Pressure dependence of critical temperature of bulk FeSe from spin fluctuation theory*

The critical temperature of the 8K superconductor FeSe is extremely sensitive to pressure, rising to a maximum of 40K at about 10GPa [1]. We test the ability of the current generation of fluctuation exchange pairing theories to account for this effect, by downfolding the density functional theory electronic structure for each pressure to a tight binding model. The Fermi surface found in such a procedure is then used with fixed Hubbard parameters to determine the pairing strength using the random phase approximation for the spin singlet pairing vertex. We find that the evolution of the Fermi surface captured by such an approach is alone not sufficient to explain the observed pressure dependence, and discuss alternative approaches.\newline [1] S. Margadonna, \textit{et al.}, Phys. Rev. B \textbf{80}, 064506 (2009); S. Medvedev, \textit{et al.}, Nat. Mater. \textbf{8}, 630 (2009).