Crystal-field excitations and gap opening in Tm: YBa2Cu4O8 by inelastic neutron scattering

✍ Scribed by G. Amoretti; R. Caciuffo; P. Santini; O. Francescangeli; E.A. Goremychkin; R. Osborn; G. Calestani; M. Sparpaglione; L. Bonoldi

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 756 KB
• Volume: 221
• Category: Article
• ISSN: 0921-4534
• DOI: 10.1016/0921-4534(94)90227-5

✦ Synopsis

We have performed a neutron spectroscopy study of Tm doped YBa2Cu4Os 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 Ei = 25.2 meV and a 30% doped one with El = 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 profdes. 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 ~ 13 meV, in order to confirm the linewidth change which was observed for Tm 3 + CF excitations in pure and in Zn doped YBCO ( 123 ) at a temperature Tw higher than To. 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 F below Tw = 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 Tw/Tc in the three compounds measured up to now, it appears that Tw/Tc tends to 1 at a temperature Tm-~ 93 K, corresponding, within the error bar, to the transition temperature of an ideally "perfect" superconductor of this family. This behaviour of Tw has been tentatively attributed to the greater extent of 3D character in the highest-T~ compounds. Since CF spectroscopy probes the dynamical susceptibility for co= cocr, the neutron results are also discussed in comparison with NMR spectroscopy, giving complementary information on Im [)(q, co) ] for co-,0.