Optical measurements on oriented thinYBa2Cu3

Abstract

Optical transmission and reflection spectra (mid ir through uv) and Raman spectra of superior-quality 90-, 180-, 400-, and 1000-nm-thick superconducting Y-Ba-Cu-O films are reported. Characteristic excitonic bands, and in particular the absorption band at -0.37 eV reported earlier, are not observed. It therefore seems unlikely that the high-TV superconductivity in cuprates could arise from excitonmediated electron pairing.PACS numbers: 74.70.Ya, 78.30.Er The discovery of high-TV superconductivity in cuprates 1 has stimulated unprecedented research activity. Much of the effort is focused on understanding the underlying mechanism of the superconductivity, and a number of models have already been proposed. 2 " 4 Some of them are specific enough to be directly tested experimentally. However, what is usually the most powerful technique for identifying the mechanism, tunneling spectroscopy, has been greatly hampered so far by the generally bad surface quality of the presently available oxygen-annealed cuprate pellets and films. Infrared spectroscopy probes much deeper into the bulk of the sample and thus could play an important role in the clarification of the mechanism of superconductivity in these materials.The optical reflectivity spectra of La-Sr-Cu-O and YBa-Cu-O have been measured 5 " 7 over a broad frequency range (far ir through uv). However, since reflectivity depends on both the real and the imaginary parts of the dielectric function, such spectra cannot be interpreted directly in general. Orenstein et al. 5 tried to fit their data by the simple classical Drude model, e(co) = e(oo) -ca^/coico + ir); their optimal parameter values were ^=2 and 3 eV for La-Sr-Cu-O and Y-Ba-Cu-O, respectively, and r=0.5 eV and e(°°)=4.5 for both compounds. The calculated reflectivity turned out to be much too large in the mid-ir and near-ir region (0.1 to 1 eV). Orenstein et al. 5 concluded that an optically allowed transition across a gap occurs at -0.5 eV in LaSr-Cu-O; a similar result was obtained for Y-Ba-Cu-O. Therefore, they inferred that the corresponding strong absorption band (with the peak absorption coefficient of -10 5 cm -1 ) should be directly observable in transmission experiments on thin films. Finally, they found this ir feature to be absent in undoped La2Cu04, thus tying it to superconductivity. Etemad et al. 5 also found the correlation between the occurrence of the 0.5-eV peak and superconductivity in La2-^Sr x Cu04. Kamaras et al., 6 by an analogous methodology, concluded that two strong electronic transitions occur at 0.44 and 1.3 eV in La-Ba-Cu-O, and at 0.37 and 2.5 eV in Y-Ba-Cu-O. In oxygen-depleted (nonsuperconducting) YBa2Cu306.2 samples, the 0.37-eV peak was absent. They interpreted these spectral features as the charge-transfer-exciton bands, in support of the excitonic superconductivity model of Varma et al. 2 Finally, Schlesinger et al. 7 also recorded similar spectra but derived just the opposite conclusion. Inspired by the recent report 8 of singlecrystal La2Ni04 reflectivity spectra, which ...

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Through a Lattice Darkly: Shedding Light on Electron-Phonon Coupling in the HighTcCuprates

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