INFRARED-EXCITED RAMAN-SCATTERING AND PHOTOLUMINESCENCE OF DEEP INTRAGAP STATES IN SEMICONDUCTING YBA2CU3O6+X

The semiconducting parent compounds of the high-T(c) superconducting s ystem YBa2Cu3O6+x, at low oxygen content (0 < x < 0. 25) and different oxygen isotopes, have been investigated by means of infrared (IR) exc ited (1.16 eV) Raman scattering and photoluminescence (PL). The IR-exc ited Raman spectrum of the YBa2Cu3O6 is dominated by a group of three bands in the spectral range of the apex oxygen O(4) Ag mode (475 cm-1 in the O-16 compound). The Raman spectrum changes dramatically upon ox ygen doping with the appearance of a strong resonant band at 507 cm-1 accompanied by several overtones. By studying site-selective O-18-isot ope-substituted samples this band is assigned to A(g) vibrations of ap ex O(4) atoms adjacent to the O(1) oxygen atoms occupying a chain site upon doping. The energy shift of this phonon and its associated overt ones is caused by a strong lattice relaxation around the O(4)-Cu(1)-O( 4) complex due to electronic charge-transfer processes involving O(4) sites. This behavior implies a strong coupling between O(4) vibration and charge-transfer excitations. PL, with a maximum at about 1.3 eV, h as been observed in samples with very low oxygen concentrations. Peak energy and intensity are temperature and doping dependent. Both Raman and PL data are interpreted assuming the existence of a narrow band, a ssociated with the O(4)-Cu(1)-O(4) complex (in YBa2Cu3O6+) which is lo cated, for small x, within the CuO2 plane charge-transfer gap.