New features of the Mott-Hubbard insulator gap of parent HTS compounds found by resonance Raman scattering and UV-excited ordinary Raman scattering |
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Authors: | M V Klein Ran Liu D Salamon S L Cooper W C Lee D M Ginsberg and S -W Cheong |
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Affiliation: | (1) Department of Physics, Materials Research Laboratory, and Science and Technology Center for Superconductivity, University of Illinois at Urbana-Champaign, 61801-3080 Urbana, Illinois;(2) AT&T Bell Laboratories, 07974 Murray Hill, New Jersey |
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Abstract: | Optical absorption at the insulating gap in the parent phase of cuprate superconductors shows a broad exciton-like peak near 1.7 eV, followed by a gradual decrease in absorption persisting 1 eV above the gap. By using ultraviolet laser lines to excite Raman spectra, we have found a Raman peak 0.2 eV below the first absorption peak in insulating cuprates. The Raman peak is much narrower than the absorption peak and hasA
2g
symmetry. We assign it to an exciton consisting of a hole transition from Cu
to a linear combination of Cud
xy
and nearest neighbor Op
orbitals. We have also studied the resonance Raman profile for two-magnon Raman scattering in the same samples. We find a sharp resonance feature at about 2.7 eV, and little Raman intensity for photon energies at the 1.7 eV absorption peak. The state created at the peak must therefore be an inappropriate intermediate state for the double spin-flip Raman process. |
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Keywords: | Raman scattering magnon exciton charge-transfer excitation |
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