Magnetic scaling in the underdoped superconductor Hg_0.8Re_0.2Ba₂Ca₂Cu₃O₈ studied by ⁶³Cu NMR

An approach has been made to deduce the second moment contribution of the planar Cu NMR line from indirect nuclear couplings, using the middle plane sites of the trilayer superconductor, Hg_0.8Re_0.2Ba₂Ca₂Cu₃O₈(T_c=126K). We find that T₁ fluctuations of neighboring nuclear spins in the plane are the dominant causes of the spin-echo decay observed in the present case of an external field perpendicular to the c axis. Taking account of this effect in the analysis, the second moment is deduced. From the T dependences of the T₁ and second moment thus obtained, we find the dynamical exponent, z=2. The present result indicates that the boundary of z crossing from 2 (overdamped regime) to 1 (quantum critical regime) may be shifted more closely to the antiferromagnetic phase than previously assumed. It is also shown that the pseudogap is not a specific feature to the z=1 region.