  The origin of nematicity in FeSe remains a critical outstanding question
towards understanding unconventional superconductivity in proximity to nematic
order. To understand what drives the nematicity, it is essential to determine
which electronic degree of freedom admits a spontaneous order parameter
independent from the structural distortion. Here, we use X-ray linear dichroism
at the Fe K pre-edge to measure the anisotropy of the 3d orbital occupation as
a function of in situ applied stress and temperature across the nematic
transition. Along with X-ray diffraction to precisely quantify the strain
state, we reveal a lattice-independent, spontaneously-ordered orbital
polarization within the nematic phase, as well as an orbital polarizability
that diverges as the transition is approached from above. These results provide
strong evidence that spontaneous orbital polarization serves as the primary
order parameter of the nematic phase.
