The coexisting of insulating and metallic/superconducting phases and their competing effects in various underdoped cuprates

We show that the unconventional electron–phonon interactions and polaronic effects, charge inhomogeneity and charge ordering in underdoped cuprates result in the nanoscale phase separation and the occurrence of competing and coexisting of insulating and metallic/superconducting phases. We identify possible types of localized and mobile charge carriers in these systems, which segregate into insulating (carrier-poor) and metallic/superconducting (carrier-rich) regions as a result of their specific ordering. We found that the coexistence of two competing insulating and metallic phases can persist in the lightly doped cuprates on a local scale, while the coexistence of three competing insulating, metallic and superconducting phases is expected in the underdoped cuprates on a global scale. We demonstrated that the competing effects of these coexisting insulating and metallic/superconducting phases are manifested in the unusual temperature dependences of the magnetic susceptibility and resistivity and in the suppression of superconductivity in various underdoped cuprates.