  In the early days of high temperature superconductivity it was already
recognized that magnetic properties of these materials are intimately related
to the superconducting ones . When doped, the long-range ordered
antiferromagnetic background of pristine copper-oxide insulators melts away and
makes room for a spin liquid and superconductivity. By resonant inelastic x-ray
scattering (RIXS) in the soft regime we probe the hitherto inaccessible
dynamical multiple-spin correlations of the magnetic background in a series of
parent compounds and in high Tc materials [NCCO (Nd2-xCexCuO4) and LSCO
(La2-xSrxCuO4)]. High resolution measurements allows the clear observation of
dispersing bimagnon excitations. In the undoped compounds the theory, fits the
data on these coherent spin excitations without free parameters. In nearly
optimally doped LSCO we observe the appearance of a new collective excitation
at an energy of 250 +/- 60 meV having the signature of a coupled
bimagnon-charge mode. It has a strongly reduced dispersion and lies in a so far
unexplored region of momentum and energy space in the mid-infrared.
