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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.

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.

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]

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

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.

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; 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)

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).

In this paper, we present a systematic study of the structural and electronic properties of the alkali/Si(111):B interface, combining low-energy electron diffraction and (angle-resolved) photoemission spectroscopy. We show that potassium (K), rubidium (Rb), and caesium (Cs) ultrathin films, including solid solutions K-Rb, share the same physics. First of all, the $2\sqrt{3}\ifmmode\times\else\texttimes\fi{}2\sqrt{3}R30\ifmmode^\circ\else\textdegree\fi{}$ surface reconstruction, evidenced only very recently using potassium is shown to be a common feature at the saturation coverage. A $3\ifmmode\times\else\texttimes\fi{}3$ reconstruction is also observed for coverages just below saturation. From an electronic point of view, the band features found with K are generalized. As a distinct point, we evidence that the surface state is constituted by two subbands which have the reconstruction symmetry. At low temperature (55 K), a systematic shift of the surface state toward higher binding energies is observed together with a reduction of its linewidth. A close inspection of the data reveals fine, systematic effects depending on the alkali. Indeed as substituting from K to Cs, the surface-state binding energy is reduced continuously whereas the bandwidth increases. K differs from Rb and Cs, by a smaller reduction of the linewidth and a higher energy shift upon cooling. These features are interpreted consistently within the bipolaron model, which was invoked previously to explain the insulating nature of the K/Si:B surface state. Polaronic effects are shown to be enhanced with K, as already demonstrated on alkali/GaAs(110) interfaces.

We present an overview of NMR data on the inorganic spin-Peierls compound CuGeO 3 in its uniform and dimerised phase, and focus on its high magnetic field incommensurate phase. In this latter 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.

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.

The structure of the chlorine induced reconstruction of Ag(111) has been studied by a combination of low-temperature scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and density-functional theory (DFT). We demonstrate that previously observed mysterious LEED pattern arises as a result of diffraction from a system of small $(15--30\text{ }\text{\AA{}})$ triangular antiphase domains with a new $(3\ifmmode\times\else\texttimes\fi{}3)$ superstructure. In our model supported by DFT calculations, within a $(3\ifmmode\times\else\texttimes\fi{}3)$ unit cell the upper silver layer reconstructs forming a couple of three-atom triangles placed in fcc and hcp sites of the substrate. Chlorine atoms occupy fourfold hollow sites between these triangles. The corner holes, which look like depressions in the STM images, are also occupied by chlorine atoms.

We present a scanning tunneling microscopy investigation of the dynamics of hot electrons and holes in Ag pyramidal nanostructures. The geometry of the nanostructure leads to a strong reduction of the decay mechanism into the bulk states and then to a large reflection coefficient of the surface electronic waves. Therefore, in contrast to quantum corrals and adatom islands which show a dominant lossy scattering contribution to the linewidth, the narrow observed structures in the differential conductivity spectra reveal the expected linewidth minimum at the Fermi energy. The electron-phonon contribution to the lifetime is shown to be dominant, in agreement with previous photoemission measurements.

We report on new low-energy electron diffraction, scanning tunneling microscopy, and angle-resolved photoemission spectroscopy studies of $\mathrm{\text{alkali-metal}}/\mathrm{Si}(111)$ previously established as having a Mott-insulating ground state at surface. The observation of a strong temperature dependent Franck-Condon broadening of the surface band together with the novel $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\ensuremath{\rightarrow}2(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ charge and lattice ordering below 270 K evidence a surface charge density wave in the strong electron-phonon coupling limit ($g\ensuremath{\approx}8$). Both the adiabatic ratio $\ensuremath{\hbar}{\ensuremath{\omega}}_{0}/t\ensuremath{\approx}0.8$ and the effective pairing energy ${V}_{\mathrm{eff}}=U\ensuremath{-}2g\ensuremath{\hbar}{\ensuremath{\omega}}_{0}\ensuremath{\approx}\ensuremath{-}800\text{ }\text{ }\mathrm{meV}$ are consistent with the possible formation of a bipolaronic insulating phase consisting of alternating doubly occupied and unoccupied dangling bonds as expected in the Holstein-Hubbard model.

