Polarised XAS study of anomalous temperature dependence of aggregation of itinerant holes and pair formation in a YBa2Cu3O7-d single crystal

A polarised X-ray absorption study is made on an underdoped YBCO substantially detwinned single crystal to study the anomalous temperature dependence of itinerant holes, charge aggregation and pair formation along the E∥b and E∥a directions. The measurements were made at the O K and Cu L3 edges varying the temperature from 10 K to room temperature (∼300 K) using a liquid helium cryostat. In earlier reports on the YBCO reported observing the charge aggregation and the Cooper pair formation taking place only once each at temperatures and , both greater than Tc we observe these phenomena repeating themselves at two more temperatures, still in the normal state though. We have observed much smaller magnitude of the fluctuations of the itinerant holes in the ‘a’-direction as compared to ‘b’-direction in our experiment. Our results at T<40 K appear to go against the hypothesis that treats stripe formation essential to superconductivity in the cuprate perovskite systems.