Raman-active phonons of a twin-free YBa2Cu3O7 crystal: A complete polarization analysis.

A complete Raman analysis has been performed on a high-quality YBa{sub 2}Cu{sub 3}O{sub 7} crystal made free of twins by a mechanical detwinning process. Marked {ital a}-{ital b} anisotropy is observed in the intensity of the {ital A}{sub {ital g}} phonons at 116, 335, and 495 cm{sup {minus}1}. At room temperature, the {alpha}{sub {ital y}{ital y}} component of the polarizability tensor of the 116-cm{sup {minus}1} phonon is strongly coupled to an electronic continuum, the {alpha}{sub {ital x}{ital x}} component is less strongly coupled, and the coupling of the {alpha}{sub {ital z}{ital z}} component is not apparent in the Raman line shape. {ital B}{sub 2{ital g}} and {ital B}{sub 3{ital g}} phonon pairs are reproducibly found at 70 and 83 cm{sup {minus}1}, 142 and 140 cm{sup {minus}1}, 210 and 303 cm{sup {minus}1}, and 579 and 526 cm{sup {minus}1}. Comparison of the observed {ital B}{sub 2{ital g}}-{ital B}{sub 3{ital g}} pairs to literature calculations of frequencies is made.