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Epitaxial bilayer silicon oxide is a transferable two-dimensional material predicted to be a wide band gap semiconductor, with potential applications for deep UV optoelectronics, or as a building block of van der Waals heterostructures. The prerequisite to any sort of such applications is the knowledge of the electronic band structure, which we unveil using angle-resolved photoemission spectroscopy and rationalize with the help of density functional theory (DFT) calculations. We discover dispersing bands related to electronic delocalization within the top and bottom planes of the material, with two linear crossings reminiscent of those predicted in bilayer AA-stacked graphene, and semi-flat bands stemming from the chemical bridges between the two planes. This band structure is robust against exposure to air, and can be controlled by exposure to oxygen. We provide an experimental lower-estimate of the band gap size of 5 eV and predict a full gap of 7.36 eV using DFT calculations.

International audience; We report on the formation of a two-dimensional supramolecular Kondo lattice made of organic molecules comprising only C, N, and H atoms, namely metal-free phthalocyanines 2HPc (C 32 H 18 N 8), adsorbed on a Ag(111) surface. Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS), ultraviolet photoemission spectroscopy (UPS), and density functional theory (DFT) are used to investigate the electronic structure of the system for low molecular density and commensurate self-assemblies. Abrikosov− Suhl resonances that are characteristic of the arising of a Kondo effect are observed by using both STS and UPS at low temperature. Whereas freestanding 2HPc is a no-spin-bearing molecule, it acquires an unpaired electron in its πconjugated lowest unoccupied molecular orbital (LUMO) upon adsorption on Ag. As a result, the Kondo effect can be interpreted as originating from the interaction between this unpaired π-spin and the Ag Fermi sea. Extra side resonances are observed in STS spectra that are a manifestation of the coupling between the π electron and molecular vibrational excitations, qualifying the effect as the molecular vibrational Kondo effect. The variation of the Kondo temperature T K of single molecules and upon two-dimensional self-assembly is discussed. This study brings new insights into the Kondo-related physics in correlation with molecular spins and low dimensionality. Moreover, it may open new routes to the synthesis of organic molecular spintronic devices.

International audience; By means of angle-resolved photoemission spectroscopy and density functional theory calculations, we investigate the electronic structure of a Br or I atom overlayer on the Cu(111) surface produced by the well-known Ullmann coupling reaction. We found that the iodine adsorbate induces two spin–orbit split highly dispersive bands, which are well-separated from the bulk Cu 3d bands, whereas the bromine-induced bands are flat and largely hybridized with the copper states. Also, our measured constant energy maps show that the I-induced bands have a parabolic shape in the whole surface Brillouin zone, which is confirmed by our calculations. Overall, the agreement between theory and experiments is excellent, giving new insights into the electronic structure of halogen atoms on noble metals and their possible influence on the molecular electronic structure.

Silicon oxide can be formed in a crystalline form, when prepared on a metallic substrate. It is a candidate support catalyst and possibly the ultimately-thin version of a dielectric host material for two-dimensional materials (2D) and heterostructures. We determine the atomic structure and chemical bonding of the ultimately thin version of the oxide, epitaxially grown on Ru(0001). In particular, we establish the existence of two sub-lattices defined by metal-oxygen-silicon bridges involving inequivalent substrate sites. We further discover four electronic bands below Fermi level, at high binding energies, two of them forming a Dirac cone at K point, and two others forming semi-flat bands. While the latter two correspond to hybridized states between the oxide and the metal, the former relate to the topmost silicon-oxygen plane, which is not directly coupled to the substrate. Our analysis is based on high resolution X-ray photoelectron spectroscopy, angle-resolved photoemission spectroscopy, scanning tunneling microscopy, and density functional theory calculations., Comment: Main part : 31 pages, 6 figures / Supporting information : 13 pages, 11 figures

