Mathieu Le Tacon (Max-Planck Institut, Stuttgart) présentera un séminaire intitulé : "Resonant x-ray scattering investigations of high-temperature superconductors" Mardi 19 juin à 16h15 Salle E424
Motivated by the search for the mechanism of high-temperature superconductivity, an intense research effort has been focused on the evolution of the spin excitation spectrum upon doping from the antiferromagnetic (AF) to the superconducting states of the cuprates. Because of technical limitations, however, the experimental investigation of doped cuprates has been largely focused on low energy excitations (2Cu3O7, exhibits damped spin excitations - paramagnons - with dispersions and spectral weights similar to those of magnons in undoped AF cuprates over much of the Brillouin zone. This is in excellent agreement with the spin excitations obtained by exact diagonalization of the t-J Hamiltonian on finite-sized clusters. A numerical solution of the Eliashberg equations based on our experimental findings for YBa2Cu3O7 reproduces its TC within a factor of two. The discovery of a well-defined, surprisingly simple spin excitation branch over a wide range of doping levels thus strongly supports magnetic Cooper pairing models for the cuprates.
In parallel to the observation of magnetic excitations, we found, in underdoped compounds, a clear enhancement of elastic scattering around incommensurate wave vectors (0.31,0,L) and (0,0.31,L) indicative of the coexistence of charge ordering and superconducting state. Further investigation revealed that the charge order survives in the pseudogap state of these underdoped compounds, and indicates a competition between the superconducting instability and charge ordering.