# Orbital-dependent scattering in Sr2RuO4 and FeSe

Hybrid talk - Physics East 217

I will present two examples where strain-tuning provides information on the $k$-dependence of scattering in metals. In the first, Sr$_2$RuO$_4$, uniaxial stress along the $a$ axis drives a Lifshitz transition of the largest Fermi surface, from an electron-like to an open geometry. We measured the Hall coefficient across this transition, and found that for temperatures $\sim$2~K and above, it becomes more \textit{electron-like}, opposite to the expectation from the change in Fermi surface topology. We show that this result can be explained by orbital differentiation in the electron-electron scattering rate, along with a drastic reduction in the $xy$ scattering rate across the transition. For FeSe, I will present the resistive anisotropy in the nematic state, and argue that here, too, orbital differentiation is important: the resistive anisotropy appears to be controlled by scattering within the $yz$ orbital. As temperature is reduced, the nematic distortion appears to proceed beyond the point where spin fluctuations within the $yz$ band are strongest, which is a challenge to one set of theories on the origin of the nematicity.

This talk is part of the Condensed Matter Physics Seminars series.