bibtex: ISI:000393443200020 bibtex\location:'1155 16TH ST, NW, WASHINGTON, DC 20036 USA',publisher:'AMER CHEMICAL SOC',type:'Article',affiliation:'Sicot, M (Reprint Author), Univ Lorraine, CNRS, Inst Jean Lamour, UMR 7198, BP 70239, F-54506 Vandoeuvre Les Nancy, France. Sicot, Muriel; Kierren, Bertrand; Fagot-Revurat, Yannick; Moreau, Luc; Granet, Julien; Malterre, Daniel, Univ Lorraine, CNRS, Inst Jean Lamour, UMR 7198, BP 70239, F-54506 Vandoeuvre Les Nancy, France. Tristant, Damien; Gerber, Iann C., Univ Toulouse, CNRS, INSA, LPCNO,UPS, 135 Ave Rangueil, F-31077 Toulouse, France. Tristant, Damien, Univ Toulouse, CNRS, CEMES, UPR 8011, 29 Rue Jeanne Marvig,BP 94347, F-31055 Toulouse, France. Cherioux, Frederic, Univ Bourgogne Franche Comte, CNRS, Inst FEMTO ST, 15B Ave Montboucons, F-25030 Besancon, France.','author-email':'muriel.sicot@univ-lorraine.fr',da:'2018-12-05','doc-delivery-number':'EJ7ZY','funding-acknowledgement':'Midi-Pyrenees Region; PRES Universite de Toulouse; Calcul en Midi-Pyrenees initiative-CALMIP [p0812]; GENCI-CINES [x2015096649]; CNRS','funding-text':'D. Tristant thanks the Midi-Pyrenees Region and the PRES Universite de Toulouse for Ph.D. funding. I. C. Gerber and D. Tristant also acknowledge the Calcul en Midi-Pyrenees initiative-CALMIP (Project p0812) for allocations of computer time. Part of this work was also performed using HPC resources from GENCI-IDRIS and GENCI-CINES (Project x2015096649). I. C. Gerber thanks CNRS for financial support.','journal-iso':'J. Phys. Chem. C','keywords-plus':'INITIO MOLECULAR-DYNAMICS; TOTAL-ENERGY CALCULATIONS; WAVE BASIS-SET; EPITAXIAL GRAPHENE; PHTHALOCYANINE MOLECULES; GRAPHITE; ADSORPTION; METALS; INTERCALATION; ARCHITECTURES','number-of-cited-references':'68','orcid-numbers':'Gerber, Iann/0000-0001-5091-2655','research-areas':'Chemistry; Science & Technology - Other Topics; Materials Science','researcherid-numbers':'Gerber, Iann/K-1622-2016','times-cited':'5','unique-id':'ISI:000393443200020','usage-count-last-180-days':'7','usage-count-since-2013':'63','web-of-science-categories':'Chemistry, Physical; Nanoscience & Nanotechnology; Materials Science, Multidisciplinary'\; International audience; The properties of two-dimensional supramolecular self-assemblies on surfaces depend on the fine balance between molecule substrate and molecule molecule interactions. In this article, we study the growth of 1,3,5-tri(4'bromophenyl)benzene (TBB) monolayer on graphene epitaxially grown on Ir(111) by means of low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) in interaction with the Gr/Ir(111) substrate, using density combined with a fully atomistic description of the molecules functional theory (DFT). In order to figure out the impact of the underlying metallic layer upon the self-assembling behavior of the molecules and their properties, we compare our results with those theoretically obtained on pristine graphene or experimentally achieved on highly oriented pyrolytic graphite (HOPG). We demonstrated that the use of the Ir layer allows the formation of large extended, continuous, and two-dimensional supramolecular networks lying even over Ir step edges like a carpet. In addition, we highlighted the obtention of two structural polymorphs never observed' on HOPG. In the light of DFT simulations, we assumed that the formation of these polymorphs is driven by the balance between molecule molecule interactions, due to halogen bonds (XB), and the tailored molecule surface interactions due to the presence of Ir layer.

Ullmann coupling or, more generally, dehalogenative aryl-aryl coupling, is one of the most widely exploited chemical reactions to obtain one- and two-dimensional polymers on metal surfaces. It is generally described as a two-step reaction: (i) dehalogenation, resulting in the formation of a stable intermediate organometallic phase and subsequent (ii) C-C coupling. The topology of the resulting polymer depends on the number and positions of the halogen atoms in the haloaromatic precursor, although its orientation and order are determined by the structure of the intermediate phase. Hitherto, only one intermediate structure, identified as an organometallic (OM) phase, has been reported for such a reaction. Here we demonstrate the formation of two distinct OM phases during the temperature-induced growth of poly(para-phenylene) from 1,4-dibromobenzene precursors on Cu(110). Beyond the already known linear-OM chains, we show that a phase reorganization to a chessboard-like 2D-OM can be activated in a well-defined temperature range. This new intermediate phase, revealed only when the reaction is carried out at low molecular coverages, was characterized by X-ray photoelectron spectroscopy, scanning tunneling microscopy and near-edge X-ray absorption fine structure spectroscopy, and modeled by density functional theory calculations. Our data show that the 2D-OM remains stable after cooling down the sample and is stabilized by four-Cu clusters at each node. The observation of such unexpected intermediate phase shows the complexity of the mechanisms underlying on-surface synthesis and broadens the understanding of Ullmann coupling, which continues to be astonishing despite its extensive use.

