Tm-169 NMR Study of the High-T-c Superconductor TmBa2Cu3O6.92

The ground state of Tm3+ ion (4f(2), H-3(6)) in a crystal electric field (CEF) of TmBaCuO is a singlet, separated from the lowest excited states by an energy gap of 100 divided by 130 cm(-1). The pulsed NMR of Tm-169 nuclei ( spin I = 1/2) in a magnetically-oriented TmBa2Cu3O6.92 powder is studied at temperatures 1.5 divided by 4.2 K. Two sorts of Tm nuclei are observed: rapidly relaxing Tm1 (>= 80% of total amount, having relaxation times T-1((1))approximate to 35 ms, T-2((1)) = 80 divided by 250 mu s) and slowly relaxing Tm2 nuclei (<= 20%, T-1((2)) approximate to 350ms, T-2((2)) = 300 divided by 1000 mu s). The Tm-169 NMR spectra are described by the spin-Hamiltonian H-1 =-h Sigma gamma iHiIii (i = x, y, z), where |gamma((1))(x)/2 pi| = 5.3(1), |gamma((1))(y)/2 pi| = 6.6(1), |gamma((2))(x)/ 2 pi|=|gamma((2))(y)/2 pi| = 5.1(1), |gamma((1))(z)/2 pi| = |gamma((2))(z)/2 pi| = 2.3(1) kHz/Oe, and axes x, y, z coincide with the crystal axes a, b, c. The Tm1 and Tm2 nuclei are identified as those belonging to orthorhombic ( CEF of D-2h symmetry) and tetragonal (D-4h) phases of the TmBaCuO compound, respectively. Two-exponential and nearly temperature-independent nuclear relaxation is observed at temperatures 1.5 divided by 4.2 K. The smallest T-2((1))-values (approximate to 80 mu s) found for the orthorhombic (superconducting) phase in an external field H perpendicular to the crystal c-axis agree fairly well with those one could expect due to the dipole-dipole interaction of thulium nuclei. However, the slowing down of this rapid relaxation of the nuclear transverse magnetization in a field H parallel to cc as well as the origin of unusually large T-2((2))-values remain unclear.