EW METAL/SUPERCONDUCTOR-INSULATOR TRANSITIONS AND THEIR EFFECTS ON HIGH-TCSUPERCONDUCTIVITY INUNDERDOPED AND OPTIMALLY DOPED CUPRATES

Abstract.Anew approach to the metal/superconductor-insulator transition in doped cuprates by studying the polaron formation and localization of doped charge carriers (holes) in them and the possibility of transforming a metallic or superconducting system into an insulatorwas developed. Amore suitable criterion for such a phase transition by comparing the bandwidth (or Fermi energy) of large polarons with their binding energies in the cuprateswas derived. The possibility of the metal/superconductor-insulator transition and phase separation in doped cuprates resulting in the formation of competing metallic/superconducting and insulating phases in underdoped, optimally doped and even in overdoped high-Tccuprateswas predicted. Then the possible detrimental and beneficial effects of the different disorders (e.g. polaron formation and charge-density-wave transition) and the coexisting insulating and superconducting phases on the critical temperature 𝑇𝑐of the superconducting transition of underdoped and optimally doped cuprateswas examined. The actual superconducting transition temperature 𝑇𝑐in these materials using the theory of Bose-liquid superconductivity, and not theBardeen-Cooper-Schrieffer-like theory of Fermi-liquid superconductivity, which is incapable of predicting the relevant value of 𝑇𝑐in high-𝑇𝑐cuprateswas determined. We find thatthe suppressing of the polaronic and charge-density-waveeffects in optimally doped cuprates results in the enhancement of 𝑇𝑐, while some lattice defects (e.g., anion vacancies) in the cuprates may strongly affect, on 𝑇𝑐and enhance high-𝑇𝑐superconductivity in them.