International audience; Noble metals exhibit a Shockley state that can be confined in one direction due to the interaction with steps in vicinal surfaces. We observe here that self-organized Co islands on Au(788) can also confine the surface state to the orthogonal direction. The two-dimensional confinement of the surface state is studied by scanning tunneling spectroscopy as well as photoemission. Several confined states are revealed by the standing-wave pattern which appears between the Co nanodots. The band dispersion shows the opening of three gaps. These gaps report on the magnitude of the surface potential, which is drastically different from that of the Au substrate. These results show how self-organization modulates the electronic potential, leading to a periodical confinement of the surface state.

The self-organized growth of nanostructures on surfaces could offer many advantages in the development of new catalysts, electronic devices and magnetic data-storage media. The local density of electronic states on the surface at the relevant energy scale strongly influences chemical reactivity, as does the shape of the nanoparticles. The electronic properties of surfaces also influence the growth and decay of nanostructures such as dimers, chains and superlattices of atoms or noble metal islands. Controlling these properties on length scales shorter than the diffusion lengths of the electrons and spins (some tens of nanometres for metals) is a major goal in electronics and spintronics. However, to date, there have been few studies of the electronic properties of self-organized nanostructures. Here we report the self-organized growth of macroscopic superlattices of Ag or Cu nanostructures on Au vicinal surfaces, and demonstrate that the electronic properties of these systems depend on the balance between the confinement and the perturbation of the surface states caused by the steps and the nanostructures' superlattice. We also show that the local density of states can be modified in a controlled way by adjusting simple parameters such as the type of metal deposited and the degree of coverage.

We have studied by scanning tunneling microscopy (STM) the effect of the reconstruction of a stepped Au(1 1 1) surface on the growth of silver sub-monolayer deposition. For narrow terraces, the reconstruction is disturbed and its pattern changes, Ag growth is therefore influenced. Thus growth of Ag on Au(7 8 8) vicinal surface can be controlled and leads to the formation of a highly ordered superlattice of nanostructures. Moreover, we show by tunneling conductance images that Ag islands exhibit electronic confinement effects of the Shockley surface state. Due to the homogeneity of their shapes and sizes, all the nanostructures of the self-assembled superlattice should exhibit similar electronic properties.

An overview of structural, chemical, and electronic properties of epitaxial $\mathrm{Ag}∕\mathrm{Au}(111)$ ultrathin films investigated by scanning tunneling microscopy and scanning tunneling spectroscopy and angle-resolved photoelectron spectroscopy is presented. New insights are exhibited: (i) a short-range ordered surface reconstruction is clearly observed for deposition at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; (ii) self-organized ordering of Ag islands is obtained at $T\ensuremath{\simeq}80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this whole temperature range, the $\mathrm{Ag}∕\mathrm{Au}(111)$ interface is shown to be (nearly) abrupt. An annealing of the elaborated room temperature leads to a strong intermixing and favors the formation of a chemically disordered surface alloy. The Shockley state parameters have been fully characterized for both interfaces. Surprisingly, the Rashba parameter is shown to scale the binding energy of the surface state in both cases. A simple one-dimensional model, taking into account the exponential decrease of the surface state wave function, allows us a quantitative understanding of the evolution of the surface state parameters. Indeed, the strength of the spin-orbit splitting is shown to be proportional to the number of heavy atoms probed by the surface state wave function, revealing its atomic character. Therefore, the strength of the Rasba coupling is shown to be tuned by adjusting the number of Ag epitaxial layers or the $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Au}$ alloy composition.

Submitted to PRL; Opening of several gaps in the surface band structure of vicinal Au(23 23 21) surface has been evidenced by high resolution photoemission spectroscopy whereas the energy dependence of the surface electronic density have been determined by scanning tunneling spectroscopy. From a methodological point of view, the values of the gaps and the phase of the electronic density allow to estimate the reconstruction potential which yields the electronic Bragg diffraction, the gap formation and the modulation of the electronic density. These spectroscopic behaviors give a pedagogical illustration of one of the basic concepts of solid state physics.

A Comment on the Letter by F. Schiller et al., Phys. Rev. Lett. 94, 016103 (2005). The authors of the Letter offer a Reply.

