Evidence for a spin acoustic surface plasmon from inelastic atom scattering

Abstract

Closed-shell atoms scattered from a metal surface exchange energy and momentum with surface phonons mostly via the interposed surface valence electrons, i.e., via the creation of virtual electron-hole pairs. The latter can then decay into surface phonons via electron-phonon interaction, as well as into acoustic surface plasmons (ASPs). While the first channel is the basis of the current inelastic atom scattering (IAS) surface-phonon spectroscopy, no attempt to observe ASPs with IAS has been made so far. In this study we provide evidence of ASP in Ni(111) with both Ne atom scattering and He atom scattering. While the former measurements confirm and extend so far unexplained data, the latter illustrate the coupling of ASP with phonons inside the surface-projected phonon continuum, leading to a substantial reduction of the ASP velocity and possibly to avoided crossing with the optical surface phonon branches. The analysis is substantiated by a self-consistent calculation of the surface response function to atom collisions and of the first-principle surface-phonon dynamics of Ni(111). It is shown that in Ni(111) ASP originate from the majority-spin Shockley surface state and are therefore collective oscillation of surface electrons with the same spin, i.e. it represents a new kind of collective quasiparticle: a Spin Acoustic Surface Plasmon (SASP).

From Quantum Paraelectric/Ferroelectric Perovskite Oxides to High Temperature Superconducting Copper Oxides -- In Honor of Professor K.A. Müller for His Lifework

“…The results of Ref. [142] initialized an EPR and magnetic susceptibility study of Na-doped WO 3 samples, suggesting traces of possible superconductivity [143]. In a later investigation of a related compound, namely lithium intercalated WO 2.9 , a small superconducting fraction was observed [144].…”

“…Inelastic HAS spectroscopy, besides measuring single surface phonon frequencies and the related λ Qν , allows for the observation of low-energy collective electronic excitations like phasons, amplitons and acoustic surface plasmons (ASP) [1,19,20,143] due to the direct "mechanical" interaction (Pauli repulsion) of He atoms with the surface electron density. When the ASP dispersion curve (linear in the long-wavelength limit [144]) enters the surface-projected phonon density, there are avoided crossings with the surface phonon dispersion curves and a robust renormalization of ASP phase velocity, all driven by e-ph interaction.…”

Measuring the Electron–Phonon Interaction in Two-Dimensional Superconductors with He-Atom Scattering

“…Inelastic HAS spectroscopy, besides measuring single surface phonon frequencies and the related λ Qν , allows for the observation of low-energy collective electronic excitations like phasons, amplitons and acoustic surface plasmons (ASP) [1,19,20,143] due to the direct "mechanical" interaction (Pauli repulsion) of He atoms with the surface electron density. When the ASP dispersion curve (linear in the long-wavelength limit [144]) enters the surface-projected phonon density, there are avoided crossings with the surface phonon dispersion curves and a robust renormalization of ASP phase velocity, all driven by e-ph interaction.…”

Electronic Temperature and Two-Electron Processes in Overbias Plasmonic Emission from Tunnel Junctions

“…As the tunneling current in a wide bias voltage range is dominated by transitions to the Ag(111) surface state band electrons, , the characteristic lifetime of these excited surface state electrons can be used to trace the relation between the observed electronic temperature and the corresponding tunneling current. This inelastic lifetime is due to the inelastic processes of electron-photon scattering , and electron–electron interaction at finite temperatures, and the corresponding self-consistent calculations are described in section SI5 in the Supporting Information. The average energy of hot electrons above the Fermi energy at a given electronic temperature can be calculated as (see section SI4 in the Supporting Information).…”

“…As the tunneling current in a wide bias voltage range is dominated by transitions to the Ag(111) surface state band electrons, 33,34 the characteristic lifetime of these excited surface state electrons can be used to trace the relation between the observed electronic temperature and the corresponding tunneling current. This inelastic lifetime is due to the inelastic processes of electron-photon scattering 35,36 and electron− electron interaction at finite temperatures, 36 and the (a) Emission edges as a function of the tunneling current for a sample temperature of 4.9 K. Electroluminescence spectra were recorded with a bias voltage of 2 V. For comparison, the rate according to eq 2 is also plotted using the actual temperature of the junction (4.9 K, dashed lines) or the fitted temperature (solid lines). The overbias emission becomes more intense with increasing current.…”