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Intense paramagnon excitations in a large family of high-temperature superconductors


In the search for the mechanism of high-temperature superconductivity, intense research has been focused on the evolution of the spin excitation spectrum on doping from the antiferromagnetic insulating to the superconducting state of the cuprates. Because of technical limitations, the experimental investigation of doped cuprates has been largely focused on low-energy excitations in a small range of momentum space. Here we use resonant inelastic X-ray scattering to show that a large family of superconductors, encompassing underdoped YBa2Cu4O8 and overdoped YBa2Cu3O7, exhibits damped spin excitations (paramagnons) with dispersions and spectral weights closely similar to those of magnons in undoped cuprates. The comprehensive experimental description of this surprisingly simple spectrum enables quantitative tests of magnetic Cooper pairing models. A numerical solution of the Eliashberg equations for the magnetic spectrum of YBa2Cu3O7 reproduces its superconducting transition temperature within a factor of two, a level of agreement comparable to that of Eliashberg theories of conventional superconductors.

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Figure 1: Scattering geometry and typical Cu L3-edge RIXS response of cuprates.
Figure 2: Experimental data.
Figure 3: Dispersion, linewidth and intensity of the magnetic excitations.
Figure 4: Results of model calculations.


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The authors acknowledge P. Bourges, J. van den Brink, D. Haug, J. P. Hill, B. J. Kim, D. Manske, D. F. McMorrow, Ch. Rüegg, S. Sachdev, G. Sawatzky, Y. Sidis and R. Zehyer for discussions, and B. Pingault for YBa2Cu4O8 sample surface preparation. This work was carried out at the ADRESS beamline using the SAXES instrument jointly built by the Paul Scherrer Institut (Villigen, Switzerland), Politecnico di Milano (Italy) and École polytechnique fédérale de Lausanne (Switzerland). Part of this research project has been supported by the European Commission under the Seventh Framework Programme: Research Infrastructures (grant agreement no 226716) and the European project SOPRANO under Marie Curie actions (grant no PITNGA-2008-214040).

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M.L.T., G.G., L.B., V.H. and B.K. managed the project. Y.T.S., G.L.S. and C.T.L. grew the single-crystalline samples; G.M.D.L. and M.S. prepared the thin films. M.L.T., G.G., M.M.S., V.H., M.M., M.B., S.B-C. and L.B. carried out the experiment assisted by K.J.Z., C.M. and T.S. M.L.T. and L.B. analysed the data. J.C., G.K. and M.W.H. carried out the theoretical calculations. M.L.T. and B.K. wrote the paper with comments from all co-authors.

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Correspondence to M. Le Tacon or B. Keimer.

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Le Tacon, M., Ghiringhelli, G., Chaloupka, J. et al. Intense paramagnon excitations in a large family of high-temperature superconductors. Nature Phys 7, 725–730 (2011).

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