Interface-induced superconductivity and strain-dependent spin density wave in FeSe/SrTiO3 thin films

Abstract: The record of superconducting transition temperature(Tc) has long been 56K for the iron-based high temperature superconductors(Fe-HTS's). Recently, in single layer FeSe films grown on SrTiO3 substrate, signs for a new 65K Tc record are reported. Here with in-situ photoemission measurements, we substantiate the presence of the spin density wave(SDW) in FeSe films, a key ingredient of Fe-HTS that was missed in FeSe before, which weakens with increased thickness or reduced strain. We demonstrate that the superconductivity occurs when the electrons transferred from the oxygen-vacant substrate suppress the otherwise most pronounced SDW in single layer FeSe. Besides providing a comprehensive understanding of FeSe films and directions to further enhance its Tc, we establish the phase diagram of FeSe vs. lattice constant that contains all the essential physics of Fe-HTS's. With the simplest structure, cleanest composition and single tuning parameter, it is ideal for testing theories of Fe-HTS's.

• The paper demonstrates that electron transfer from the SrTiO3 substrate elevates FeSe’s superconducting transition temperature to around 65 K by suppressing SDW order.
• It utilizes in-situ ARPES and LEED techniques to detail the thickness-dependent evolution of the electronic structure and Fermi surface topology in FeSe films.
• The findings highlight interfacial engineering as a key strategy for tuning superconductivity and magnetism in iron-based superconductors.

Interface-Induced Superconductivity and Spin Density Wave Behavior in FeSe/SrTiO$_3$ Thin Films

The paper "Interface-induced superconductivity and strain-dependent spin density wave in FeSe/SrTiO$_3$ thin films" presents a comprehensive study on the interplay between superconductivity and magnetic order in FeSe thin films grown on SrTiO$_3$ substrates. This work focuses on revealing the superconducting properties and spin density wave (SDW) characteristics in this heterostructure, employing techniques such as in-situ angle-resolved photoemission spectroscopy (ARPES) and low energy electron diffraction.

Summary of Key Findings

The research explores the superconducting transition temperature ($T_c$) of FeSe films, noting an increase to approximately 65 K when grown on SrTiO$_3$. This enhancement in $T_c$, compared to the bulk FeSe with a $T_c$ of around 8 K, is attributed primarily to electron transfer from oxygen vacancies in the SrTiO$_3$ substrate, which reduces the prominence of the SDW observed in the single-layer FeSe films. The existence of pronounced SDW behavior in FeSe films is demonstrated, a feature not previously identified in bulk FeSe due to experimental limitations.

By thoroughly examining the electronic structure of FeSe films of varying thickness, the study establishes that superconductivity is favored at the interface and diminishes with increasing film thickness. This is evidenced by the thickness dependence of the electron Fermi pockets and SDW signatures, with a transition to bulk FeSe electronic properties observed as the film thickness exceeds 35 monolayers (ML).

Electronic Structure and Parametric Variations

The authors report that the electronic structure of FeSe transforms considerably depending on the lattice constants, with strain from the SrTiO$_3$ substrate inducing notable modifications. Specifically, a distinct lattice expansion is seen in the monolayer FeSe, which compresses to bulk FeSe lattice constants with increased thickness. This structural variation is depicted in the derived phase diagram, outlining the connection between the lattice constant and $T_c$, as well as the SDW characteristic temperature $T_A$. The suppression of the SDW and the coexistence with superconductivity highlights a key feature aligning with observations in iron pnictides.

The observed Fermi surface topology, characterized by electron pockets at the Brillouin zone corners, resembles features found in K$_x$Fe$_2$Se$_2$, yet differs with the absence of a hole pocket at the zone center, similar to the phase-separated states seen in aforementioned materials. The study further documents the dependency of $T_A$ and SDW strength on lattice strain, showcasing the richness of phase evolution within these films.

Implications and Future Directions

The implications of these findings are twofold, both practical and theoretical. Practically, this research suggests potential avenues to further augment the $T_c$ of FeSe-based materials by substrate engineering to modulate electron concentration and lattice strain. Theoretically, FeSe films serve as a model system with a simple yet rich phase diagram, providing an accessible platform to test theories of high-temperature superconductivity.

This study also postulates potential operations to achieve higher $T_c$ in heavily electron-doped FeSe by expanding the lattice constants while perfectly optimizing electron transfer from substrates. There’s anticipation for new interfacial engineering techniques to explore even broader ranges of superconducting and magnetic behaviors in similar heterostructural systems.

Conclusion

The paper thus provides significant insights into the interaction between interfacial effects and intrinsic magnetic properties of thin films. By establishing the influence of SDW order in FeSe films and connecting it with the enhanced superconducting $T_c$, this research paves the way for future discoveries in iron-based superconductors through controlled growth conditions and precise electronic tuning. As a part of the growing research body, it contributes to deepening the understanding of emergent superconductivity phenomena in complex material systems.