OPTICAL PHONONS IN YBA2CU4O8 AND Y2BA4CU7O15-DELTA

We present Raman spectra of high-temperature superconducting single crystals of YBa2Cu4O8 and Y2Ba4Cu7O15-, differing from the well-known YBa2Cu3O7- by having double CuO chains. By performing polarization-dependent measurements, we were able to identify the symmetry of the observed phonons. In both materials, we detected four out of six Raman-allowed modes of the double Cu(1)-O(1) chains and assign the peaks to Cu(1) Ag (250 cm-1), O(1) Ag (605 cm-1), Cu(1) B3g (314 cm-1), and O(1) B2g (228 cm-1) in addition to the 5 Ag phonons (in YBa2Cu4O8 near 104, 153, 341, 438, and 500 cm-1), which are also present in the well-known YBa2Cu3O7-. Contrary to previous reports of Raman scattering on double-CuO2-plane superconductors, B2g and B3g phonons can unambiguously be observed. They are, in fact, stronger than some of the Ag phonons. The peaks at 605, 314, 250, and 228 cm-1 are more than twice as large in YBa2Cu4O8 than in Y2Ba4Cu7O15- thus confirming that they are chain related. The Raman intensities of the Ag modes at 250, 500, and 605 cm-1 are much larger in yy (along the chains) than in xx polarization, proving that the crystals are untwinned and stressing the importance of the chains for the Raman activity of these modes. When lowering the temperature below Tc=74 K the B1g-like O(2)-O(3) phonon near 104 cm-1 as well as the Ba Ag phonon at 341 cm-1 exhibit a rather sharp softening. The Ba mode shows a pronounced Fano line asymmetry, while that of the O(2)-O(3) line is rather weak. © 1990 The American Physical Society.