Superconductivity Basics: 18 flashcards
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1. What is a superconductor?
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A material that exhibits zero electrical resistance and perfect diamagnetism below a critical temperature.
2. Define critical temperature (Tc).
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The temperature below which a material becomes superconducting, showing zero resistance.
3. What is the Meissner effect?
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The expulsion of all magnetic fields from the interior of a superconductor when it transitions to the superconducting state.
4. What is the critical magnetic field (Hc)?
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The maximum magnetic field that a superconductor can withstand before losing its superconducting state.
5. What is the critical current density (Jc)?
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The maximum current density that a superconductor can carry without losing its superconducting property.
6. What is a Type I superconductor?
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A superconductor that exhibits a sharp transition to the normal state at a single critical magnetic field, showing the Meissner effect perfectly.
7. What is a Type II superconductor?
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A superconductor that transitions gradually through a mixed state (vortex state) between lower and upper critical fields, allowing partial magnetic field penetration.
8. What is the BCS theory?
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The theory that explains conventional superconductivity as resulting from the formation of Cooper pairs of electrons mediated by lattice vibrations (phonons).
9. What is a Cooper pair?
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A pair of electrons with opposite spins and momenta that are bound together by phonon exchange, enabling superconductivity.
10. What is the energy gap in a superconductor?
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An energy gap around the Fermi level where no single-electron states exist, requiring energy to break Cooper pairs.
11. What is a high-temperature superconductor?
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A superconductor that operates above the boiling point of liquid nitrogen (77 K), typically cuprate ceramics.
12. What is the London penetration depth?
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The characteristic length scale over which an external magnetic field decays to zero inside a superconductor.
13. What is the coherence length in a superconductor?
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The characteristic length scale over which the superconducting order parameter (and Cooper pair density) varies.
14. What is the isotope effect in conventional superconductors?
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The critical temperature (Tc) varies with the isotopic mass of the lattice atoms, indicating that lattice vibrations (phonons) are involved in the pairing mechanism.
15. What is a Josephson junction?
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A device made of two superconductors separated by a thin insulating barrier, allowing supercurrent to flow via quantum tunneling.
16. What is a SQUID?
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A Superconducting Quantum Interference Device, used to measure extremely small magnetic fields with high sensitivity.
17. What is flux quantization in a superconducting ring?
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The magnetic flux through a superconducting loop is quantized in multiples of the flux quantum, Φ0 = h/(2e).
18. What is the primary application of superconductors in MRI machines?
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They are used to generate strong, stable magnetic fields for imaging, typically using niobium-titanium (NbTi) superconducting magnets.
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