MAGNETIC AND TRANSPORT-PROPERTIES OF 214 OXIDES - DEPENDENCE ON DOPING

We consider how doping can be described in terms of the charge-transfe r insulator concept. We discuss and compare a few models for the band structure for the doped charges. This has led us to the conclusion tha t the band structure stability problem is one of the main issues in an y correspondence between results for the t-J model and, say, the three -band model for the slightly doped layered oxides. The stability crite rion is formulated and its implications discussed. Provided a phenomen ological conduction band is chosen to satisfy the criterion of stabili ty, a detailed picture of how dopants influence the spin wave spectrum at T = 0 is presented. The basic physics for the destruction of the a ntiferromagnetic (AF) long-range order is rather model-independent: th e long-range order (at T = 0) disappears due to the Cerenkov effect wh en the Fermi velocity first exceeds the spin wave velocity. We then di scuss the overall spectrum of spin excitations and see that the spin w ave attenuation for x < x(c), T = 0 due to Landau damping appears in t he range of magnon momenta k(x) = 2ms +/- a square-root x. We also ar gue that in the presence of superconductivity, the Cerenkov effect is eliminated due to the gap in the spectrum. This may restore the role o f the AF fluctuations as the main source of dissipation at the lowest temperatures. A brief discussion of how interaction with magnons may a ffect the hole spectrum concludes the paper.