Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3

Abstract: Single unit cell films of iron selenide (1UC FeSe) grown on SrTiO3 (STO) substrates have recently shown superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 K), a record for iron-based superconductors. Towards understanding why Cooper pairs form at such high temperatures, a primary question to address is the role, if any, of the STO substrate. Here, we report high resolution angle resolved photoemission spectroscopy (ARPES) results which reveal an unexpected and unique characteristic of the 1UC FeSe/STO system: shake-off bands suggesting the presence of bosonic modes, most likely oxygen optical phonons in STO, which couple to the FeSe electrons with only small momentum transfer. Such coupling has the unusual benefit of helping superconductivity in most channels, including those mediated by spin fluctuations. Our calculations suggest such coupling is responsible for raising the superconducting gap opening temperature in 1UC FeSe/STO. This discovery suggests a pathway to engineer high temperature superconductors.

• The paper demonstrates that interfacial electron-phonon interactions raise Tc from 8 K in bulk FeSe to nearly 70 K in 1UC films on SrTiO3.
• The study employs high-resolution ARPES to reveal replica bands shifted by approximately 100 meV, indicating coupling with oxygen optical phonons.
• Theoretical analysis estimates an electron-phonon coupling constant of about 0.5, supporting new strategies for engineering high-Tc superconductors.

Interfacial Mode Coupling in FeSe/SrTiO$_3$ and Its Role in Enhancing Superconducting Transition Temperature

The paper "Interfacial mode coupling as the origin of the enhancement of T$_{c}$ in FeSe films on SrTiO$_{3}$" presents a comprehensive study using high-resolution angle-resolved photoemission spectroscopy (ARPES) to investigate the superconducting properties of single-unit-cell (1UC) FeSe films when interfaced with SrTiO$_3$ (STO) substrates. A focal point of the study is the significant increase in superconducting transition temperature (T$_{c}$) of FeSe films from 8 K in bulk form to near 70 K when in monolayer form on STO, addressing the mechanisms that promote such enhancement.

Findings

The paper attributes this enhancement primarily to the coupling between the FeSe electrons and interfacial bosonic modes, hypothesized to be oxygen optical phonons intrinsic to the STO substrate. Notably, these modes engage in electron–phonon interactions characterized by low momentum transfer, an unconventional trait that seems to facilitate superconductivity across numerous channels, including spin fluctuation-mediated mechanisms.

Experimental Observations

From ARPES data, a standout observation was the presence of replica bands in the 1UC FeSe/STO system, discernible by an approximate 100 meV shift relative to the primary electron bands. These findings are unprecedented in solid-state systems and can be linked to a Taylor-like electron excitation mechanism originating from the STO substrate's phonon modes. Furthermore, the study noted that these replica bands are absent in films thicker than the 1UC, aligning with the detected lack of superconducting behavior in such multi-layer constructs.

Theoretical Insights

The theoretical calculations performed, addressing both the magnitudes of the electron-phonon (e-ph) coupling and its efficiency in enhancing T$_{c}$, reveal that the coupling constant can be as substantial as $\lambda \approx 0.5$ based on intensity assessments of the replica bands. This degree of e-ph interaction is significant, derived from the limited frequency of the phonon modes implicated. Utilizing a mean-field approximation, enhancements via interfacial coupling were projected to elevate Cooper pair formation temperatures substantially, reinforced by a proposed heterostructure modification that may double the phononic influence and further boost T$_{c}$.

Implications

The findings from this research offer insights into the engineering of high-temperature superconductors by underlining the importance of substrate-induced interfacial phononic interactions, potentially informing strategies to surpass existing superconductive thresholds. The low momentum transfer characteristic of the e-ph coupling introduces a paradigm where designing lattice dynamics at interfaces could become pivotal in new superconducting technologies.

Future Directions

Prospective developments could include experiments on FeSe films coupled with varied phonon-rich substrates or low-dimensional heterostructures, assessing the integrative effect of differing phononic bands on electronic properties. Further computational modeling could refine our understanding, especially concerning spin fluctuation mediations in these systems. Moreover, investigations into other transition metal-based superconductor systems might leverage the findings here, exploring new avenues in superconductor design using hybrid interfacial engineering.

The results presented in this paper clarify some fundamental interactions within high-T$_{c}$ superconductors, potentially steering future research toward actionable enhancements in superconducting technologies.