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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. [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]

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 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_{0}/t approximately 0.8 and the effective pairing energy V_{eff} = U - 2gvariant Planck's over 2piomega_{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.