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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]

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]

We present a methodology to determine the periodic potential of a surface from angle-resolved photoemission spectroscopy. By mapping the band structure and measuring the energy gap widths at high symmetry points of the surface Brillouin zone, the first Fourier components of the potential can be estimated. This method has been applied in the case of the reconstructed Ag/Cu(111) surface where controlled potassium atom doping has been used to precisely tune the band gap energies. This potential determination allows one to calculate the local density of states which is found to be in very good agreement with scanning tunneling spectroscopy data. [ABSTRACT FROM AUTHOR]

The non-centro-symmetric semiconductor BiTeI exhibits two distinct surface terminations that support spin- split Rashba surface states. Their ambipolarity can be exploited for creating spin-polarized p-n junctions at the boundaries between domains with different surface terminations. We use scanning tunneling microscopy (STM) and spectroscopy (STS) to locate such junctions and investigate their atomic and electronic properties. The Te- and I-terminated surfaces are identified owing to their distinct chemical reactivity and an apparent height mismatch of electronic origin. The Rashba surface states are revealed in the STS spectra by the onset of a van Hove singularity at the band edge. Eventually, an electronic depletion is found on interfacial Te atoms, consistent with the formation of a space-charge area in typical p-n junctions. [ABSTRACT FROM AUTHOR]

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 (x√-3 x x√-3)R30° 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 Å. [ABSTRACT FROM AUTHOR]

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 A depending on the initial chlorine coverage. The driving force of the superstructure formation is attributed to the substrate-mediated elastic interaction. [ABSTRACT FROM AUTHOR]

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 °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. [ABSTRACT FROM AUTHOR]

In this work, we present a combined low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT) study of chlorine adsorption on Au( 111) at low coverages. Our STM study ofCl/Au(111) system has shown that at submonolayer coverages (θ <0.1 ML) chlorine atoms form chainlike structures with abnormally short distances of 3.8 Å between them. Our DFT calculations have shown that chlorine atoms can interact with each other through distortion of the substrate and this indirect elastic interaction is strong enough to affect their arrangement in the chainlike structures. [ABSTRACT FROM AUTHOR]

We take advantage of complementary high-resolution experimental techniques and theoretical tools to get insight into the α-Sn/Ge(111) triangular lattice surface consisting of sp electrons. We report a (3×3) phase, characterized by a charge ordering settled by electronic correlation, which appears between the known metallic-(3×3) and Mott insulator phases. We identify the atomistic mechanism behind the stabilization of this phase and interpret these findings on the basis of theoretical calculations. We disentangle the role of the various degrees of freedom in the stability of the different phases found and describe the stepwise surface changes between the metallic and Mott insulating phases. [ABSTRACT FROM AUTHOR]

We investigate the structural and electronic properties of strained Ge/Si(111)-(5×5) surfaces by means of scanning tunneling microscopy and high-resolution angle-resolved photoemission spectroscopy. The homogeneous (5×5) reconstructed overlayers are characterized by three electronic surface states, similar to the Si(111)-(7×7) surface. The dispersion of the dangling bond related surface state exhibits the same periodicity as that of the (5×5) reconstruction. Moreover, a careful analysis of the shape and width of this surface state provides striking evidence of electron-phonon coupling at low temperatures. By considering the spectral function within a simple Debye model, we determine both the Debye energy and the electron-phonon coupling strength. The latter value is further confirmed by analyzing the temperature-dependent phonon broadening of the dangling bond related surface state linewidth. [ABSTRACT FROM AUTHOR]

Spin-orbit coupling (SOC) related to the minority interface resonance states (IRSs) of Fe(001) has been evidenced in full-epitaxial Fe(001)/MgO/Fe magnetic tunnel junctions (MTJs) from magnetotransport experiments correlated to theoretical ab initio calculations. We show that the SOC effect strongly depends on the symmetry composition of the 1RS, which has been skillfully engineered via the interfacial chemical structure in the Fe(001)/MgO stack. The SOC scattering is enhanced between the majority Δ1) and minority Δ5 band near the Fermi level when presenting a Δ5 symmetry dominant 1RS at the Fe/MgO interface. Our results validate the interplay between SOC and IRSs, which gives further understanding of the mechanisms responsible for the tunneling anisotropic magnetoresistance and the large perpendicular magnetic anisotropy at the Fe/MgO interface. [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]

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]

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]

We study the electronic structure of HgBa&#95;{2}Ca&#95;{2}Cu&#95;{3}O&#95;{8+δ} (Hg1223; T&#95;{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&#95;{dd}=6.5±0.5  eV) and O site (U&#95;{pp}=1.0±0.5  eV). Cu&#95;{2}O&#95;{7}-cluster calculations with nonlocal screening explain the Cu 2p core level PES and Cu L-edge XAS spectra, confirm the U&#95;{dd} and U&#95;{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.

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 alkali-metal/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] x sqrt[3] --> 2(sqrt[3] x sqrt[3]) charge and lattice ordering below 270 K evidence a surface charge density wave in the strong electron-phonon coupling limit (g approximately 8). Both the adiabatic ratio variant Planck's over 2piomega&#95;{0}/t approximately 0.8 and the effective pairing energy V&#95;{eff} = U - 2gvariant Planck's over 2piomega&#95;{0} approximately -800 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.

High-resolution photoemission of the Sn 4d core level of Sn/Ge(111)-(3x3) resolves three main components in the line shape, which are assigned to each of the three Sn atoms that form the unit cell. The line shape found is in agreement with an initial state picture and supports that the two down atoms are inequivalent. In full agreement with these results, scanning tunnel microscopy images directly show that the two down atoms are at slightly different heights in most of the surface, giving rise to an inequivalent-down-atoms (3x3) structure. These results solve a long-standing controversy on the interpretation of the Sn 4d core-level line shape and the structure of Sn/Ge(111)-(3x3).

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.