Extrinsic carrier self-trapping and metal-insulator transitions in doped high-T_c cuprates

摘要

The self-trapping of hole carriers near the dopants (or impurities) and the metal-insulator transitions (MITs) were studied theoretically. The ground state energies of the carriers interacting with impurities and lattice vibrations as well as with each other in the three-dimentional polar cuprates are calculated variationally within the continuum model and adiabatic approximation. We have shown that the extrinsic self-trapped states (i.e. bound states of the impurity and large (bi)polarons) are formed in the charge-transfer gap of the lightly doped cuprates and these localized impurity states form the narrow in-gap impurity bands. The MITs in the cuprates occur when the in-gap impurity bands merge with the oxygen valence band at some critical hole concentrations. We have obtained the quantitative criterion for the MIT driven by the extrinsic carrier self-trapping.