International audience; Cerium intermetallics have raised a lot of interest for the past forty years thanks to their very unusual and interesting electronic and magnetic properties. This can be explained by the peculiar electronic configuration of Ce (4f1) that allows different oxidation states leading to singular behavior such as quantum phase transitions, heavy-fermion behavior and the Kondo effect. In this work, we used a mixed-valence molecular analogue to study the Kondo effect down to the atomic scale by means of scanning tunneling microscopy/spectroscopy (STM/STS) for which new many-body effects are expected to emerge due to reduced dimensionality and specific chemical environment of the 4f-ion. For that purpose, double-decker molecular complexes hosting a Ce ion were synthesized and adsorbed onto Ag and Cu (111) surfaces forming two-dimensional lattices. As a result, we observed a zero-bias conductance resonance on Ag only indicative of a Kondo effect arising from the coupling between a molecular spin and the conducting electrons of the metallic surface. The emergence of the Kondo effect is discussed in terms of intermolecular and molecule/substrate interactions. This work expands the little knowledge to date on the structural and related electronic properties of Ce-based molecular systems on surfaces. In particular, it shows that Ce-based double deckers are good platforms to obtain insight into 4f-induced many-body effects down to the nanometer scale and in two-dimensional lattices. Moreover, this outcome has a strong impact for future applications of molecular devices in which both metals are commonly used as electrical contacts.

Équipe 102 : Surfaces et Spectroscopies; International audience; Both the optical and structural properties of Ce-doped SiO1.5 thin films were investigated. The Ce-related blue luminescence, which can be seen even at room temperature for as-grown films, exhibits a rather complex evolution with the annealing temperature. In particular, a strong decrease is observed when the films are annealed at 900 degrees C. Structural characterizations combining scanning transmission electron microscopy and atom probe tomography reveal the formation of Si- and Ce-rich clusters at this temperature, thus demonstrating that the decreasing Ce-related luminescence is due to concentration induced quenching. For annealing temperatures higher than 900 degrees C, the Ce-related luminescence increases. The different structural characterizations provide clear experimental evidence of a phase separation occurring at the nanoscale between pure Si nanocrystals and Ce-rich clusters having a stoichiometry close to the cerium silicates Ce2Si2O7 or Ce-4.667(SiO4)(3)O. The latter compounds are optically active thereby explaining the increased Ce-related luminescence observed at the highest annealing temperature.

The electronic structure of the reentrant superconductor $\mathrm{Eu}({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14}$)${}_{2}{\mathrm{As}}_{2}$ (${T}_{c}=22$ K) with coexisting ferromagnetic order (${T}_{M}=18$ K) is investigated using angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy. We study the in-plane and out-of-plane band dispersions and Fermi surface (FS) of $\mathrm{Eu}{({\mathrm{Fe}}_{0.86}{\mathrm{Ir}}_{0.14})}_{2}{\mathrm{As}}_{2}$. The near-${E}_{F}$ Fe-$3d$-derived band dispersions near the $\mathrm{\ensuremath{\Gamma}}$ and $X$ high-symmetry points show changes due to Ir substitution, but the FS topology is preserved. From momentum-dependent measurements of the superconducting gap measured at $T=5$ K, we estimate an essentially isotropic $s$-wave gap ($\mathrm{\ensuremath{\Delta}}\ensuremath{\sim}5.25\ifmmode\pm\else\textpm\fi{}0.25$ meV), indicative of strong-coupling superconductivity with $2\mathrm{\ensuremath{\Delta}}/{k}_{B}{T}_{c}\ensuremath{\simeq}5.8$. The gap gets closed at temperatures $T\ensuremath{\ge}10$ K, and this is attributed to the resistive phase which sets in at ${T}_{M}=18$ K due to the ${\mathrm{Eu}}^{2+}$-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound.

Ullmann coupling is the most common approach to form surface-confined one- and two-dimensional conjugated structures from haloaryl derivatives. The dimensions of the formed nanostructures can be controlled by the number and location of halogens within the molecular precursors. Our study illustrates that the type of halogen plays an essential role in the design, orientation, and extent of the surface-confined organometallic and polymeric nanostructures. We performed a comparative analysis of five 1,4-dihalobenzene molecules containing chlorine, bromine, and iodine on Cu(110) using scanning tunneling microscopy, fast-X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies. Our experimental data identify different molecular structures, reaction temperatures and kinetics depending on the halogen type. Climbing image nudged elastic band simulations further clarify these observations by providing distinct diffusion paths for each halogen species. We show that in addition to the structure of the building blocks, the halogen type has a direct influence on the morphology of surface-confined polymeric structures based on Ullmann coupling.

