  Spatial uctuations of spin density wave (SDW) and charge density wave (CDW)
in chromium have been compared by combining coherent and simultaneous X-ray
diffraction experiments. Despite their close relationship, spatial fluctuations
of the spin and of the charge density waves display a very different behavior:
the satellite reflection associated to the charge density displays speckles
while the spin one displays an impressive long-range order. This observation is
hardly compatible with the commonly accepted magneto-elastic origin of CDW in
chromium and is more consistent with a purely electronic scenario where CDW is
the second harmonic of SDW. A BCS model taking into account a second order
nesting predicts correctly the existence of a CDW and explains why the CDW is
more sensitive to punctual defects.
