Orbital transmutation in the nematic state of FeSe: the consequences for Raman and ARPES experiments*

In this talk I will discuss nematicityinduced change of the orbital composition of low-energy excitations in FeSe, and how it affects Raman and ARPES probes. I will argue that deep in the nematic state, hole and electron pockets become nearly mono-orbital, consistent with polarized ARPES measurements. I show that this leads to strong reduction of Raman intensity in B_1g channel (in 1Fe zone), as observed in the measurements. The reduction comes about because for nearly mono-orbital pockets, B_1g Raman response gets reduced by vertex corrections, which enforce charge conservation. I further discuss ARPES experiments at the M point (π,π). Deep in the nematic state, the two excitations, best visible in ARPES, have been identified as having xz and yz orbital character. However, they remain split above the nematic transition, in apparent contradiction to the fact that in the tetragonal phase the xz and yz orbitals are degenerate. We show that these experimental data are naturally explained by orbital transmutation of the excitations at M between tetragonal and nematic phases.