Discrete Superconducting Phases in FeSe-Derived Superconductors

Area of Science:

• Condensed Matter Physics
• Materials Science
• Superconductivity

Background:

• Unconventional superconductors typically exhibit a superconducting dome in their phase diagrams.
• Iron selenide (FeSe) is the simplest iron-based superconductor, making it a key material for fundamental studies.
• Understanding the factors controlling superconductivity in FeSe is crucial for designing new superconducting materials.

Purpose of the Study:

• To investigate the emergence of discrete superconducting phases in FeSe thin flakes.
• To explore the effect of carrier concentration tuning on the superconducting properties of FeSe.
• To compare the superconducting phase diagram of FeSe derivatives with other unconventional superconductors.

Main Methods:

• Fabrication of FeSe thin flakes.
• Continuous tuning of carrier concentration using solid ionic gating with Li and Na ion intercalation.
• Substitution of S for Se and Cu for Fe to probe the structural and chemical sensitivity of superconductivity.

Main Results:

• Observation of a series of discrete superconducting phases in FeSe thin flakes, distinct from a typical superconducting dome.
• These discrete phases remain robust against 20% sulfur substitution for selenium.
• Superconductivity is highly sensitive to minor substitutions (2% Cu for Fe), emphasizing the critical role of the intact iron site.

Conclusions:

• The superconducting phase diagram of FeSe derivatives is unique and differs significantly from those of other unconventional superconductors.
• The findings highlight the importance of carrier concentration and the integrity of the iron site in stabilizing superconductivity in FeSe.
• This work provides new insights into the complex phase behavior of iron-based superconductors.