Interfacial Phonons in Sr2VO3FeAs (August 2017)
The implications of interfacial phonons in the FeSe/STO system described above led to the proposal  that superconductivity could be enhanced in heterostructures of iron-based superconductors and oxide layers. The bulk system Sr2VO3FeAs is a naturally occurring version of such a structure where FeAs monolayers are sandwiched between (Mott) insulating Sr2VO3FeAs layers. In collaboration with Prof. Jhinhwan Lee's group, we examined the possibility for interface interactions in this material using Scanning Tunneling Microscopy. Quasiparticle Interference imaging the material's electronic structure revealed a shallow electron-like band where superconductivity occurs below Tc. In addition to this band, we also found a shadow band reminiscent of the replica structures observed in the FeSe/STO interface. We can capture these features well using a theoretical model that includes coupling to small q oxygen phonon modes in the Sr2VO3FeAs layers. Detailed comparisons between the model and the experimental data indicated that these phonons could account for as much as half of the total pairing in this interesting material. These findings, published in Phys. Rev. Lett. , also highlight the general concept of interface engineering as a viable route to superconductivity at high temperatures.
FeSe thin films on SrTiO3 Substrates (January 2016)
A flurry of scientific interest has been generated by the discovery of a dramatically enhanced superconductivity in iron selenide (FeSe) monolayers grown on strontium titanate (SrTiO3 or STO) substrates . On its own, bulk FeSe has a very modest superconducting transition temperature Tc ~ 8 K. When a single monolayer of FeSe is grown on an STO substrate, however, Tc is increased close to the boiling point of liquid nitrogen (77 K), which is a significant threshold for technological applications.
Understanding the origin of the enhanced superconductivity in this interface system promises new routes to high-Tc superconductivity. An important clue was recently uncovered by an angle-resolved photoemission spectroscopy (ARPES) study that focused on the role of the substrate . This study observed replica bands in the electronic structure of the FeSe monolayer, offset in energy from the main bands by ~100 meV (top figure). These replicas are interpreted as shake-off states resulting from coupling between the FeSe electrons and a high-energy optical oxygen phonon in the STO substrate. Moreover, these replica bands are complete copies of the main bands; they possess the same effective mass and terminate at the same momentum points in the Brillouin zone. This is a key observation that implies that the electron-phonon (e-ph) interaction responsible for forming the replicas is strongly peaked in the forward scattering direction (small momentum transfers q). This momentum dependence is unique in that it will enhance superconductivity in most pairing channels, including those mediated by spin fluctuations [4,5].
In a follow-up work, we examined the consequences of such an e-ph interaction using Migdal-Eliashberg theory. We found that this interaction can reproduce the size, shape, and intensity of the observed replica bands. We then used this information to determine the superconducting Tc provided by this interaction. Surprisingly, we found that the strong forward scattering results in a significant transition temperature, far greater than that predicted by conventional BCS theory (bottom figure). For a coupling strength consistent with the ARPES data, we found that nearly all of the observed Tc is accounted for. If this scenario is correct, we predict a fully gapped s-wave superconducting state will be formed.
We have recently examined  the phononic properties of the STO mode that is we believe is relevant to the interface system. This study was motivated by the fact that when the e-ph interaction is strong for only a small subset of phonon modes, one expects anomalously broad phonon linewidths and potential instabilities towards phases that compete with superconductivity. In this follow-up work, we showed that indeed the bare e-ph coupling would lead to such a case; however, long range Coulombic screening dramatically reducing these effects in the FeSe/STO interface. These results show that a combination of the e-ph and e-e interaction should be considered when modeling this fascinating system.
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