Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor (1202.5849v1)

Abstract: The latest discovery of high temperature superconductivity signature in single-layer FeSe is significant because it is possible to break the superconducting critical temperature ceiling (maximum Tc~55 K) that has been stagnant since the discovery of Fe-based superconductivity in 2008. It also blows the superconductivity community by surprise because such a high Tc is unexpected in FeSe system with the bulk FeSe exhibiting a Tc at only 8 K at ambient pressure which can be enhanced to 38 K under high pressure. The Tc is still unusually high even considering the newly-discovered intercalated FeSe system A_xFe_{2-y}Se_2 (A=K, Cs, Rb and Tl) with a Tc at 32 K at ambient pressure and possible Tc near 48 K under high pressure. Particularly interesting is that such a high temperature superconductivity occurs in a single-layer FeSe system that is considered as a key building block of the Fe-based superconductors. Understanding the origin of high temperature superconductivity in such a strictly two-dimensional FeSe system is crucial to understanding the superconductivity mechanism in Fe-based superconductors in particular, and providing key insights on how to achieve high temperature superconductivity in general. Here we report distinct electronic structure associated with the single-layer FeSe superconductor. Its Fermi surface topology is different from other Fe-based superconductors; it consists only of electron pockets near the zone corner without indication of any Fermi surface around the zone center. Our observation of large and nearly isotropic superconducting gap in this strictly two-dimensional system rules out existence of node in the superconducting gap. These results have provided an unambiguous case that such a unique electronic structure is favorable for realizing high temperature superconductivity.

• The paper reveals a unique Fermi surface topology in single-layer FeSe, with electron pockets near the M point and the absence of hole pockets at Γ.
• The paper finds that the effective electron mass is lighter and the electron doping level is near optimal, indicating weaker electron correlations in FeSe.
• The paper challenges conventional pairing models with its isotropic, nodeless superconducting gap, suggesting alternative mechanisms driven by interface effects and local interactions.

Electronic Origin of High Temperature Superconductivity in Single-Layer FeSe Superconductor

This paper provides comprehensive insight into the mechanisms underpinning high-temperature superconductivity (HTS) in the single-layer FeSe system, a strictly two-dimensional (2D) iron selenide (FeSe) compound. The significance of this paper lies in its potential for advancing our understanding of superconductivity phenomena and exploring the structural simplification offered by single-layer FeSe.

Key Findings and Methodology

The authors employ high-resolution angle-resolved photoemission spectroscopy (ARPES) to investigate the electronic structure responsible for superconductivity in single-layer FeSe films grown on SrTiO$_3$ substrates. The research identifies key distinctions in the Fermi surface topology when compared to other iron-based superconductors. Notably, the Fermi surface consists solely of electron pockets around the M point without the typical hole pockets at the Γ point. This Fermi surface topology distinguishes FeSe and suggests an alternative pairing mechanism.

Results and Implications

1. Unique Fermi Surface Topology:
   • A significant finding is the absence of hole-like Fermi surfaces around Γ, which are prevalent in most iron-based superconductors. The Fermi surface consists exclusively of electron pockets near the M point.
   • The paper reports a superconducting gap size measured at low temperatures (~20 K) to be ~15 meV, indicating isotropic and nodeless characteristics.
2. Electron Mass and Doping Considerations:
   • The effective electron mass in single-layer FeSe is considerably lighter (2.7m$_e$) than that of (Tl$_{0.58}$Rb$_{0.42}$)Fe$_{1.72}$Se$_2$ (6.1m$_e$), suggesting weaker electron correlations in FeSe.
   • The electron doping level of ~0.09 electrons/Fe is closer to the optimal doping level previously observed in other electron-doped systems, suggesting this may be a factor in achieving higher T$_c$.
3. Pairing Mechanism and Theoretical Implications:
   • The identified Fermi surface topology, with its absence of Γ point Fermi surfaces, challenges traditional electron-pairing models that rely on interband scattering between hole and electron pockets.
   • The nodeless gap structure argues against d-wave pairing, limiting the necessity for alternative models such as local magnetic interactions or orbital fluctuations.

Future Directions

The findings introduce several avenues for further research. Primarily, they highlight the need to resolve the discrepancies between ARPES and tunneling spectroscopy in the determination of superconducting gap features. Additionally, consideration of the role of interface effects between FeSe and SrTiO$_3$ is critical, given the possibility of these influences enhancing superconductivity. The isolation of single-layer behavior versus interface phenomena remains paramount. Moreover, understanding why superconductivity manifestation is strictly confined to the first FeSe layer may lead to enhanced superconductor designs with higher T$_c$ by engineering substrate interactions.

Conclusion

The paper provides crucial insight into the high-T$_c$ superconductivity mechanism in Fe-based systems, emphasizing the importance of precise Fermi surface topology and electron band structure. While it raises questions about conventional pairing mechanisms, it opens discussions on alternatives that may lead to the design of novel superconducting materials. The potential applications of this research in developing HTS materials with simpler structures and higher transition temperatures hold significant promise for the field of condensed matter physics.