BCS Quasiparticle Formation

© David Pines.

Because pairs of normal state quasiparticles, depicted here by the blue lines, can scatter in and out of the condensate, a macroscopically occupied state made up of pairs of particles of opposite spin and momentum, shown here in purple as , a BCS quasiparticle of momentum p and spin up, depicted here by the red double line, will necessarily be a mixture of a quasiparticle of momentum p and spin up, and a quasihole of momentum -p and spin down. The condensate strength, , is the superconducting order parameter; it is also the energy gap found in the quasiparticle spectrum; as the temperature increases above absolute zero, that strength and gap decrease, until both vanish at the superconducting transition temperature, T_c. (Unit: 8)