High <em>T_c</em> via Spin Fluctuations from Incipient Bands: Application to Monolayers and Intercalates of FeSe

We investigate superconductivity in a two-band system with an electronlike and a holelike band, where one of the bands is away from the Fermi level (or “incipient”). We argue that the incipient band contributes significantly to spin-fluctuation pairing in the strong coupling limit where the system is close to a magnetic instability and can lead to a large T_c. In this case, T_c is limited by a competition between the frequency range of the coupling (set by an isolated paramagnon) and the coupling strength itself, such that a domelike T_c dependence on the incipient band position is obtained. The coupling of electrons to phonons is found to further enhance T_c. Furthermore, the results are discussed in the context of experiments on monolayers and intercalates of FeSe.. Authors: Linscheid, A. [University of Florida, Gainesville, FL (United States)]; Maiti, S. [University of Florida, Gainesville, FL (United States)]; Wang, Y. [University of Tennessee, Knoxville, TN (United States)]; Johnston, S. [University of Tennessee, Knoxville, TN (United States)]; Hirschfeld, P. J. [University of Florida, Gainesville, FL (United States)]. DOE Contract: FG02-05ER46236. Subjects: 75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; Free-electron model; High-temperature superconductors; Multiband superconductivity; Paramagnetism; Perturbation theory; Pnictides; Random phase approximation; Superconducting gap