PHYSICAL REVIEW MATERIALS 2, 066002 (2018) Plasmon spectroscopy: Robust metallicity of Au wires on Si(557) upon oxidation Z. Mamiyev, 1, 2 T. Lichtenstein, 1 C. Tegenkamp, 1, 2 C. Braun, 3 W. G. Schmidt, 3 S. Sanna, 4 , * and H. Pfnür 1, 2 , † 1 Institut für Festkörperphysik, Leibniz Universität Hannover, Appelstraße 2, 30167 Hannover, Germany 2 Laboratorium für Nano- und Quantenengineering (LNQE), Leibniz Universität Hannover, Schneiderberg 39, 30167 Hannover, Germany 3 Lehrstuhl für Theoretische Materialphysik, Universität Paderborn, 33095 Paderborn, Germany 4 Institut für Theoretische Physik, Justus-Liebig-Universität Gießen, Heinrich-Buff-Weg 16, D-35392 Gießen, Germany (Received 20 April 2018; revised manuscript received 31 May 2018; published 15 June 2018) We investigated initial steps of oxidation of the Si(557)-Au system by plasmon spectroscopy and first-principles calculations. The measurements, performed using an electron energy loss instrument with simultaneous high resolution in energy and momentum, reveal that metallicity is preserved under all oxidation conditions that are experimentally accessible in UHV. Corresponding simulations, performed within density functional theory, confirm this finding: Only the oxidation of the Si environment of the Au chains turned out to be strongly exothermic, with similar binding energy for adsorption on different structural elements. While large and site specific changes of the band structure were observed, the upper edge of the excitation spectrum of electron-hole pairs, to which plasmon dispersion is most sensitive, remains almost unchanged during the various steps of oxidation, due to the opposite and largely compensating contributions of different adsorption configurations. This investigation not only proves the robustness of metallicity of the gold chains upon oxidation of the surrounding environment of Si atoms, but also demonstrates the usefulness of plasmon spectroscopy in characterizing the electronic excitation spectrum of quasi-one-dimensional systems and unoccupied band structure. DOI: 10.1103/PhysRevMaterials.2.066002 I. INTRODUCTION Quasi-one-dimensional (1D) electronic systems have at- tracted a great deal of interest due to highly unusual properties such as quantization of conductance, extremes of electronic correlation manifested by spin-charge separation, charge and spin density waves [1,2], triplet superconductivity, and Lut- tinger liquid behavior [3–5]. Due to their inherent instability, however, structural embedding and understanding of the cou- pling to other dimensions is of high relevance and raises the question of how much of 1D properties survives under experi- mentally accessible conditions. Fortunately, many 1D proper- ties can still be observed in these quasi-1D systems [6–10]. The close relationship between low-energy plasmons and metallicity is well established in all dimensions [11]. Partic- ularly in two and one dimensions, the plasmonic dispersion goes to zero in the long wavelength limit [12]. In this limit, a linear dispersion for plasmons in quasi-1D metallic wires is predicted [13]. Atomic wires thus are ideal candidates for directed energy transport on the nanoscale. Such dispersions have indeed been found for regular arrays of atomic wires on insulating substrates [14–16]. Moreover, confinement effects in these metallic subunits on the surface lead to the formation of intersubband excitations [15–18]. The vicinal Si(111) surfaces represent an interesting play- ground in this context, since either single or double Au chains are formed on these surfaces, depending on the step orientation. While, e.g., on Si(553) and Si(775), double chains are observed * Simone.Sanna@theo.physik.uni-giessen.de † pfnuer@fkp.uni-hannover.de [8,19–22], the (335) and (557)-oriented Si-Au systems form wires with only a single atomic Au chain on each terrace [19,23]. The origin of this remarkable difference can only be due to the different type of step edge, which (formally) exhibits two dangling bonds per step edge atom compared to only one on the Si(553)-Au and Si(775)-Au surfaces. Common to all these structures is a Si-honeycomb chain located at the step edges [8,19,20]. While plasmon dispersions in such systems turned out to be purely 1D [24–26], a clear dependence of slopes on terrace widths and structural motif of the gold chains was found, taken as indications of dimensional crossover, as well as plasmonic coupling between the wires in the ordered arrays [26–29]. In this paper we extend previous studies of such sys- tems [16,26,29,30] and test the robustness specifically of the Si(557)-Au system, containing a single atomic chain per ter- race, against oxidation of its environment. This study has some relevance since oxidation is the first modification that will take place when such samples are brought into the environment. As a further purpose of the present study, we extend our tests of the predictions of quasi-1D plasmon theory [25,28] and in particular the close relationship between the plasmon dispersion and the continuum of electron-hole pair excitations characteristic for a metallic system, which we started very recently [29]. We demonstrated that the comparison of this continuum, as derived from band structure calculations, with experimental data of plasmon dispersion can yield direct information about the form of the occupied as well as of the unoccupied band structure in the vicinity of the Fermi level [29]. Using different Si(111) vicinal surfaces as test systems, and combining experimental electron energy loss spectroscopy with quantitative density-functional theory (DFT) calculations, 2475-9953/2018/2(6)/066002(6) 066002-1 ©2018 American Physical Society