DECREASE OF THE HOMOGENEOUS WIDTH OF THE TM3+ ELECTRON-ENERGY LEVELS IN TMBA2CU3O6+X AT THE SUPERCONDUCTING TRANSITION

Enhanced NMR of Tm-169 (I = 1/2) enhanced NMR has been studied in TmBa2CU3O6+x, X = 0, 0.2, 0.4, at temperatures 1.2-40K. The nuclear magnetic relaxation rates at T > 20K increase exponentially as 1/T1(2) = R1(2)exp(-DELTA/kT), with DELTA = 176 K being the energy of the first excited state of the Tm3+ ion, while the R1(2) pre-exponential factors are inversely proportional to the homogeneous width GAMMA(o) of this doublet state. The pre-exponential factors measured at the frequencies of 48 and 69 MHz show homogeneous broadening of the 4f-levels due to the decreasing dipole-dipole transfer of the electron excitations from ion to ion at increasing x because of crystal structure disordering. A sharp decrease of GAMMA(o) (by about 0.9K) is observed in the superconducting compound with x = 0.4 (T(c) onset at 20K) at low temperatures and is attributed to the weakening of the 4f-electron-phonon interaction. At the temperatures around 6K the same sample displays enhancement of the transverse relaxation due to the relatively slow (W almost-equal-to 10(9)s-1) fluctuations of the internal magnetic fields caused by Cu2+ ions in CuO2 planes.