Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

Abstract

FeSe undergoes a transition from a tetragonal to a slightly orthorhombic phase at 90\,K, and becomes a superconductor below 8\,K. The orthorhombic phase is sometimes called a nematic phase because quantum oscillation, neutron, and other measurements detect a significant asymmetry in $x$ and $y$. How nematicity affects superconductivity has recently become a matter of intense speculation. Here we employ an advanced \emph{ab-initio} Green's function description of superconductivity and show that bulk tetragonal FeSe would, in principle, superconduct with almost the same T$_{c}$ as the nematic phase. The mechanism driving the observed nematicity is not yet understood. Since the present theory underestimates nematicity, we simulate the full nematic asymmetry by artificially enhancing the orthorhombic distortion. For benchmarking, we compare theoretical spin susceptibilities against experimentally observed data over all energies and relevant momenta. When the orthorhombic distortion is adjusted to correlate with observed nematicity in spin susceptibility, the enhanced nematicity causes spectral weight redistribution in the Fe-3d$_{xz}$ and d$_{yz}$ orbitals, but it leads to at most 10-15$\%$ increment in T$_{c}$. This is because the d$_{xy}$ orbital always remains the most strongly correlated and provides most of the source of the superconducting glue. Nematicity suppresses the density of states at Fermi level; nevertheless T$_{c}$ increases, in contradiction to both BCS and BEC theories. We show how the increase is connected to the structure of the particle-particle vertex. Our results suggest while nematicity may be intrinsic property of bulk FeSe, is not the primary force driving the superconducting pairing.