Singlet-ground-state paramagnetic centers in CuO[sub 2] layers as seen from [sup 169]Tm NMR in TmBa[sub 2]Cu[sub 3]O[sub 6+[ital x]] superconductors

The [sup 169]Tm nuclear spin-lattice relaxation in oxygen-deficient TmBa[sub 2]Cu[sub 3]O[sub 6+[ital x]] compounds, as quenched and room-temperature annealed, has been measured at low temperatures. The results are consistent with the existence of paramagnetic centers in the CuO[sub 2] double layer, which have a nonmagnetic (singlet) ground state separated from an excited magnetic state by an energy gap of the order of 1 K.