Insulator-superconductor-metal transitions induced by spin fluctuations inthe paramagnetic phase of doped insulators

The band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding subst itution atoms, and the phase diagram of the paramagnetic states as a functi on of the degree x of doping at zero temperature is studied. The Hamiltonia n contains electronic correlations on impurity orbitals and hybridization b etween them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the for mation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an i nsulator state of the doped compound for x < x(thr, 1), (2) lead to a super conducting state for x(thr, 1) < x < x(thr, 2), and (3) decay as x > x(thr, 2) increases further, and the doped compound transforms into a paramagneti c state of a "poor" metal with a high density of localized electronic state s at the Fermi level. (C) 2000 MAIK "Nauka/Interperiodica".