  Continued improvement of the energy resolution of resonant inelastic x-ray
scattering (RIXS) spectrometers is crucial for fulfilling the potential of this
technique in the study of electron dynamics in materials of fundamental and
technological importance. In particular, RIXS is the only alternative tool to
inelastic neutron scattering capable of providing fully momentum resolved
information on dynamic spin structures of magnetic materials, but is limited to
systems whose magnetic excitation energy scales are comparable to the energy
resolution. The state-of-the-art spherical diced crystal analyzer optics
provides energy resolution as good as 25 meV but has already reached its
theoretical limit. Here, we demonstrate a novel sub-10meV RIXS spectrometer
based on flat-crystal optics at the Ir-L$_3$ absorption edge (11.215$\sim$ keV)
that achieves an analyzer energy resolution of 3.9$\sim$meV, very close to the
theoretical value of 3.7$\sim$meV. In addition, the new spectrometer allows
efficient polarization analysis without loss of energy resolution. The
performance of the instrument is demonstrated using longitudinal acoustical and
optical phonons in diamond, and magnon in Sr$_3$Ir$_2$O$_7$. The novel
sub-10$\sim$meV RIXS spectrometer thus provides a window into magnetic
materials with small energy scales.
