Anisotropy of the dielectric function in YBa2Cu3O6

Abstract

Experimental data for the anisotropy of the dielectric tensor ε=${\epsilon }_1$+i${\epsilon }_2$ in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_6$ are presented and interpreted. The spectra in the range between 1.7 and 5.2 eV were obtained by rotating-analyzer ellipsometry using a single crystal. It is found that the component of ${\epsilon }_2$ in the ab plane (${\epsilon }_2^a$) displays a sharp peak at 4.1 eV, while the component along the c axis (${\epsilon }_2^c$) has a less pronounced peak at 3.8 eV, a small peak at 4.2 eV, and a broad maximum near 5 eV. Similar structures were found in the interband spectra calculated using the local-density approximation. The 4-eV optical structure is caused by nearly parallel initial- and final-state bands whose wave-functions are located, respectively, in the O-Cu-O dumbbells and in the BaO-Cu-OBa triple layers. Intraionic 3d→4p transitions in dumbbell Cu contribute 90% of the optical matrix elements.