Hg199 and Cu63 NMR in superconducting HgBa2CuO4+δ oriented powder

Abstract

Hg199 NMR measurements have been performed both in the normal and in the superconducting state for an oriented HgBa2CuO4+δ superconducting powder sample with Tc=96 K. The large anisotropic Knight shift of Hg199, 199Kax=-0.15% at room temperature, is explained by the chemical shift related to the linear Hg-O(2) bonding configuration. Both 199Kiso and 199Kax decrease below Tc and scale linearly with each other in the whole temperature range investigated. The Hg199 Knight shift 199K slowly decreases with decreasing temperature on approaching Tc in the normal state, reflecting the decrease of the uniform spin susceptibility χ′(0,0) with lowering temperature. The Hg199 spin-echo decay can be fit by the product of a Gaussian component (TG−1) and an exponential one (TL−1). The Gaussian component TG−1 which is dominant above Tc, is shown to be due mainly to an indirect nuclear interaction via the conduction electrons (holes) and is found to be directly proportional to the spin contribution (199Ksp) of the Knight shift. The exponential component TL−1 becomes dominant well below Tc and is ascribed to the effect of thermal motion of flux lines. The Hg199 nuclear spin-lattice relaxation rate T1−1 in the normal state shows a Korringa behavior well above Tc with (T1T)−1=0.1 sec−1 K−1. Reduction of (T1T)−1 with decreasing temperature is observed starting about 10 K above Tc and is consistent with the decrease of χ′(0,0) in the normal state observed in K(T) and TG−1. 199Ksp(T) was extracted using the Korringa relation and below Tc, is found to fit the d-wave pairing scheme with a superconducting gap parameter 2Δ0=3.5kBTc. The d-wave pairing is also supported by the temperature dependence of Hg199 T1−1 in the superconducting state. The Cu63 T1−1 and T2−1 measurements have been performed in the normal state. In contrast to the Korringa behavior of Hg199 T1−1 in the normal state, the preliminary results show the increase of the Cu63 (T1T)−1 with decreasing temperature, indicating the enhancement of the antiferromagnetic fluctuations of Cu2+ moments common in the high-Tc cuprates. The reduction of Cu63 (T1T)−1 is observed starting above Tc and is compared with the decrease of Hg199 Ksp, TG−1, and (T1T)−1 in the normal state. The Cu63 nuclear spin-spin relaxation T2−1 is found to follow an exponential decay in the normal state and to decrease with decreasing temperature similar to the Hg199 Ksp and TG−1. © 1996 The American Physical Society.

Source-reported materials — not catalogue approval

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