Electronic Structures and Surface Reconstructions in Magnetic Superconductor RbEuFe₄As₄

Area of Science:

• Condensed Matter Physics
• Materials Science
• Quantum Materials

Background:

• The pnictide RbEuFe4As4 exhibits superconductivity at 36 K.
• Superconductivity coexists with long-range magnetic ordering of Europium (Eu) 4f spins below 15-19 K.

Purpose of the Study:

• To investigate the electronic properties and surface structure of RbEuFe4As4 using scanning tunneling experiments.
• To understand the interplay between superconductivity, magnetic ordering, and electronic structure.

Main Methods:

• Scanning tunneling microscopy (STM) on cold-cleaved single crystals.
• Comparison of experimental tunneling spectra with density functional theory (DFT) calculations.

Main Results:

• Observed coexistence of large Rb-terminated and small Eu-terminated terraces with 1x2 and √2x√2 reconstructions.
• Discovered a hidden electronic order with a ~5 nm periodicity on √2x√2 surfaces.
• Measured a superconducting gap of ~7 meV, showing significant quasiparticle state filling.
• DFT calculations confirmed Eu 4f orbitals are ~1.8 eV below the Fermi level, not participating in Cooper pair formation.

Conclusions:

• The study elucidates the complex electronic and magnetic landscape of RbEuFe4As4.
• Eu 4f orbitals do not directly contribute to superconductivity, despite the coexistence of magnetic order.
• A novel hidden electronic order was identified, offering new avenues for research in correlated electron systems.