Comparative studies of the NMR of thulium in Tm123 and Tm124 compounds: Evidence for structural and electronic phase separation

Abstract

The nuclear magnetic relaxation of ¹⁶⁹Tm in TmBa₂Cu₃O_6+x (x=0.1–1.0, Δ x=0.1) and TmBa₂Cu₄O₈ is studied at temperatures below 5 K. In all the samples, the Tm spin-lattice relaxation proceeds via intrinsic paramagnetic centers (PCs) like Cu²⁺ or copper-oxygen spin-polarized clusters. The experimental data for TmBa₂Cu₃O_6+x support the idea of the structural (chemical) micro-phase separation in oxygen-deficient 123 compounds. Apparently, the samples with x⩾0.4 contain hole-poor nonsuperconducting regions, enriched with PCs, and hole-rich (PC-poor) superconducting regions. The volume fraction f_n of the PC-rich phase reaches a maximum value of 0.85 at x=0.4 and decreases monotonically with increasing x (f_n=0.5, 0.3, and 0.25 at x=0.5, 0.6, and 0.7, respectively). The Tm spin-lattice relaxation in the underdoped TmBa₂Cu₄O₈ compound indicates that this sample, in contrast to oxygen-deficient TmBa₂Cu₃O_6+x, has a homogeneous composition. However, the Tm spin-spin relaxation measurements reveal two sorts of the Tm nuclear spins in Tm124, having different NMR spectra and different relaxation times T₂. The latter result is evidence of electronic phase separation in CuO₂ phases.