We study the electronic structure of HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} (Hg1223; T_{c}=134 K) using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Resonant valence band PES across the O K edge and Cu L edge identifies correlation satellites originating in O 2p and Cu 3d two-hole final states, respectively. Analyses using the experimental O 2p and Cu 3d partial density of states show quantitatively different on-site Coulomb energy for the Cu site (U_{dd}=6.5±0.5 eV) and O site (U_{pp}=1.0±0.5 eV). Cu_{2}O_{7}-cluster calculations with nonlocal screening explain the Cu 2p core level PES and Cu L-edge XAS spectra, confirm the U_{dd} and U_{pp} values, and provide evidence for the Zhang-Rice singlet state in Hg1223. In contrast to other hole-doped cuprates and 3d-transition metal oxides, the present results indicate weakly correlated oxygen holes in Hg1223.

Motivated by the different behaviour of Sn/Si(1 1 1) and Sn/Ge(1 1 1) in their metal-insulator transition, we have explored the possibility of growing Sn on an ultra-thin Ge layer strained on top of a Si(1 1 1) substrate. We have demonstrated by scanning tunneling microscopy and low energy electron diffraction that a ( 2 3 × 2 3 ) R30° reconstruction can be stabilized under adequate growth conditions. The size of the reconstructed domains increases progressively up to a coverage of 1.3 monolayer of Sn, as determined by a combined study of scanning tunneling microscopy and core level spectroscopy. This coverage differs from that of Sn/Si(1 1 1) and Sn/Ge(1 1 1) exhibiting Mott phases. Angle resolved photoemission shows that the highly strained reconstruction is a band insulator, with a surface state dispersing roughly between 1300 and 2300 meV of binding energy.

International audience; Te capping layer on topological insulator (TI) surface provides an efficient way for the protection of surface from oxidation. However, when using Te to protect the surface of (Bi1−xSbx)2Te3 ternary topological insulators, it is still unclear for the influence of Te on the surface chemical composition and the surface state bands after the procedure of capping and desorption of Te. Here, we have performed a systematic study of the surface morphology, crystallinity, chemical composition as well as the band structure of (Bi1−xSbx)2Te3 after the desorption of the Te capping layer. Our results confirm a good recovery of the surface state bands of the (Bi1−xSbx)2Te3 ternary topological insulators with different Bi and Sb compositions. The chemical composition of (Bi1−xSbx)2Te3 remains almost unchanged after the desorption of Te. This study proves that the Te capping layer works as a suitable protection for ternary (Bi1−xSbx)2Te3 TI layers, allowing for ex-situ transfer of TI samples in air. This opens the way for the development of metal/TI hybrid structures for advanced spintronic applications.

Équipe 102 : Surfaces et Spectroscopies; International audience; Adsorption of molecular chlorine on Au(111) has been studied with a low-temperature (5 K) scanning tunneling microscope in combination with density functional theory calculations. The formation of AuCl2 quasi-molecules was detected at chlorine coverage exceeding 0.33 ML. The self-organization of the AuCl2 species into the ordered ``honeycomb'' structure was clearly demonstrated for coverages close to saturation.

The surface structure of alkali doped Si(1 1 1):B ultra-thin films has been studied by low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). A comparative study of K/Si(1 1 1)-3 × 1 and K/Si(1 1 1):B- 2 3 × 2 3 R 30 interfaces allowed us to determine the saturation coverage to be 0.5 monolayer in the later case. The 2 3 -surface reconstruction is shown to be a common property of pure K, Rb, Cs materials and K 0.4 Rb 0.6 alloys but progressively disappears if Rb is replaced by Ca. Taking into account the existence of two distinct boron sites in the ratio 1/3 as seen from B-1s core levels spectra, LAPW-DFT calculations have been carried out in order to optimize the atomic structure. As a result, alkali adatoms are shown to form trimers leading to a large modulation of the Si B bonds accompanied by an inhomogeneous doping of the dangling bonds in agreement with voltage dependent STM images.

A joint experimental and theoretical study is presented for the electronic structures of copper oxides including $\mathrm{C}{\mathrm{u}}_{2}\mathrm{O}$, CuO, and the metastable mixed-valence oxide $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3}$. The optical band gap is determined by experimental optical absorption coefficient, and the electronic structure in valence and conduction bands is probed by photoemission and electron energy loss spectroscopies, respectively. The experimental results are compared with many-body $GW$ calculations utilizing an additional on-site potential for $d$-orbital energies that facilitates tractable and predictive computations. The side-by-side comparison between the three oxides, including a band insulator $(\mathrm{C}{\mathrm{u}}_{2}\mathrm{O})$ and two Mott/charge-transfer insulators (CuO, $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3})$ leads to a consistent picture for the optical and band-structure properties of the Cu oxides, strongly supporting indirect band gaps of about 1.2 and 0.8 eV in CuO and $\mathrm{C}{\mathrm{u}}_{4}{\mathrm{O}}_{3}$, respectively. This comparison also points towards surface oxidation and reduction effects that can complicate the interpretation of the photoemission spectra.

