RESONANT RAMAN-SCATTERING IN INSULATING YBA2CU3O6 AS A PROBE OF ITS ELECTRONIC-STRUCTURE

In order to investigate the electronic structure as well as the origin of the intensities of Raman phonons in the antiferromagnetic insulator YBa2Cu3O6, we have measured Raman intensities in absolute units as a function of laser wavelength. The results yield information not only about the energies of interband transitions but also about their orbital character and real-space sites. For xx polarization, the vibrations of atoms in the CuO2 planes exhibit resonances near 1.7, 2.1, and 2.5 eV, which are absent in zz polarization. Both peaks in the resonance profiles can be attributed to "charge-transfer transitions" from the plane oxygen p(x) or p(y) bands to the upper Hubbard band having mainly plane copper d(x2-y2) character. From the absence of any intensity for the Ba and apical oxygen phonons, we infer that their orbitals, and also plane oxygen p(z) orbitals, do not contribute to the xx-polarized dielectric response in the investigated energy range. We predict, however, large intensities for the apical oxygen phonon at laser energies near 4.1 eV. For zz polarization, we observe a broad resonance around 2.1 eV, although no critical point has been identified at this energy in the dielectric function. This structure can be assigned to transitions from the "dumbbell" Cu d3z2-r2-O p(z) band to the upper Hubbard band of the CuO2 planes. The former has only very small matrix elements to the final-state bands; still, large zz-polarized Raman intensities can be observed, since this band is strongly modulated by the motion of the apical oxygen atom. It is, hence, justified to consider the planes as isolated units in the insulating material. A large Hubbard splitting is only present for the plane bands whereas the chain Cu atoms seem to be rather unaffected by correlation effects. These results should encourage calculations of the Raman intensities with reasonable models of the electronic structure which include correlations. Under resonance conditions, four infrared-active LO phonons of E(u) symmetry become Raman active via Frohlich interaction. We show that a small concentration of crystal imperfections is sufficient to make this scattering mechanism independent of the direction of the laser wave-vector transfer k(L).