Heater Tungsten DELTA 05

In neutral graphene, two prominent cusps known as Kohn anomalies are found in the phonon dispersion of the highest optical phonon at q=Γ (LO branch) and q=K (TO branch), reflecting a significant electron-phonon coupling (EPC) to undoped Dirac electrons. In this work, high-resolution electron energy loss spectroscopy is used to measure the phonon dispersion around the Γ point in quasifreestanding graphene epitaxially grown on Pt(111). The Kohn anomaly for the LO phonon is observed at finite momentum q∼2kF from Γ, with a shape in excellent agreement with the theory and consistent with known values of the EPC and the Fermi level. More strikingly, we also observe a Kohn anomaly at the same momentum for the out-of-plane optical phonon (ZO) branch. This observation is the first direct evidence of the coupling of the ZO mode with Dirac electrons, which is forbidden for freestanding graphene but becomes allowed in the presence of a substrate. Moreover, we estimate the EPC to be even greater than that of the LO mode, making graphene on Pt(111) an optimal system to explore the effects of this new coupling in the electronic properties.

A. Politano, F. de Juan, G. Chiarello, H. A. Fertig
Phys. Rev. Lett. 115, 075504

We have investigated plasmonic excitations at the surface of Bi2Se3(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q∥, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q∥. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q∥∼0.04  Å−1), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi2Se3. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators.

A. Politano,V. M. Silkin, I. A. Nechaev, M. S. Vitiello, L. Viti, Z. S. Aliev, M. B. Babanly, G. Chiarello, P. M. Echenique, E. V. Chulkov
Phys. Rev. Lett. 2015, 115, 216802

Using high resolution electron energy loss spectroscopy (HREELS), we have characterized the fingerprint spectra of clean, hydrogenated, methanol-dosed and air-exposed n-doped Ge(1 0 0). On clean Ge(1 0 0) 2 × 1, we report the observation of a surface phonon peak between ∼28–35 meV. The position and shape of this peak is sensitive to the presence of low surface coverage of hydrogen and oxygen. By adsorbing molecular hydrogen on the n-doped Ge, this peak shifts towards the elastic peak, and becomes attenuated. The HREELS fingerprint spectrum of air-exposed Ge is similar to that created by dosing Ge with methanol. Methanol undergoes dissociation into methyl radicals and hydroxyl species on Ge surfaces at room temperature and oxidizes the Ge surface readily.

L. C. Wei, S. J. Mei, N. L. Ma, W. Chen, L. K. Ping
Surface Science 575, pp. 51-59ff.

The initial stage oxygenation of the Ge(100) surfaces has been studied using density functional theory and high resolution electron energy loss spectroscopy. The sequences of the addition of dissociated O2 on the Ge−Ge bridged and backbonded sites were considered in order to correlate the energetics of the reactions predicted by first principles calculations with the surface vibrational modes observed in our experiments. Our results suggest that a one O2-per-dimer site reaction is more favorable than the dissociative chemisorption of O2 across two dimer sites. The first insertion of one O into the Ge backbond is apparently barrierless; further thermal activation allows the second O bridging the dimer site to be inserted into the second Ge backbond in the same dimer.

J. M. Soon, C. W. Lim, K. P. Loh, N. L. Ma, P. Wu
Phys. Rev. B 72, 115343

High-resolution electron energy-loss spectroscopy (HREELS) has been used to investigate the vibrational properties of the newly discovered, well-ordered oxygen (1×1) overlayer on Ru(001). The totally symmetric RuO stretch has been mapped out along the Γ̄–K̄–M̄–K̄direction of the surface Brillouin zone (SBZ). The stretching mode exhibits a strong downward dispersion of about 100 cm−1 along Γ̄–K̄ starting at 660 cm−1, with a subsequent upward dispersion along K̄–M̄. The dispersion observed for small k‖ is explained by dominating long-range dynamical dipole–dipole coupling. From comparisons of experimental data in the vicinity of the Γ̄ point with calculations for dipole–dipole coupling, the frequency of the isolated oscillator (singleton frequency), as well as the vibrational and electronic polariziabilities, can be estimated to be 598 cm−1, 0.8 Å3 and 1.7 Å3, respectively. However, the short-range interactions deviate significantly from dynamical dipole–dipole interactions as is documented by the discrepancy between the calculated and the observed dispersion between K̄ and M̄.

T. Moritz, D. Menzel, W. Widdra
Surface Science 427-428, pp. 64-68

Ozone (O3) was synthesized in the condensed phase induced by electron bombardment of multilayer films of molecular oxygen condensed at temperatures below 30 K on metal surfaces. O3 formation was demonstrated by the observation of the asymmetric stretching ( ν3) and bending ( ν2) normal modes of vibration in a high-resolution electron energy-loss spectroscopy experiment, and by characteristic changes in electron-stimulated desorption of O−. The threshold electron energy for the O3 formation is found at 3.5±0.2eV. It corresponds to the formation of O(3P) associated with O−(2P) by dissociative electron attachment at condensed O2, followed by the third body reaction O+O2+O2→O3+O2. Above 5.1 eV bombarding energy, dissociative excitation of the O∗2 ( c1Σ−u,C3Δu,A3Σ+u,B3Σ−u) states is the main source of atomic oxygen O(3P) or O(1D) involved in the O3 synthesis.

S. Lacombe, F.Cemi, K. Jacobi, M. N. Hedhili, Y. Le Coat, R. Azria, M. Tronc
Phys. Rev. Lett. 79, 1146