Low dimensional systems are characterized by at least one spatial dimension of only some atoms. Such size reduction has often important consequences for physical properties. Electronic correlation and electron–phonon coupling can originate Mott insulators or charge density waves (CDWs), both phenomena enhanced by dimensionality reduction. Interfaces offer a natural way of reducing the dimensionality. Among all the surfaces, semiconducting surfaces are particularly well adapted for electronic correlation studies. In them, correlation is enhanced because of the low dimension, the electronic localization in dangling bonds and the large inter-orbital distances in reconstructions. Despite these factors favoring correlation, eventually stronger than in bulk systems, the field is by far much less developed. We review here the discovery of correlated surfaces, while studying the Schottky barrier of alkalis on Si or GaAs, and coetaneous studies on SiC. We summarize then the studies on K/Si(111):B, whose was considered a surface Mott insulator with an important electron-phonon coupling. The recent discovery of a symmetry has been first interpreted as evidence of a bipolaronic insulator, but new findings have finally proven it to be a band insulator. We will then focus on the model system Sn/Ge(111). The last unclear issue about the (3 × 3) reconstruction at 150 K was related to the Sn 4d core level. High-resolution photoemission has clarified the core level deconvolution while refining the structural model. This metallic reconstruction is sensitive to electronic correlations which trigger a phase transition to a Mott phase below 30 K.

Very recently, new LEED, STM and ARPES measurements on the (1/3)ML K/Si(1 1 1):B interface have shed the light on the role of phonons and force to reconsider the nature of the ground state, formerly proposed to be a Mott insulator. In this paper, we present simulations of the one-electron spectral function A ( E ) . These are based on the atomic Holstein–Hubbard model, which is relevant in the strong electron–phonon coupling limit. The exact spectral function has been analysed carefully with special attention on the gap behaviour as a function of the on-site repulsion. We show that the spectral funcion is the same for opposite values of the effective on-site repulsion. The observed reconstruction favours the negative- U e f f case, which corresponds to a bipolaronic charge-ordered ground state. Finally, the ARPES lineshape is reproduced correctly with fitting parameters in agreement with the literature.

We report here on low energy electron diffraction (LEED), Auger spectroscopy, scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES) studies of ultrathin films of K/Si(111)-√3×√3R30:B. A potassium-induced surface state is evidenced being maximum at the saturation coverage. ARPES data clearly evidence the folding of the K-induced surface band near the expected kF attesting of a large Mott gap Δ=500 meV and a very narrow measured bandwidth W=140 meV. This clearly signs the strongly correlated nature of this material. Nevertheless, by decreasing the temperature, a novel insulator to insulator surface phase transition is observed characterized by a doubling of the unit cell together with a stabilization of the surface band. In addition, the linear temperature dependence of the ARPES linewidth suggests these materials are characterized by a strong electron-phonon coupling. Therefore, such a phase transition can be explained if the electron-phonon coupling is large enough to compensate the strong on-site repulsion defining a bi-polaronic insulating ground state instead of a Mott-Hubbard one. [DOI: 10.1380/ejssnt.2009.259]

A self-ordered nanoporous lattice formed by individual chlorine atoms on the Au(111) surface has been studied with low-temperature scanning tunneling microscopy, low-energy electron diffraction, and density functional theory calculations. We have found out that room-temperature adsorption of 0.09--0.30 monolayers of chlorine on Au(111) followed by cooling below 110 K results in the spontaneous formation of a nanoporous quasihexagonal structure with a periodicity of 25--38 \AA{} depending on the initial chlorine coverage. The driving force of the superstructure formation is attributed to the substrate-mediated elastic interaction.

Spectroscopic effects associated with the superperiodic surface structure have been observed in Au(1 1 1) vicinal surfaces and nanostructured systems. In the vicinal Au(23 23 21) surface, high resolution angle resolved photoemission spectroscopy shows the opening of several gaps in the surface band structure, whereas scanning tunneling spectroscopy reveals the energy dependence of the electronic density. These combined spectroscopic data allow to determine the reconstruction potential by deducing their first Fourier components. We also demonstrate that due to the peculiar growth on this Au vicinal surface, we can obtain a self-assembled superlattice of triangular Ag islands. The high ordering of the nanostructures leads to homogenous electronic properties.

Équipe 102 : Surfaces et Spectroscopies; International audience; Adsorption of chlorine atoms on the Cu(111) surface has been studied with low-temperature scanning tunneling microscopy (LT-STM) in a combination with density functional theory (DFT) calculations. At submonolayer coverage range (theta

Dynamical phase transitions are a challenge to identify experimentally and describe theoretically. Here, we study a new reconstruction of Sn on silicon and observe a reversible transition where the surface unit cell divides its area by a factor of 4 at 250 degrees C. This phase transition is explained by the 24-fold degeneracy of the ground state and a novel diffusive mechanism, where four Sn atoms arranged in a snakelike cluster wiggle at the surface exploring collectively the different quantum mechanical ground states.

