when:
  • Critical current is exceeded

Applications of Superconductors

• MRI machines use superconducting magnets
• Maglev trains use superconducting magnetic levitation
• Used in power transmission cables to reduce energy loss

High-Temperature Superconductors

• Many have critical temperature above 77 K
  • Allows cooling using liquid nitrogen
• Often made of ceramic oxides

Advanced Superconductivity Concepts

• Very high-temperature superconductivity in some materials:
  • Requires extremely high pressure
• Hydride-based superconductors:
  • Show superconductivity at high temperatures
  • But only under extremely high pressure
• Magnetic field alone cannot create superconductivity

Core Properties

• Superconductors show zero electrical resistance below critical temperature
• They exhibit perfect diamagnetism (Meissner effect)
• Superconductors have zero resistance, not infinite resistance

Meissner Effect (Key Concept)

• Occurs only below critical temperature
• It distinguishes superconductors from perfect conductors
• Implies complete expulsion of magnetic field lines from the interior

Theory (Quantum Nature)

• Cooper pairs:
  • Paired electrons
  • Act like bosons
  • Enable condensation into a single quantum state

Destruction of Superconductivity (Critical Limits)

Superconductivity is destroyed if:

• Temperature > Critical Temperature
• Magnetic Field > Critical Field
• Current > Critical Current