Growth, morphology and electronic properties of epitaxial graphene on vicinal Ir(332) surface.

Superlattice induced minigaps in graphene band structure due to underlying one-dimensional nanostructuration has been demonstrated. A superperiodic potential can be introduced in graphene if the substrate is periodically structured. The successful preparation of a periodically nanostructured substrate in large scale can be obtained by carefully studying the electronic structure with a spatial averaging technique such as high-energy resolution photoemission. In this work, we present two different growth methods such as temperature programmed growth (TPG) and chemical vapor deposition (CVD) studied by scanning tunnelling microscopy (STM) and low energy electron diffraction (LEED). In both methods, we show that the original steps of Ir(332) have modified with (111) terraces and step bunching after graphene growth. Graphene grows continuously over the terrace and the step bunching areas. We observe that while TPG growth does not give rise to a well-defined surface periodicity required for opening a bandgap, the CVD growth does. By combining with angle-resolved photoemission spectroscopy (ARPES) measurements, we correlate the obtained spatial periodicity to observed band gap opening in graphene.

Evidence for Weakly Correlated Oxygen Holes in the Highest-T_{c} Cuprate Superconductor HgBa_{2}Ca_{2}Cu_{3}O_{8+δ}.

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.

Ultrafast Atomic Diffusion Inducing a Reversible (2sqrt[3]×2sqrt[3])R30°â†”(sqrt[3]×sqrt[3])R30° Transition on Sn/Si(111)∶B.

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.

Nonpareil Yb behavior in YbMn6Ge(6-x)Sn(x).

We investigate the temperature dependence of the Yb valence in YbMn6Ge1.8Sn4.2 and YbMn6Ge1.6Sn4.4 using resonant inelastic x-ray scattering experiments. Yb is found to be in an intermediate valent state in the whole investigated temperature range (10-450 K). We thus prove that the unusually high magnetic ordering temperature of Yb (∼60 and 90 K for x=4.2 and 4.4, respectively) involves an intermediate valent Yb, an unprecedentedly observed phenomenon. Further, an anomalous increase in the Yb valence is observed upon cooling. A scenario is proposed to explain this unusual behavior. It is based on the presence of magnetically ordered Mn moments and on an Anderson Hamiltonian with a Zeeman term modeling the magnetic interactions.

Giant alkali-metal-induced lattice relaxation as the driving force of the insulating phase of alkali-metal/Si(111):B.

Ab initio density-functional theory calculations, photoemission spectroscopy (PES), scanning tunneling microscopy, and spectroscopy (STM, STS) have been used to solve the 2sqrt[3]×2sqrt[3]R30 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.

Phonon contribution to the lifetime of surface state quasiparticles confined in nanopyramids.

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.

Understanding the insulating nature of alkali-metal/Si(111):B interfaces.

We have recently revisited the phase diagram of alkali-metal/Si(111):B semiconducting interfaces previously suggested as the possible realization of a Mott-Hubbard insulator on a triangular lattice. The insulating character of the 2√[3] × 2√[3]R30 surface reconstruction observed at the saturation coverage, i.e. 0.5 ML, has been shown to find its origin in a giant alkali-metal-induced vertical distortion. Low energy electron diffraction, photoemission spectroscopy and scanning tunneling microscopy and spectroscopy experiments coupled with linear augmented plane-wave density functional theory calculations allow a full understanding of the k-resolved band structure, explaining both the inhomogeneous charge transfers into an Si-B hybridized surface state and the opening of a band gap larger than 1 eV. Moreover, √[3] × âˆš[3]R30, 3 × 3 and 2√[3] × 2√[3]R30 surface reconstructions observed as a function of coverage may reveal a filling-controlled transition from a half-filled correlated magnetic material to a strongly distorted band insulator at saturation.