Replica bands in FeSe monolayers on STO

The recent observation of replica bands in FeSe monolayers on STO in angle-resolved photoemission spectroscopy (ARPES) [1] has triggered intense discussions concerning the influence of FeSe electron coupling with STO phonon on the enhanced superconductivity. To obtain narrow replica bands tracing closely the dispersion of main bands rather than broad shake-off features or kinks, one has to require the coupling to be strongly peaked at q=0. However, whether the uniform electric field generated by q=0 STO phonon can strengthen the Cooper pairing in FeSe monolayer is still in discussion. We follow up with a new interpretation of the replica bands demonstrating they are largely due to the energy loss process of the escaping photoelectron, resulted from the well-known strong coupling of external propagating electrons to q=0 Fuchs-Kliewer (FK) surface phonons in STO [2]. Photoelectron energy loss on FeSe monolayer is calculated using the demonstrated successful semiclassical dielectric theory in describing low energy high-resolution electron energy loss spectroscopy (HREELS) [2]. We reduce the loss probability in HREELS by a factor 2 since the electron traveling path in ARPES is 1/2 of the HREELS one. Our calculation turns out to be able to reproduce the replica intensity and other experimental features in detail very well without the need for any fitting parameter [3]. This strongly suggests that the observed replica bands are mostly due to extrinsic photoelectron energy loss and not a result of the electron-phonon interaction of the Fe d electrons with the STO phonons. Therefore, the mechanism of the enhanced superconductivity in these monolayers remains an open question although other phonons than the FK types may still contribute.

[1] J. Lee et al., Nature (London) 515, 245 (2014).
[2] H. Ibach and D. L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations (1982).
[3] F. Li and G. A. Sawatzky, Phys. Rev. Lett. 120, 237001 (2018).