CRYSTAL-FIELD EXCITATIONS AND GAP OPENING IN TM-YBA2CU4O8 BY INELASTIC NEUTRON-SCATTERING

We have performed a neutron spectroscopy study of Tm doped YBa2Cu4O8 u sing the direct geometry time-of-flight chopper spectrometer HET of th e ISIS facility at the Rutherford Appleton Laboratory. Two samples wer e 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 b een performed by means of a perturbation procedure, allowing us to obt ain a reliable starting set of crystal field (CF) parameters for the f itting of the experimental profiles. We have investigated the temperat ure behaviour of the linewidth F for the two strong transitions from t he ground singlet to the quasi-doublet centered at approximately 13 me V, 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 transitio ns between the excited states at high temperature, we have observed al so in YBCO(124) an abrupt decrease of GAMMA below T(w) congruent-to 12 5 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, co rresponding, 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 chara cter in the highest-T(c) compounds. Since CF spectroscopy probes the d ynamical susceptibility for omega = omega(CF), the neutron results are also discussed in comparison with NMR spectroscopy, giving complement ary information on Im [chi(q, omega)] for omega-->0.