Y89NMR evidence for a fermi-liquid behavior inYBa

Abstract

We report NMR shift AK and T\ data of 89 Y taken from 77 to 300 K in YBa 2 Cu 3 0 6 +x for 0.35 < x < 1, from the insulating to the metallic state. A Korringa law and therefore a Fermi-liquid picture is found to apply for the spin part K 5 of AK. The spin contribution x*ix 9 T) to Xm is singled out, as the T variation of AK scales linearly with the macroscopic susceptibility # m . This implies that Cu(3<i) and Oilp) holes do not have independent degrees of freedom. Their hybridization, which has a a character, hardly varies with doping. These results put severe constraints on theoretical models of high-r c cuprates.The interplay between the magnetic properties of Cu(3rf) holes and the charge transport mediated by the Oilp) holes is still a highly controversial question in high-TV materials. While T\ nuclear-spin-lattice relaxation data on 89 Y and 63 Cu at the superconducting transition 1,2 indicate that both hole systems are involved in the superconducting pairing, some authors suggest that the persistence of 2D antiferromagnetic spin fluctuations 3 above T c indicates that the Cui3d) spins could somehow be decoupled from the charge transport mediated by the Oilp) holes. 4 This could result for instance from weak Cui3d)-Oilp) n bonding at the Fermi level. 5 Such ideas have recently led Johnston 6 to attempt a partition of the macroscopic susceptibility % m in high-T c materials into a ^-dependent part attributed to the lattice of Cu 2+ spins and a Pauli term associated with charge carriers. In order to gain local insight on these properties, NMR measurements are highly desirable. 89 Y nuclei are direct probes 7,8 of the susceptibility of the CuCh planes. We report here 9 an extensive experimental study of both the NMR shift K s and the nuclear relaxation rate T{~1 of 89 Y, which allows for the first time a direct comparison of the static and dynamic susceptibilities in these materials from the metallic to the semiconducting state. These results provide evidence that both K s and iT\T) ~l are T dependent in the metallic state as soon as x departs from unity. A Korringa relation between T\T and K s is established and indicates that a Fermi-liquid picture holds. Our data suggest that this behavior might extend as well into the semiconducting state. Further, K S9 which probes the T dependence of x on the oxygen sites, 7,8 is found to scale with # m , which is dominated by % on the copper sites. 10 Therefore the T variation of Xm should in no case be attributed solely to the Cu 2_H spins and Cu(3rf) and 0(2/?) holes do not have independent degrees of freedom.NMR data have taken on the same powdered ceramic samples as in Ref. 7, which had been deoxygenated at low T (~500°C) and immediately sealed in Pyrex vials in a 4 He atmosphere. Our initial JC™1 sample (now la-beled x -B 1 -e) had been left about two weeks in air before being sealed and has probably lost some oxygen. Samples kept now for more than one year in such vials did not show any change in their properties. The positions AK of the NMR line relative to a YCI...

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