Special SSO Seminar with Shua Sanchez of MIT

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. In this talk, I will discuss recent experiments performed at the Advanced Photon Source in which we combined simultaneous applied uniaxial strain, transport measurements, and x-ray techniques to address this question. We used 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. Further, we demonstrate that the orbital polarizability diverges with the elastoresistivity as the transition is approached from above. These results provide strong evidence that the orbital polarization serves as the primary order parameter of the nematic phase.