title: Anisotropic softening of magnetic excitations along the nodal direction in superconducting cuprates.
abstract: The high-Tc cuprate superconductors are close to antiferromagnetic order. Recent measurements of magnetic excitations have reported an intriguing similarity to the spin waves--magnons--of the antiferromagnetic insulating parent compounds, suggesting that magnons may survive in damped, broadened form throughout the phase diagram. Here we show by resonant inelastic X-ray scattering on Bi(2)Sr(2)CaCu(2)O(8+δ) (Bi-2212) that the analogy with spin waves is only partial. The magnon-like features collapse along the nodal direction in momentum space and exhibit a photon energy dependence markedly different from the Mott-insulating case. These observations can be naturally described by the continuum of charge and spin excitations of correlated electrons. The persistence of damped magnons could favour scenarios for superconductivity built from quasiparticles coupled to spin fluctuations. However, excitation spectra composed of particle-hole excitations suggest that superconductivity emerges from a coherent treatment of electronic spin and charge in the form of quasiparticles with very strong magnetic correlations.
authors.affiliation: Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Guarise
authors.affiliation: Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: B
authors.initials: B
authors.lastname: Dalla Piazza
authors.affiliation: Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: H
authors.initials: H
authors.lastname: Berger
authors.affiliation: Département de Physique de la Matière Condensée, Université de Genève, 24 Quai Ernest-Ansermet, 1211 Geneva, Switzerland.
authors.forename: E
authors.initials: E
authors.lastname: Giannini
authors.affiliation: Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland.
authors.forename: T
authors.initials: T
authors.lastname: Schmitt
authors.affiliation: 1] Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland [2] RIKEN Centre for Emergent Matter Science (CEMS), Wako 351-0198, Japan.
authors.forename: H M
authors.initials: HM
authors.lastname: Rønnow
authors.affiliation: Department of Physics &Astronomy, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z1.
authors.forename: G A
authors.initials: GA
authors.lastname: Sawatzky
authors.affiliation: Institute for Theoretical Solid State Physics, IFW Dresden, D01171 Dresden, Germany.
authors.forename: J
authors.initials: J
authors.lastname: van den Brink
authors.affiliation: Institut für Theoretische Physik III, Ruhr-Universität Bochum, D-44801 Bochum, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Altenfeld
authors.affiliation: 1] Institut für Theoretische Physik III, Ruhr-Universität Bochum, D-44801 Bochum, Germany [2] National University of Science and Technology 'MISiS', Moscow 119049, Russian Federation.
authors.forename: I
authors.initials: I
authors.lastname: Eremin
authors.affiliation: Institute of Condensed Matter Physics (ICMP), Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland.
authors.forename: M
authors.initials: M
authors.lastname: Grioni