X-Ray Raman Scattering Establishes HTSC Energy Scales
One signature of the onset of superconductivity is the formation of electron pairs. A team of Swedish and Dutch scientists has used x-ray spectroscopy (resonant x-ray Raman scattering) to investigate electronic transitions within copper atoms in Sr2CuO2Cl2, an insulating model compound for the copper-based high-temperature superconductors (HTSCs). Contrary to earlier conjecture, they found that these transitions have energies that are too high to be directly involved in the electron-pairing mechanism for high-temperature superconductivity, the source of which is one of the great unsolved problems of condensed-matter physics.
The common feature of HTSCs is a set of parallel copper-oxygen planes. In these planes, each Cu2+ ion is surrounded by four oxygen atoms (red). The 3d orbitals of copper are shown in blue (ground state: x2 - y2) and green (excited states: z2, xz, yz, and xy). In Raman scattering, the absorption of an x ray (shown in yellow) and the emission of an x ray (orange) result in the excitation of the copper d orbital from the ground state to an excited state with a different orientation.
The Raman shift (energy of emitted photon relative to the large elastic scattering peak) and polarization dependence in x-ray resonant Raman spectra give the energies of the transitions and the orientations of the orbitals involved.
With the brightness afforded by the ALS, the Swedish/Dutch group has been able to use resonant x-ray Raman spectra for measuring excitation energies that were not known before, thereby shedding light on an important problem in superconductivity. They believe that this technique can be used to address many other problems, such as colossal magnetoresistance, in complex materials dominated by strong Coulomb interactions between electrons (highly correlated systems).
Research conducted by P. Kuiper (Hogskolan i Halmstad, Sweden); J. Guo, C.
Sathe, L. Duda, and J. Nordgren (Uppsala University, Sweden); and F. de Groot
and G. Sawatzky (Groningen University, The Netherlands), using Beamline 7.0.
Funding: National Research Council, Sweden.
Publication about this experiment: P. Kuiper et al., Phys. Rev. Lett. 80, 5204 (1998).
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