Results of measurements of the temperature dependence of the absorption spectra of YBa2Cu3O6+x films in the antiferromagnetic and metallic (superconducting, Tc=88 K) phases are presented. The temperature evolution of the absorption bands, which are diagnostic for the electron and spin correlations and the degree of metallization (oxygen–copper hybridization), is investigated in the energy region 1.3–2.6 eV. It is shown that in the metallic phase in the pseudogap state below T*=120±10 K there is a sharp rise of these absorption bands characteristic of the antiferromagnetic (AF) phase, including the electron–two-magnon (A+J) band, which reflects the development of strong AF correlations. In contrast to the insulator, here the (A+J) band has a fine structure due to the creation of both interacting and noninteracting magnons, and it coexists with the bands due to the strong metallization. The results demonstrate phase separation at Tc<T<T*, with the formation of a stripe superstructure consisting of metallic and insulating (with short-range antiferromagnetic ordering) regions. This superstructure is preserved in the superconducting state, and an additional enhancement of the AF correlations occurs at the superconducting transition. Taken together, the results provide independent optical evidence of the undoubted participation of AF correlations in the superconductivity.
It has been established as a result of successful direct experimental studies of the symmetry of the superconducting order parameter that the pairing symmetry in the compounds YBCO, GdBCO, Tl2201, and Bi2212 is of the d-wave type. In this paper, experimental and theoretical results concerning the d-wave superconducting state and the peculiarities of magnetic properties of the d-type superconductor such as pseudogap, charge ordering, and unusual structure of vortex excitations differing from that for s-states are analyzed. Several mechanisms of d-pairing and related microscopic theories are discussed and possible experiments for their verification are proposed.
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