Epitaxial Ag ultra-thin films grown on Au(1 1 1) have been studied by scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES). In the submonolayer range, the spectroscopic measurements reveal the existence of two parabolic dispersive surface Shockley bands associated with the Au substrate and the Ag islands, respectively. The surface state in the Ag region is shifted by about 160 meV towards the Fermi level. High resolution measurements on Au(1 1 1) reveal the splitting of each band in two spin–orbit contributions. A reduction of this spin–orbit splitting is observed for a 1 ML Ag film. Moreover, an annealing of the interface leads to an interdiffusion and to an additional shift of the surface state.

Combined high-resolution ARPES with STM measurements are presented to investigate the Ag/Cu(111) interface. In the submonolayer range, the irreversible moire to triangular structural transformation is revealed by the change of the surface state energy. A transient state, seen in our ARPES measurements, is connected to a nanoscopic pre-formed triangular area evidenced by STM. In addition, a careful analysis of the annealing effect on a 1.6 monolayer film suggests that the presence of the second layer limits the entrance of vacancies in the topmost Cu layer and prevents the formation of the triangular dislocation loop pattern. These results emphasize how to use surface states as a sensitive probe of surface and interface reconstructions from their spectroscopic signature.

The atomic structure and surface electronic properties of Ag epitaxial ultra-thin films on Cu(111) substrate have been studied by scanning tunneling microscopy (STM) and angle-resolved photoemission (ARPES). In the submonolayer range, Ag films exhibit a surface reconstruction which depends on the preparation temperature. At T=150 K, a 9×9 moire superstructure is observed resulting from the superposition of Ag and Cu lattices whereas at room temperature a triangular structure is stabilized. Photoemission spectroscopy shows that the surface state energy reflects the morphology and the structure of the Ag films. Below one monolayer, two parabolic dispersions indicate the presence of confined surface states in the Cu terraces and in the Ag islands. The different surface state energies associated with the two Ag layer reconstructions allow to study the irreversible transition from the moire to the triangular structure.

Shockley surface states have been studied in Ag epitaxial ultra-thin films on Cu(111) by angle-resolved photoemission (ARPES), scanning tunneling microscopy (STM) and spectroscopy (STS). The morphology of these films is characterized by large Ag islands growing along the step edges of the substrate. In the submonolayer range, the two spectroscopic techniques indicate the existence of two surface states associated with the Cu terraces and the Ag islands suggesting the confinement of the electronic states. Moreover, two different atomic reconstructions depending on the preparation temperature have been evidenced and lead to different surface state energies.

Temperature-dependent high-resolution ultra-violet photoemission spectroscopy (PES) measurements combined with tight-binding linear muffin-tin orbital (LMTO) atomic-sphere approximation (TB-LMTO-ASA) band-structure calculations are carried out to study the polar ordering in the low-dimensional ferroelectric CuInP2Se6. In the paraelectric phase, the energy-dependent PES spectra are in good agreement with our LMTO calculations. Below Tc ? 230 K, evidence for a strong redistribution of the experimental density of occupied states (DOS) is given. The LMTO calculations, accounting for the off-centre position of Cu atoms in the low-T phase, allow us to interpret this behaviour in terms of a redistribution of the partial Cu 3d DOS. This strong interplay between electronic and crystallographic instabilities is analysed in the light of a second-order Jahn?Teller effect acting as the driving force of the ferroelectric transition.

At magnetic fields where an oscillatory structure in the Hall constant RH = ρxy/B points to the localization of electrons, we investigate the NMR-induced change in the transverse magnetoresistivities. Because the nuclear hyperfine field BHF acts only on the spin of the electrons, the role of spin-splitting can be seperated from that of the external magnetic field for the magnetic field dependence of RH. This experiment shows that the oscillatory structure in RH is directly related to a redistribution in the occupation of the two spin states in the lowest Landau level.

In the spin-Peierls compound CuGeO$_{3}$, the commensurate-incommensurate transition concerning the modulation of atomic position and the local spin-polarization is fully monitored at T=0 by the application of an external magnetic field ($H$) above a threshold value $H_{c}\simeq $ 13 Tesla. The solitonic profile of the spin-polarization, as well as its absolute magnitude, has been precisely imaged from $^{65}Cu$ NMR lineshapes obtained for $h=(H-H_{c})/H_{c}$ varying from 0.0015 to 2. This offers a unique possibility to test quantitatively the various numerical and analytical methods developed to solve a generic Hamiltonian in 1-D physics, namely strongly interacting fermions in presence of electron-phonon coupling at arbitrary band filling., Comment: 3 pages, 4 eps figures, RevTeX, submitted to Physical Review Letter

