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Register a new userCharacterization of localized hole states in Pr_(1+x)Ba_(2-x)Cu_(3)O_(6+y) by nuclear magnetic resonance
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We investigated the charge distribution in Pr_{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 determined for the different oxygen coordinations of Cu(1) on the chains confirm the model that the hole states are localized in the O 2p - pi-orbitals of the CuO_(2)-planes. The intensity and line-shape analysis of the Cu(1)- and Pr-resonances in the oxygen doping series (x approx 0, y=0..1) and in the Pr/Ba solid-solution series (at y=1) allows us to assign the Pr-resonance to Pr at RE-sites. Therefore, we can investigate the charge distribution in the CuO_(2)-Pr-CuO_(2)-layers by crystal field analysis of the Pr-signal. To consistently describe our results with neutron scattering data we propose that two Pr-states are present in the RE-layer, and ascribe them to Pr with and one without a hole localized in the oxygen coordination shell. NMR detects only the former, with a virtually nonmagnetic singlet ground state, while space-integral techniques are dominated by the latter, due to a quasi-doublet ground state and antiferromagnetism of its Pr-moments at low temperature.
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We have performed ^{75}As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) on single crystalline Ba_{1-x}K_{x}Fe_{2}As_{2} for x = 0.27-1. ^{75}As nuclear quadruple resonance frequency ({ u}_{Q}) increases linearly with increasing x. The Knight shift K in normal state shows Pauli paramagnetic behavior with slight temperature T dependence. The value of K increases gradually with increasing x. By contrast, nuclear spin- lattice relaxation rate 1/T_{1} in normal state has a large T-dependence, which indicates existence of large antiferomagnetic (AF) spin fluctuations for all x. The T-dependence of 1/T_{1} shows a gap-like behavior below approximately 100 K for 0.6 < x < 0.9. These behaviors are well explained by the change of band structure with expansion of hole Fermi surfaces and shrink and disappearance of electron Fermi surfaces at Brillouin zone (BZ) with increasing x. The anisotropy of 1/T_{1}, represented by a ratio of 1/T_{1ab} to 1/T_{1c}, is always larger than 1 for all x, which indicates that the stripe-type AF fluctuations is dominant in this system. The K in superconducting (SC) state decreases, which corresponds to appearance of spin-singlet superconductivity. The T dependence of 1/T_{1} in SC state indicates multiple-SC-gap feature. A simple two gap model analysis shows that the larger superconducting gap gradually decreases with increasing x from 0.27 to 1 and smaller gap decreases rapidly and nearly vanishes for x > 0.6 where the electron pockets in BZ disappear.
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