Characterization of localized hole states in Pr_(1+x)Ba_(2-x)Cu_(3)O_(6+y) by nuclear magnetic resonance

摘要

We investigated the charge distribution inPr_{1+x}Ba_{2-x}Cu_3O_{6+y}-crystals at liquid-He temperature by means of^{63,65}Cu- and ^{141}Pr-NMR and NQR. The electric field gradients determinedfor the different oxygen coordinations of Cu(1) on the chains confirm the modelthat the hole states are localized in the O 2p - pi-orbitals of theCuO_(2)-planes. The intensity and line-shape analysis of the Cu(1)- andPr-resonances in the oxygen doping series (x approx 0, y=0..1) and in the Pr/Basolid-solution series (at y=1) allows us to assign the Pr-resonance to Pr atRE-sites. Therefore, we can investigate the charge distribution in theCuO_(2)-Pr-CuO_(2)-layers by crystal field analysis of the Pr-signal. Toconsistently describe our results with neutron scattering data we propose thattwo Pr-states are present in the RE-layer, and ascribe them to Pr with and onewithout a hole localized in the oxygen coordination shell. NMR detects only theformer, with a virtually nonmagnetic singlet ground state, while space-integraltechniques are dominated by the latter, due to a quasi-doublet ground state andantiferromagnetism of its Pr-moments at low temperature.