RESUMEN
In pnictide RbEuFe4As4, superconductivity sets in at 36 K and coexists, below 15-19 K, with the long-range magnetic ordering of Eu 4f spins. Here we report scanning tunneling experiments performed on cold-cleaved single crystals of the compound. The data revealed the coexistence of large Rb-terminated and small Eu-terminated terraces, both manifesting 1 × 2 and 2×2 reconstructions. On 2×2 surfaces, a hidden electronic order with a period â¼5 nm was discovered. A superconducting gap of â¼7 meV was seen to be strongly filled with quasiparticle states. The tunneling spectra compared with density functional theory calculations confirmed that flat electronic bands due to Eu 4f orbitals are situated â¼1.8 eV below the Fermi level and thus do not contribute directly to Cooper pair formation.