1. H. H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK
2. Condensed Matter Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, USA
3. Department of Physics and Astronomy, The University of Tennessee, Knoxville, Tennessee 37996-1200, USA
4. ISIS Facility, Rutherford Appleton Laboratory, Chilton, Oxon OX11 0QX, UK
5. Department of Ceramic Engineering, University of Missouri-Rolla, Rolla, Missouri 65409-0330, USA

Correspondence to: S. M. Hayden¹ Email: S.Hayden@bristol.ac.uk

Abstract

In conventional superconductors, lattice vibrations (phonons) mediate the attraction between electrons that is responsible for superconductivity¹. The high transition temperatures (high-T_c) of the copper oxide superconductors has led to collective spin excitations being proposed as the mediating excitations in these materials². The mediating excitations must be strongly coupled to the conduction electrons, have energy greater than the pairing energy, and be present at T_c. The most obvious feature in the magnetic excitations of high-T_c superconductors such as YBa₂Cu₃O_6+x is the so-called 'resonance'^{3, 4, 5, 6}. Although the resonance may be strongly coupled to the superconductivity^{3, 4, 5, 6, 7, 8}, it is unlikely to be the main cause, because it has not been found in the La_2-x(Ba,Sr)_xCuO₄ family and is not universally present in Bi₂Sr₂CaCu₂O₈₊ (ref. 9). Here we use inelastic neutron scattering to characterize possible mediating excitations at higher energies in YBa₂Cu₃O_6.6. We observe a square-shaped continuum of excitations peaked at incommensurate positions. These excitations have energies greater than the superconducting pairing energy, are present at T_c, and have spectral weight far exceeding that of the 'resonance'. The discovery of similar excitations in La_2–xBa_xCuO₄ (ref. 10) suggests that they are a general property of the copper oxides, and a candidate for mediating the electron pairing.

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