We have performed a neutron spectroscopy study of Tm doped YBa2Cu4O8 using the direct geometry time-of-flight chopper spectrometer HET of the ISIS facility at the Rutherford Appleton Laboratory. Two samples were examined: a 10% doped sample with incident energy E(i)=25.2 meV and a 30% doped one with E(i) = 120 meV. The analysis of the spectra has been performed by means of a perturbation procedure, allowing us to obtain a reliable starting set of crystal field (CF) parameters for the fitting of the experimental profiles. We have investigated the temperature behaviour of the linewidth F for the two strong transitions from the ground singlet to the quasi-doublet centered at approximately 13 meV, in order to confirm the linewidth change which was observed for Tm3+ CF excitations in pure and in Zn doped YBCO (123) at a temperature T(w) higher than T(c). After correcting for the effect of the transitions between the excited states at high temperature, we have observed also in YBCO(124) an abrupt decrease of GAMMA below T(w) congruent-to 125 K, which could be attributed to the opening of an energy gap in the spectrum of the spin excitations in the CuO2 planes. By comparing the ratio T(w)/T(c) in the three compounds measured up to now, it appears that T(w)/T(c) tends to 1 at a temperature T(cm) congruent-to 93 K, corresponding, within the error bar, to the transition temperature of an ideally ''perfect'' superconductor of this family. This behaviour of T(w) has been tentatively attributed to the greater extent of 3D character in the highest-T(c) compounds. Since CF spectroscopy probes the dynamical susceptibility for omega = omega(CF), the neutron results are also discussed in comparison with NMR spectroscopy, giving complementary information on Im [chi(q, omega)] for omega-->0.
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