Energy spectrum, magnetic characteristics, and spin dynamics of lightly doped 2:1:4 oxides

Abstract

Analyses of ${}_{}{}^{63}{}_{}{}^{}\mathrm{Cu}$ and ${}_{}{}^{17}{}_{}{}^{}\mathrm{O}$ relaxation rates, as measured by NMR probes, prove unambiguously that in conducting ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and ${\mathrm{La}}_{1.85}$ ${\mathrm{Sr}}_{0.15}$ ${\mathrm{CuO}}_4$ the ${\mathrm{Cu}}^{2+}$ ions still possess a magnetic moment that exhibits antiferromagnetic fluctuations. We discuss the corresponding characteristics in the low-doped regime from the side of the Mott-insulator state, where the local moments are expected to be even more pronounced. Both neutrons and NMR probes measure the imaginary part of the generalized magnetic susceptibility. We have investigated the contribution to the dissipation due to added holes. Then, depending on the hole spectrum chosen, both bulk and staggered static susceptibilities in the high-temperature paramagnetic phase are strongly concentration dependent. The corresponding exchange-integral components in the absence of holes can be strongly enhanced, but their signs do not always lead to an instability. At the same time, the imaginary part of the generalized susceptibility, as determined by the hole spectrum, behaves independently and can provide strong peaks at some wave vectors even if the static part has no singular behavior there. We identify the single model energy spectrum, which is compatible with the properties of 2:1:4 materials, and discuss the ‘‘metallization’’ of ${\mathrm{La}}_{2\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{CuO}}_4$ with the increase in strontium concentration. While the transport properties can be understood in terms of carriers present in the conducting bands, the static magnetic susceptibility behavior reveals the overwhelming role of spin-spin correlations. Shortcomings of the model are also discussed.