In a magnetic field, spin-ladders undergo two zero-temperature phase transitions at the critical fields Hc1 and Hc2. An experimental review of static and dynamical properties of spin-ladders close to these critical points is presented. The scaling functions, universal to all quantum critical points in one-dimension, are extracted from (a) the thermodynamic quantities (magnetization) and (b) the dynamical functions (NMR relaxation). A simple mapping of strongly coupled spin ladders in a magnetic field on the exactly solvable XXZ model enables to make detailed fits and gives an overall understanding of a broad class of quantum magnets in their gapless phase (between Hc1 and Hc2). In this phase, the low temperature divergence of the NMR relaxation demonstrates its Luttinger liquid nature as well as the novel quantum critical regime at higher temperature. The general behaviour close these quantum critical points can be tied to known models of quantum magnetism.

We present an overview of NMR data on the inorganic spin-Peierls compound CuGeO 3 focusing on its high magnetic field incommensurate phase. In this phase, the analysis of the NMR line shape provide the first detailed microscopic picture of the local spin density corresponding to a magnetic soliton lattice. While on a qualitative level the temperature and magnetic field dependence of this soliton lattice correspond to the theoretical predictions, quantitative disagreements may provide important information on the effects of quasiparticle interactions, interchain coupling and magnetoelastic coupling. The field dependence of the line shape also indicates that the incommensurate phase may be divided into sub-phases with one hypothetical transition line situated near 26 T.

We present an extensive NMR study of the spin-1/2 antiferromagnetic Heisenberg ladder $\mathrm{Cu}{}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}\mathrm{Cl}{}_{4}$ in a magnetic field range 4.5--16.7 T. By measuring the proton NMR relaxation rate $1/{T}_{1}$ and varying the magnetic field around the critical field ${H}_{c1}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\ensuremath{\Delta}/g{\ensuremath{\mu}}_{B}\ensuremath{\approx}7.5\mathrm{T}$, we have studied the transition from a gapped spin liquid ground state to a gapless magnetic regime which can be described as a Luttinger liquid. We identify an intermediate regime $T\ensuremath{\ge}|{H\ensuremath{-}H}_{c1}|$, where the spin dynamics is (possibly) only controlled by the $T\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ critical point ${H}_{c1}$.

The ${}^{1}\mathrm{H}$ hyperfine shift $K$ and NMR relaxation rate ${T}_{1}^{\ensuremath{-}1}$ have been measured as a function of temperature in the $S\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1/2$ Heisenberg antiferromagnetic ladder compound ${\mathrm{Cu}}_{2}({\mathrm{C}}_{5}{\mathrm{H}}_{12}{\mathrm{N}}_{2}{)}_{2}{\mathrm{Cl}}_{4}$. The presence of a spin gap $\ensuremath{\Delta}\ensuremath{\simeq}{J}_{\ensuremath{\perp}}{\ensuremath{-}J}_{\ensuremath{\parallel}}$ in this strongly coupled ladder ( ${J}_{\ensuremath{\parallel}}l{J}_{\ensuremath{\perp}}$) is supported by the $K$ and ${T}_{1}^{\ensuremath{-}1}$ results. By comparing ${T}_{1}^{\ensuremath{-}1}$ at two different ${}^{1}\mathrm{H}$ sites, we infer the evolution of the spectral functions ${S}_{z}(q,{\ensuremath{\omega}}_{n})$ and ${S}_{\ensuremath{\perp}}(q,{\ensuremath{\omega}}_{n})$. When the gap is significantly reduced by the magnetic field, two different channels of nuclear relaxation, specific to gapped antiferromagnets, are identified and are in agreement with theoretical predictions.

Équipe 102 : Surfaces et Spectroscopies; International audience; We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 +/- 10 K.

Équipe 102 : Surfaces et Spectroscopies; International audience; Boron vacancies at the Si( 111):B-root 3 surface are model systems in the comprehension of strongly correlated semiconductor surfaces. By using scanning tunneling spectroscopy, the origin of the single-vacancy electronic structure is addressed. It is shown to originate from the localization of a well-identified dangling-bond surface state with significant B character. The bivacancy defect, which is characterized by energy-split bonding and antibonding states, is interpreted within the textbook diatomic molecule picture. From the hopping parameter, we determine the bandwidth of the surface state from which the impurity state derives and evaluate the strength of many-body effects. Our analysis supports the realization of the Mott-Hubbard insulator state in half-filled dangling-bond surface states on root 3-reconstructed surfaces, as proposed recently for SiC(0001), Sn/Ge(111), and Sn/Si(111)

