The observation of replica bands by angle-resolved photoemission spectroscopy enables the study of electron-phonon coupling at low carrier densities, particularly in monolayer FeSe/SrTiO3. Theoretical work suggests that the electrons in the ultra-thin FeSe layer couple to optical phonons in the SrTiO3 substrate that thereby contributes to the enhanced superconducting pairing temperature. So far, the inherent fragility of such single-layer thick materials and the weak intensity of replica features has limited the quantitative evaluation of their nature.

Figure 1: (left, top) Replica band topology in single layer FeSe/SrTiO3 along a cut at M and the corresponding second-derivative showing a second-order replica. (left, bottom) Photon energy dependence of the replica band intensities in a waterfall plot and the extracted ratios between first and second order replicas. (right, top) Observation of second-order replica bands in single-layer FeSe/SrTiO3. Waterfall plot of the spectra at M with blue, red, yellow, and green markers to track the main and replica bands (right, bottom) Band positions based on fits to the observed peak positions. Determination of the electron-phonon coupling constant λ and comparison to theoretical behavior. Grey regions indicate the experimental uncertainty.

To overcome this challenge, the PARADIM in-house research team developed a system to transfer the sensitive samples from ultrahigh vacuum directly into an inert environment glovebox for transport to the Advance Light Source for beamline ARPES measurements.  Using this approach, PARADIM scientists were able to observe interfacial replica bands in far greater quantitative detail than had previously been possible. A detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allowed them to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ = 0.19 ± 0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO3. Furthermore, the methodology employed in this work can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants.

What has been achieved:

PARADIM’s in-house research team reports a quantitative examination of photoemission replica bands in monolayer FeSe/SrTiO3, where the appearance of replica bands has motivated theoretical work suggesting that the interfacial coupling of electrons in the FeSe layer to optical phonons in the SrTiO3 substrate might contribute to the enhanced superconducting pairing temperature. By performing a quantitative examination of replica bands in monolayer FeSe/SrTiO3, we are able to conclusively demonstrate that the replica bands are indeed signatures of intrinsic electron-boson coupling, and not associated with final state effects as had been alternatively theorized.  Furthermore, a detailed analysis of the energy splittings and relative peak intensities between the higher-order replicas, as well as other self-energy effects, allows us to determine that the interfacial electron-phonon coupling in the system corresponds to a value of λ = 0.19 ± 0.02, providing valuable insights into the enhancement of superconductivity in monolayer FeSe/SrTiO3. The methodology employed in this work can also serve as a new and general approach for making more rigorous and quantitative comparisons to theoretical calculations of electron-phonon interactions and coupling constants in other systems where interfacial electron-phonon coupling is present.

This work not only resolves the longstanding debate regarding the origin of photoemission replica bands in FeSe/SrTiO3 by demonstrating that they arise from intrinsic electron-phonon coupling, but also suggests a new generalized methodology for extracting more quantitative information about electron phonon coupling constants in quantum materials through comparisons to theory.

Unique Feature(s) of the MIP that Enabled this Achievement:

Realizing this discovery was made possible only by the unique combination of MBE, ARPES, and in situ R vs. T measurements, and the ability to transfer sensitive films from ultrahigh vacuum into an inert environment glovebox for transport to synchrotron beamline.