We present a detailed photoemission study of the surface state dispersion in 0--10 ML thin Ag films on Au(111). The dispersion of the $L$-gap Shockley-type surface state changes monotonically from the one of Au towards the one of Ag with increasing layer thickness indicating an extension of the wave function into the solid. High resolution photoemission (PES) enables an insight into decreasing spin-orbit splitting of the surface state in Ag-covered-Au(111). The photoemission data have been well reproduced by ab initio band structure calculations using the WIEN2K code. The dependence of the binding energy on the increasing silver film thickness is indicative of the localization of the surface state at the surface by comparing it with the calculated electron density of the surface state. Measurements on 10 ML $\mathrm{Ag}∕\mathrm{Au}(111)$ reveal the $d$-band states of Ag, but one still observes $sp$-related quantum well states of silver within the projected $sp$-band gap of gold.

International audience; We proposed a method based on scanning tunneling spectroscopy of the Shockley states in thin Ag films on Au111 to get structural and chemical information and to image the buried stepped substrate. Using the thickness dependence of the energy of the Shockley surface state band, we have been able to determine the local thickness of the film over the surface and to derive the growth mode of this system. We also evidenced the alloying mechanism which occurs at the buried substrate step edges under annealing. Our method could in principle be applied to any epitaxial metallic systems having Shockley-type surface states.

We have studied the modification of Au(111) surface state parameters with Ag deposition. We unambiguously evidence a continuous decrease in the spin-orbit splitting of the two surface state subbands upon increasing Ag coverage. An annealing leads to the formation of a chemically disordered Ag-Au alloy. This alloy formation is accompanied by an increase in the spin-orbit splitting. We established a quantitative correlation between the amplitude of this splitting and the relative amount of Au and Ag atoms probed by the surface state wave function proving the atomic character of the spin-orbit splitting. Control of the Ag-Au interface allows a continuous fine tuning of surface state properties; in particular, the $k$-dependent spin polarization.

Epitaxial Ag ultrathin films grown on Cu(111) have been studied by scanning tunneling microscopy and angle-resolved photoemission spectroscopy. In the submonolayer range, the spectroscopic measurements reveal the existence of two parabolic dispersive surface bands associated with the Cu terraces and the Ag islands suggesting the lateral confinement of the electronic states. A thickness dependence of the surface-state energy is evidenced. Moreover, the two well-known atomic reconstructions (moir\'e and triangular superstructures which depend on the preparation temperature) lead to different surface state energies. Photoemission spectra also reflect the transition from the metastable moir\'e structure into the triangular one. These results clearly demonstrate the strong sensitivity of the surface-state energy to the in-plane atomic structure.

In this paper, we present photoemission experiments on the hole-doped Haldane chain compound Y 2 - x Sr x BaNiO 5 . By using the photon-energy dependence of the photoemission cross section, we identified the symmetry of the first ionization states (d type). Hole doping in this system leads to a significant increase in the spectral weight at the top of the valence band without any change in the vicinity of the Fermi energy. This behavior, not observed in other charge-transfer oxides at low doping level, could result from the inhomogeneous character of the doped system and from a Ni 3d-O 2p hybridization enhancement due to the shortening of the relevant Ni-O distance in the localized hole-doped regions.

In this paper, we present photoemission experiments on the hole-doped Haldane chain compound Y2−xSrxBaNiO5. By using the photon energy dependence of the photoemission cross section, we identified the symmetry of the first ionisation states (d type). Hole doping in this system leads to a significant increase in the spectral weight at the top of the valence band without any change in the vicinity of the Fermi energy. This behavior, never observed in other charge transfer oxides, could result from the Ni3d–O2p hybridization enhancement due to the shortening of the relevant Ni–O distance with doping.

We present and analyze the normal phase temperature dependence of resistivity and nuclear spin-lattice relaxation of $(\mathrm{TMTSF}{)}_{2}{\mathrm{ClO}}_{4}$ under strong transverse magnetic fields. From a field-dependent scaling theory we show that the singular enhancement of the one-dimensional electron-electron umklapp scattering and antiferromagnetic spin correlations under field play a fundamental role in the large enhancement of magnetoresistance and nuclear relaxation of Bechgaard salts.