Pseudogap and Precursor Superconductivity Study of Zn doped YBCO [Hardcover]

In this thesis, the pseudogap and the precursor superconducting state, which are of great importance in clarifying the superconductivity mechanism in high-temperature cuprate superconductors, are investigated with a c-axis optical study in YBa₂(Cu_1-xZn_x)₃O_y. Testing was performed over a wide energy range with smaller temperature intervals for several Zn-substituted samples, as well as for several carrier-doping levels.

A spectral weight (SW) analysis, in which the pseudogap behavior can be separated from the superconducting condensate with the SW transfer to the high-energy region, revealed that the pseudogap is not the precursor of the superconductivity (carriers moving to the high-energy region with pseudogap opening never contribute to the superconducting condensation). Moreover, the high-energy transfer continues even below T_c for the Zn-substituted samples (in which we weaken the superconductivity), which gives evidence to the coexistence of the pseudogap and the superconducting gap below T_c.

On the other hand, the analysis of optical conductivity revealed that a precursor state to superconductivity can be defined at temperatures much higher than T_c. The superconducting carrier density (n_s) was calculated for each temperature (above and below T_c) and the results confirmed the existence of n_s at temperatures above T_c. The observed real superconducting condensate (n_s) above T_c puts a serious constraint on the theory l³°