Q6Electrical Materials

Origin: superconductivity appears in many metals, alloys and ceramic oxides when cooled below a material-specific critical temperature Tc. Its microscopic origin (BCS theory) is the formation of Cooper pairs: at sufficiently low temperature, a weak attractive interaction mediated by lattice vibrations (one electron slightly deforms the lattice, and a second electron is attracted to that deformation) binds electrons into pairs that condense into a single coherent quantum state. This paired condensate cannot exchange energy with the lattice in the small amounts ordinary scattering requires — an energy gap separates it from excited states — so the usual resistive scattering mechanism is completely suppressed.

Zero resistance: below Tc the DC resistance is not merely small but exactly zero — persistent currents induced in superconducting rings have been observed to circulate for years without measurable decay, a direct experimental demonstration that ρ = 0 in the superconducting state. The transition at Tc is abrupt, and the zero-resistance state persists only while temperature, applied magnetic field, and current density all remain below their critical values (Tc, Hc, Jc); exceeding any one of them quenches the superconductivity and restores normal resistive behavior.

Meissner effect: when a superconductor is cooled through Tc in an applied magnetic field, it actively expels the magnetic flux from its interior (B = 0 inside), behaving as a perfect diamagnet. This is a stronger statement than zero resistance alone: a hypothetical merely-perfect conductor would trap whatever flux was present when it lost resistance, whereas a superconductor ejects pre-existing flux — proving superconductivity is a distinct thermodynamic phase, not just infinite conductivity. The expulsion is accomplished by persistent surface screening currents that generate a field exactly cancelling the applied field internally. The Meissner effect underlies magnetic levitation demonstrations and defines Type-I behavior (complete expulsion up to Hc); Type-II superconductors instead admit quantized flux vortices between two critical fields Hc1 and Hc2 while remaining superconducting, which is what allows practical high-field superconducting magnets (NbTi, Nb₃Sn) used in MRI machines and research magnets.