Équipe 102 : Surfaces et Spectroscopies; International audience; We report the synthesis and first electronic characterization of an atomically thin two dimensional pi-conjugated polymer. Polymerization via Ullmann coupling of a tetrabrominated tetrathienoanthracene on Ag(111) in ultra-high vacuum (UHV) produces a porous 2D polymer network that has been characterized by scanning tunnelling microscopy (STM). High-resolution X-ray photoelectron spectroscopy (HRXPS) shows that the reaction proceeds via two distinct steps: dehalogenation of the brominated precursor, which begins at room temperature (RT), and C-C coupling of the resulting Ag-bound intermediates, which requires annealing at 300 degrees C. The formation of the 2D conjugated network is accompanied by a shift of the occupied molecular states by 0.6 eV towards the Fermi level, as observed by UV photoelectron spectroscopy (UPS). A theoretical analysis of the electronic gap reduction in the transition from monomeric building blocks to various 1D and 2D oligomers and polymers yields important insight into the effect of topology on the electronic structure of 2D conjugated polymers.

The commensurate-incommensurate phase transition in chlorine monolayer on Ag(111) has been studied with low-temperature scanning tunneling microscopy and low-energy electron diffraction. We report the observation of self-interstitial defects: two-dimensional (2D) crowdions formed at the initial stage of the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)R30${}^{\ensuremath{\circ}}$ lattice compression. After critical coverage of 0.34 monolayer (ML) the crowdions condense into a domain-wall fluid. As coverage further increases, the domain-wall crystal solidifies with average wall separation of 23 \AA{}.

From scanning tunneling spectroscopy measurements, we recently found evidence for the effect of electron-phonon interactions on the lifetime of hot electrons and holes. This information was obtained by measuring the linewidth of quantum well states resulting from the confinement of Shockley states in truncated hexagonal Ag nanopyramids. In the present paper, we show that a careful analysis allows us to obtain both the intrinsic and extrinsic broadening contributions. This latter contribution results from the lossy boundary scattering which is strongly reduced in this peculiar geometry. Indeed, we deduce the energy dependence of the reflection coefficient which remains higher, as usually observed in nanostructures like adatom islands and quantum corrals.

The structure of saturated chlorine layer on Ag(111) has been studied with low temperature scanning tunneling microscopy and density functional theory. For the first time atomic-resolution STM images of saturated chlorine coverage have been obtained. STM images demonstrate coexistence of the domain with (3 $\ifmmode\times\else\texttimes\fi{}$ 3)-like reconstruction and numerous bright objects identified as Ag${}_{3}$Cl${}_{7}$ clusters. According to our model supported by DFT calculations, clusters are formed on the boundaries between the adjacent ($3\ifmmode\times\else\texttimes\fi{}3$) antiphase domains. These boundaries have a characteristic triangular shape and are formed by six chlorine atoms chemisorbed on the triangular silver island with local periodicity (1 $\ifmmode\times\else\texttimes\fi{}$ 1).

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2√3 x 2√3R30 surface reconstruction observed previously by LEED on 0.5 ML K/Si:B. A large K-induced vertical lattice relaxation occurring only for 3/4 of Si adatoms is shown to quantitatively explain both the chemical shift of 1.14 eV and the ratio 1/3 measured on the two distinct B 1s core levels. A gap is observed between valence and conduction surface bands by ARPES and STS which is shown to have mainly a Si-B character. Finally, the calculated STM images agree with our experimental results. This work solves the controversy about the origin of the insulating ground state of alkali-metal/Si(111):B semiconducting interfaces which were believed previously to be related to many-body effects, This work has received the financial support of the French ANR SURMOTT program (ANR-09-BLAN- 0210-01) and the Spanish MICIIN under Project No. FIS2010-16046

9 páginas, 3 figuras.-- et al., The inhibition in wave propagation at band gap energies plays a central role in many areas of technology such as electronics (electron gaps), nanophotonics (light gaps) and phononics (acoustic gaps), among others. Here we demonstrate that metal surfaces featuring free-electron-like bands may become semiconducting by periodic nanostructuration. We combine scanning tunneling spectroscopy and angle-resolved photoemisssion to accurately determine the energy-dependent local density of states and band structure of the Ag/Cu(111) noble metal interface patterned with an array of triangular dislocations, demonstrating the existence of a 25 meV band gap that extends over the entire surface Brillouin zone. We prove that this gap is a general consequence of symmetry reduction in close-packed metallic overlayers; in particular, we show that the gap opening is due to the symmetry lowering of the wave vector group at the K point from C3v to C3., This work was supported in part by the Spanish MICINN (MAT2007-66050, MAT2007-63083 and Consolider NanoLight.es), the EU (NMP4-SL-2008-213669-ENSEMBLE), the Basque Government (IT-257-07) and the Centre National de la Recherche Scientifique (CNRS).