Superconductivity

Superconductivity is characterised by the sudden drop in resistance and the occurrence of the Meisner effect – expelled magnetic field.

Odd-frequency superconductivity: Due to the fermionic nature of electrons, the spatial symmetry (s-wave, p-wave, d-wave, etc.) of a superconducting gap is intimately related to the spin state (singlet or triplet) of the Cooper pairs making up the condensate. In the limit of equal-time pairing this relationship is quite simple, even-parity gaps (like s-wave, or d-wave) correspond to spin singlet states while odd-parity gaps (like p-wave or f -wave) correspond to spin triplet states. However, if the electrons are paired at unequal times the superconducting gap could be odd in time or, equivalently, odd in frequency, in which case the condensate could be even in spatial parity and spin triplet or odd in spatial parity and spin singlet.

Superconducting strontium titanate: SrTiO_3 is an unusual superconductor for two main reasons: low T_c c. 0.4 K and low carrier concentrations. The origin of the superconducting pairing is still not clear (despite being known as a superconductor since 1965), and one possible source is the ferroelectric phonons in this quantum paraelectric.