Electron-phonon interaction in In-induced structures on Si(111) from first-principles.

Electron-phonon interaction in the Si(111)-supported rectangular phases of In is investigated within the density-functional theory and linear-response. For both single-layer and double-layer structures, it is found that the phonon-induced scattering of electrons is almost exclusively determined by vibrations of In atoms. It is shown that the strength of electron-phonon coupling at the Fermi level λ(EF) increases almost twofold upon adding the second In layer. One of the reasons is that additional low-frequency modes appear in the phonon spectrum, which favors a strong enhancement of λ(EF). The agreement of the calculated parameter λ(EF) = 0.99 for a double-layer structure as well as the superconducting transition temperature Tc = 3.5 K with experimental estimates indicates that the discovered superconducting phase is probably a double-layer rectangular -In structure on Si(111) with a coverage of 2.4 ML. This conclusion is also supported by good agreement between the calculated electron band structure and ARPES measurements.

Electronic Structures and Surface Reconstructions in Magnetic Superconductor RbEuFe4As4.

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 ∼5 nm was discovered. A superconducting gap of ∼7 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 ∼1.8 eV below the Fermi level and thus do not contribute directly to Cooper pair formation.

Low-energy acoustic plasmons at metal surfaces.

Nearly two-dimensional (2D) metallic systems formed in charge inversion layers and artificial layered materials permit the existence of low-energy collective excitations, called 2D plasmons, which are not found in a three-dimensional (3D) metal. These excitations have caused considerable interest because their low energy allows them to participate in many dynamical processes involving electrons and phonons, and because they might mediate the formation of Cooper pairs in high-transition-temperature superconductors. Metals often support electronic states that are confined to the surface, forming a nearly 2D electron-density layer. However, it was argued that these systems could not support low-energy collective excitations because they would be screened out by the underlying bulk electrons. Rather, metallic surfaces should support only conventional surface plasmons-higher-energy modes that depend only on the electron density. Surface plasmons have important applications in microscopy and sub-wavelength optics, but have no relevance to the low-energy dynamics. Here we show that, in contrast to expectations, a low-energy collective excitation mode can be found on bare metal surfaces. The mode has an acoustic (linear) dispersion, different to the dependence of a 2D plasmon, and was observed on Be(0001) using angle-resolved electron energy loss spectroscopy. First-principles calculations show that it is caused by the coexistence of a partially occupied quasi-2D surface-state band with the underlying 3D bulk electron continuum and also that the non-local character of the dielectric function prevents it from being screened out by the 3D states. The acoustic plasmon reported here has a very general character and should be present on many metal surfaces. Furthermore, its acoustic dispersion allows the confinement of light on small surface areas and in a broad frequency range, which is relevant for nano-optics and photonics applications.