Introduction and Properties of Superconducting Materials: 25 flashcards
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1. What is a superconductor?
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A material that conducts electricity with exactly zero electrical resistance below its critical temperature $T_c$.
2. What are the two defining signatures of superconductivity?
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Zero electrical resistance and the Meissner effect (active expulsion of magnetic fields).
3. What is the critical temperature $T_c$?
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The temperature below which a material transitions into the superconducting state with zero resistance.
4. What is the $T_c$ of mercury, the first superconductor discovered?
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Approximately $4.2\,\text{K}$.
5. What is the approximate $T_c$ of niobium?
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About $9\,\text{K}$.
6. What are Cooper pairs?
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Two electrons bound together via lattice vibrations (phonons), behaving as a single boson-like quantum object.
7. Why does a superconductor have zero resistance?
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Cooper pairs condense into a single coherent quantum state where no scattering channel exists, so current flows without resistance.
8. What is the Meissner effect?
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The active expulsion of magnetic fields from the interior of a superconductor when it crosses $T_c$.
9. What produces the expelled magnetic field in the Meissner effect?
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Persistent surface currents that generate an opposing magnetic field.
10. How do Type I superconductors respond to magnetic fields?
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They expel magnetic fields completely up to a single critical field $H_c$, then lose superconductivity.
11. How do Type II superconductors respond to magnetic fields?
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They expel fields up to $H_{c1}$, then allow partial flux penetration as vortices until $H_{c2}$.
12. Which type of superconductor is used in most practical applications?
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Type II superconductors, such as Nb-Ti and YBCO, because they survive much stronger magnetic fields.
13. What does BCS theory explain?
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Low-temperature superconductivity via phonon-mediated electron pairing, predicting $T_c$ up to roughly $30\,\text{K}$.
14. What are high-temperature superconductors?
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Cuprate ceramics with $T_c$ above $77\,\text{K}$, the boiling point of liquid nitrogen.
15. Why is liquid nitrogen cooling significant for superconductors?
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Liquid nitrogen ($77\,\text{K}$) is far cheaper than liquid helium, making high-$T_c$ superconductors more practical.
16. What three limits must a superconductor stay below simultaneously?
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Critical temperature $T_c$, critical magnetic field, and critical current density.
17. How are superconductors used in MRI scanners?
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Nb-Ti superconducting coils generate strong, stable magnetic fields without resistive heating.
18. How do maglev trains use superconductors?
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They exploit the Meissner effect and flux pinning for frictionless levitation.
19. What are SQUIDs used for?
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Detecting extremely weak magnetic fields for medical and geological sensing.
20. What is the main barrier to widespread superconductor use?
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Cooling cost — most superconductors require expensive cryogenic cooling.
21. Has room-temperature, ambient-pressure superconductivity been confirmed?
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No — no confirmed room-temperature, ambient-pressure superconductor exists yet.
22. Why is a superconductor not just a 'very good conductor'?
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Even the best normal metals retain residual resistance; a superconductor's resistance is identically zero, a distinct phase of matter.
23. What happens to persistent currents in superconducting rings?
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They can flow for years without measurable decay.
24. What role do phonons play in superconductivity?
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Lattice vibrations (phonons) mediate the attraction that binds electrons into Cooper pairs.
25. Why can Cooper pairs condense into one quantum state?
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They behave as bosons (integer spin), so many can occupy the same lowest-energy state.
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