Optical excitation of quasiparticle pairs in the vortex core of high-Tcsuperconductors

Abstract
A far-infrared resonance has been observed in superconducting YBa2 Cu3 O7 thin films in the presence of high magnetic fields. It corresponds to the quasiparticle pair creation process inside the vortex core. The resonance frequency is ω0=1.3kTc/ħ with a linewidth 1/τ≃0.6ω. This value for ω0 implies, within BCS theory, a large energy gap in YBa2 Cu3 O7.