EVIDENCE FOR POLARONS AND ITINERANT CARRIERS IN SUPERCONDUCTIVITY FROM TIME-RESOLVED OPTICAL MEASUREMENTS ON YBA2CU3O7-DELTA

Time-resolved optical relaxation measurements on time scales from 10(- 13) to 10(-8) s as a function of energy (0.8-3 eV) and temperature giv e evidence for the existence of two different carrier relaxation proce sses, indicating the presence of two distinct electronic subsystems in the normal state in high-temperature superconductors, one exhibiting band-like relaxation properties and the other those of localized, pola ronic states. Both components of the electrodynamic response exhibit s ignificant changes at T-c, indicating that both carrier types may be i nvolved in superconductivity. The experiments also show that both pola rons and band-like carriers exist predominantly in the CuO planes, wit h an out-of-plane component of the order of 10-20% which is close to t he experimentally observed occupancy ratio of holes of O ions of the C uO5 pyramids. Spectrally, the superconducting condensate is also found to be coupled to two distinct components in frequency (for both carri ers): one below 0.1 eV and one in the range 1.5-2 eV, but with differe nt anisotropies. A natural explanation of these datas is that the two types of carriers correspond to the two phases suggested by the struct ural data and as such present the first direct evidence of two-compone nt superconductivity. The underdoped sample data can be interpreted as evidence of pre-formed pairs above T-c.