Novel mechanism of high-temperature superconductivity in the pseudo- gap state and radiation effects in under- doped and optimally doped copper oxides

Full text: The discovery of high-TC superconductivity in the ceramic copper oxides (curpates) has become powerful stimulus for developing the fundamental and applied research in the field condensed matter physics. Despite a huge amount of experimental and theoretical studies of the physical properties of the curpates semiconductors, the microscopic mechanism of superconductivity in these high-Tc superconductor (HTSC) still remains mysterious. Another puzzling feature and under doped curpates is the appearance of a pseudo gap (PG) in their excitation spectrum. In this talk, the review of the basic theoretical ideas and experimental results concerning the PG phenomena and unusual high-Tc superconductivity in the underdoped and optimally doped oxide HTSC is presented. We have critically analyzed the validity and adequacy of the existing theoretical scenarios for describing the new PG state of matter and the mechanism of high-Tc superconductivity in these HTSC systems. We have shown that most theoretical scenarios (such as, for example, the precursor superconducting (SC) fluctuations, the spin-charge separation, the nearly antiferromagnetic Fermi-liquid and marginal Fermi-liquid) based on the strong electron correlation and neglecting the important role of the electron-phonon interactions characteristic of polar oxide HTSC are faced with serious difficulties in describing the origins of the PG and SC states and their relationship. Unlike these theoretical approaches the new two-stage Fermi-Bose-liquid model going beyond the usual Fermi-liquid, BCS and Bose-Einstein condensation (BEC) theories takes into account properly the electron-phonon interactions in the strong and intermediate coupling regimes and describes consistently the fluctuationless formation of the non-coherent Cooper pairs above the SC transition temperature Tc and subsequent Cooper-pair condensation into a superfluid (SF) Bose-liquid state at T≤TC or establishment of their long-range phase coherence at T≤TC. The new approaches to the problems of the precursor BCS-like pairing of large polarons at intermediate electron-phonon coupling regime and the SF Cooper-pair condensation is developed. The novel superconductivity of Cooper-like polaron pairs in the PG state is described as their condensation into a SF Bose-liquid state which is quite different from the usual BEC state. We have also studied the radiation effects on the formation of the PG state and the condensation of the Cooper-like polaron pairs into a SF or SC Bose-liquid state in the underdoped and optimally doped oxide HTSC. It is demonstrated that the two-stage FBL scenario is more realistic scenario than other ones and in better agreements with existing well- established experimental data on underdoped and optimally doped cuprates