Phase Diagram and High Temperature Superconductivity at 65 K in Tuning Carrier Concentration of Single-Layer FeSe Films

Abstract: Superconductivity in the cuprate superconductors and the Fe-based superconductors is realized by doping the parent compound with charge carriers, or by application of high pressure, to suppress the antiferromagnetic state. Such a rich phase diagram is important in understanding superconductivity mechanism and other physics in the Cu- and Fe-based high temperature superconductors. In this paper, we report a phase diagram in the single-layer FeSe films grown on SrTiO3 substrate by an annealing procedure to tune the charge carrier concentration over a wide range. A dramatic change of the band structure and Fermi surface is observed, with two distinct phases identified that are competing during the annealing process. Superconductivity with a record high transition temperature (Tc) at ~65 K is realized by optimizing the annealing process. The wide tunability of the system across different phases, and its high-Tc, make the single-layer FeSe film ideal not only to investigate the superconductivity physics and mechanism, but also to study novel quantum phenomena and for potential applications.

• The paper demonstrates that controlled annealing adjusts carrier concentration to trigger superconductivity at 65 K in single-layer FeSe films.
• The paper employs molecular beam epitaxy and ARPES to reveal a clear transition from a non-superconducting N phase to a superconducting S phase.
• The paper highlights the critical role of interfacial effects and electronic structure evolution in enhancing high-Tc superconductivity.

Phase Diagram and High Temperature Superconductivity in Single-Layer FeSe Films

This paper explores the phase diagram and superconducting properties of single-layer FeSe films on SrTiO$_3$ substrates, presenting significant advancements in high-temperature superconductivity research. Through controlled annealing processes, the authors provide an extensive study of the evolution of superconductivity and its relationship with carrier concentration in these films. The research identifies distinct phases, notably a transition to a superconducting state with a high transition temperature of 65 K.

The FeSe films, synthesized via molecular beam epitaxy, demonstrate a tunable electronic phase characterized by notable changes in band structure and Fermi surface topology. The study emphasizes the role of charge carrier concentration adjustment through annealing in vacuum conditions, rather than traditional chemical doping. This approach reveals a distinct "S phase," where optimal annealing maximizes superconducting properties, enhancing the transition temperature to 65 K, surpassing previous records for analogous Fe-based compounds.

Utilizing angle-resolved photoemission spectroscopy (ARPES), the paper details the evolution of the electronic structure across various annealing stages. Initial annealing sequences yield a non-superconducting "N phase," exhibiting electronic features akin to antiferromagnetic parent compounds seen in other Fe-based superconductors. Further annealing transitions the system into the "S phase," characterized by an electron-like Fermi surface around the M points and a full opening of the superconducting gap upon cooling. The transition between these phases constitutes a compelling physical transformation triggered by enhanced carrier densities, highlighting a competition between phases during intermediate annealing states.

The superconducting gap is methodically analyzed, with strong empirical evidence supporting the existence of high-T$_c$ superconductivity. Measurements illustrate a gap opening across the full Fermi surface, consistent with BCS theory, and highlight a strong-coupling regime as indicated by the 2$\Delta$/$k_B$T$_c$ ratio approximating 6–7. The findings also suggest robust interfacial effects, possibly linked to the SrTiO$_3$ substrate, shaping the distinct electronic properties observed in the 2D FeSe system.

This work concludes with a schematic phase diagram highlighting the dual-phase nature of the system and the superconducting region. These findings have broad implications, not only for understanding the pairing mechanism in Fe-based superconductors but also for potential applications in quantum devices leveraging heterostructures composed of superconducting and non-superconducting phases. The study fosters future research opportunities to further elucidate the interface phenomena and optimize material systems for practical high-T$_c$ applications.