Superconducting fluctuations in Bi ₂Sr ₂Ca _1-xDy _xCu ₂O _8+δ as seen by terahertz spectroscopy

More than one hundred years ago Paul Drude derived the frequency-dependent conductivity σ (ω) of a gas of point charges, showing that the width of the spectrum was related to the scattering rate of carriers. Seventy five years later, in the late 1970's and early 1980's, it was recognized that σ (ω) of a two-dimensional superconductor above it's transition temperature T _c could be described by the Drude response of interpenetrating gases of electrons and vortices. Here we describe measurements of σ (ω) in the high-T _c superconductor system Bi ₂Sr ₂Ca _1-xDy _xCu ₂O _8+δ (BSCCO) using time-domain THz spectroscopy that provide evidence for vortices above T _c. We compare this evidence with results obtained by other probes of fluctuations in cuprate superconductors.