1. Forschungszentrum Karlsruhe, Institut für Festkörperphysik, POB 3640, D-76021 Karlsruhe, Germany
2. Laboratoire Léon Brillouin, CE Saclay, F-91191 Gif-sur-Yvette Cedex, France
3. Department of Physics, Division of Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
4. Institute for Material Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
5. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA

Correspondence to: D. Reznik^1,2 Correspondence and requests for materials should be addressed to D.R. (Email: Reznik@llb.saclay.cea.fr).

Abstract

The attempt to understand copper oxide superconductors is complicated by the presence of multiple strong interactions in these systems. Many believe that antiferromagnetism is important for superconductivity, but there has been renewed interest in the possible role of electron–lattice coupling^{1, 2, 3, 4}. The conventional superconductor MgB₂ has a very strong electron–lattice coupling, involving a particular vibrational mode (phonon) that was predicted by standard theory and confirmed quantitatively by experiment⁵. Here we present inelastic scattering measurements that show a similarly strong anomaly in the Cu–O bond-stretching phonon in the copper oxide superconductors La_2-xSr_xCuO₄ (with x = 0.07, 0.15). Conventional theory does not predict such behaviour. The anomaly is strongest in La_1.875Ba_0.125CuO₄ and La_1.48Nd_0.4Sr_0.12CuO₄, compounds that exhibit spatially modulated charge and magnetic order, often called stripe order⁶; it occurs at a wave vector corresponding to the charge order. These results suggest that this giant electron–phonon anomaly, which is absent in undoped and over-doped non-superconductors, is associated with charge inhomogeneity. It follows that electron–phonon coupling may be important to our understanding of superconductivity, although its contribution is likely to be indirect.

1. Forschungszentrum Karlsruhe, Institut für Festkörperphysik, POB 3640, D-76021 Karlsruhe, Germany
2. Laboratoire Léon Brillouin, CE Saclay, F-91191 Gif-sur-Yvette Cedex, France
3. Department of Physics, Division of Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8602, Japan
4. Institute for Material Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan
5. Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973-5000, USA

Correspondence to: D. Reznik^1,2 Correspondence and requests for materials should be addressed to D.R. (Email: Reznik@llb.saclay.cea.fr).

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