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Yb exhibits unusual physical behavior in YbMn6Ge6-xSnx with coexisting intermediate valent state and magnetism. We complete the (x,T) magnetic phase diagram of this system by using dc magnetization and powder neutron-diffraction experiments in a composition range (4.60≤x≤5.30) where the alloys were previously reported to two-phase separate. In these compounds, Mn ferromagnetically orders near room temperature while the intermediate valent Yb sublattice (υ≈2.8) obeys a Doniach-like behavior. Upon increasing the 4f conduction electron hybridization strength through Sn for Ge substitution, the Yb magnetic moment continuously reduces as a result of the enhancement in the Kondo screening. Meanwhile the Yb magnetic ordering temperature reaches its maximum TYb∼125 K in the alloy with x=4.65, before dropping upon further Sn doping. The Yb magnetic instability is located near x∼5.23, in the vicinity of which the nature of the quantum phase transition remains to be elucidated. [ABSTRACT FROM AUTHOR]

Boron vacancies at the Si(111):B-√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 √3-reconstructed surfaces, as proposed recently for SiC(0001), Sn/Ge(111), and Sn/Si(111). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper, we report on the experimental determination of alkali absolute coverage in the K/Si(111):B- 2√3 x 2√3R30° surface. To this end, we carried out a comparative study with the closest system, namely, K/Si(111)-3 x 1, for which potassium coverage has been widely demonstrated to be 1/3 of a monolayer. We used x-ray photoemission spectroscopy to count and compare the number of potassium atoms in both surfaces, together with a scanning tunneling microscopy in order to check the completion of these ultrathin layers. The analysis leads to a 1/2 monolayer coverage in the 2√3 x 2√3R30° surface, that is six potassium atoms per unit cell. Assuming this coverage, we can propose a simple model structure with potassium atoms arranged in trimers; we discuss the effect of such a reconstruction in terms of two distinct "up" and "down" Si adatom sites as well as the resulting surface electronic properties. [ABSTRACT FROM AUTHOR]

Abstract: Very recently, new LEED, STM and ARPES measurements on the (1/3)ML K/Si(111):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 . 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- 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. [Copyright &y& Elsevier]

Abstract: Electronic properties of (Ag, Co) nanostructures grown on Au(1 1 1) vicinal surfaces have been studied by angle resolved photoemission spectroscopy (ARPES), scanning tunneling microscopy and spectroscopy (STM/STS). The growth and self-assembling of Co and Ag nano dots on Au(7 8 8) surface are described. Co island growth leads to the formation of repulsive energy barriers for the surface state, and subsequently to the appearance of confined states in between each group of four Co dots. On the contrary, when Ag nano dots are grown, the potential barrier for the surface electrons is not enough to suppress their dispersive behavior. Nevertheless, inside Ag islands appear new quantized states whose energies can be tailored by varying the deposition rate of the adsorbate and/or the Miller index of the vicinal surfaces. In both systems, high homogeneity of the electronic properties is achieved over a macroscopic scale. [Copyright &y& Elsevier]

Abstract: Spectroscopic effects associated with the superperiodic surface structure have been observed in Au(111) vicinal surfaces and nanostructured systems. In the vicinal Au(232321) 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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

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. [Copyright &y& Elsevier]

The electronic structure of the reentrant superconductor Eu(Fe0.86Ir0.14)2As2 (Tc=22 K) with coexisting ferromagnetic order (TM=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 Eu(Fe0.86Ir0.14)2As2. The near-EF Fe-3d-derived band dispersions near the Γ 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 (Δ∼5.25±0.25 meV), indicative of strong-coupling superconductivity with 2Δ/kBTc≃5.8. The gap gets closed at temperatures T≥10 K, and this is attributed to the resistive phase which sets in at TM=18 K due to the Eu2+-derived magnetic order. The modification of the FS with Ir substitution clearly indicates an effective hole doping with respect to the parent compound. [ABSTRACT FROM AUTHOR]

We study the electronic structure of HgBa2Ca2Cu3O8+δ (Hg1223; Tc=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 (Udd=6.5±0.5 eV) and O site (Upp=1.0±0.5 eV). Cu2O7-cluster calculations with nonlocal screening explain the Cu 2p core level PES and Cu L-edge XAS spectra, confirm the Udd and Upp 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. [ABSTRACT FROM AUTHOR]