605 research outputs found

    Triangular Mott-Hubbard Insulator Phases of Sn/Si(111) and Sn/Ge(111) Surfaces

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    The ground state of Sn/Si(111) and Sn/Ge(111) surface őĪ\alpha-phases is reexamined theoretically, based on ab‚ąíinitioab-initio calculations where correlations are approximately included through the orbital dependence of the Coulomb interaction (in the local density + Hubbard U approximation). The effect of correlations is to destabilize the vertical buckling in Sn/Ge(111) and to make the surface magnetic, with a metal-insulator transition for both systems. This signals the onset of a stable narrow gap Mott-Hubbard insulating state, in agreement with very recent experiments. Antiferromagnetic exchange is proposed to be responsible for the observed őď\Gamma-point photoemission intensity, as well asfor the partial metallization observed above above 60 K in Sn/Si(111). Extrinsic metallization of Sn/Si(111) by, e.g.e.g. alkali doping, could lead to a novel 2D triangular superconducting state of this and similar surfaces.Comment: 4 pages, 4 figure

    Connection Between Magnetism and Structure in Fe Double Chains on the Ir(100) Surface

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    The magnetic ground state of nanosized systems such as Fe double chains, recently shown to form in the early stages of Fe deposition on Ir(100), is generally nontrivial. Using ab initio density functional theory we find that the straight ferromagnetic (FM) state typical of bulk Fe as well as of isolated Fe chains and double chains is disfavored after deposition on Ir(100) for all the experimentally relevant double chain structures considered. So long as spin-orbit coupling (SOC) is neglected, the double chain lowest energy state is generally antiferromagnetic (AFM), a state which appears to prevail over the FM state due to Fe-Ir hybridization. Successive inclusion of SOC adds two further elements, namely a magnetocrystalline anisotropy, and a Dzyaloshinskii-Moriya (DM) spin-spin interaction, the former stabilizing the collinear AFM state, the second favoring a long-period spin modulation. We find that anisotropy is most important when the double chain is adsorbed on the partially deconstructed Ir(100) -- a state which we find to be substantially lower in energy than any reconstructed structure -- so that in this case the Fe double chain should remain collinear AFM. Alternatively, when the same Fe double chain is adsorbed in a metastable state onto the (5x1) fully reconstructed Ir(100) surface, the FM-AFM energy difference is very much reduced and the DM interaction is expected to prevail, probably yielding a helical spin structure.Comment: to appear on PR

    Spectroscopic fingerprints of a surface Mott-Hubbard insulator: the case of SiC(0001)

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    We discuss the spectroscopic fingerprints that a surface Mott-Hubbard insulator should show at the intra-atomic level. The test case considered is that of the Si-terminated SiC(0001) sqrt{3}xsqrt{3} surface, which is known experimentally to be insulating. We argue that, due to the Mott-Hubbard phenomenon, spin unpaired electrons in the Si adatom dangling bonds are expected to give rise to a Si-2p core level spectrum with a characteristic three-peaked structure, as seen experimentally. This structure results from the joint effect of intra-atomic exchange, spatial anisotropy, and spin-orbit coupling. Auger intensities are also discussed.Comment: 4 pages, 2 figures, ECOSS-18 conferenc

    Atomic spin sensitive dissipation on magnetic surfaces

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    We identify the mechanism of energy dissipation relevant to spin-sensitive nanomechanics including the recently introduced magnetic exchange force microscopy, where oscillating magnetic tips approach surface atomic spins. The tip-surface exchange couples spin and atom coordinates, leading to a spin-phonon problem with Caldeira-Leggett type dissipation. In the overdamped regime, that can lead to a hysteretic flip of the local spin with a large spin-dependent dissipation, even down to the very low experimental tip oscillation frequencies, describing recent observations for Fe tips on NiO. A phase transition to an underdamped regime with dramatic drop of magnetic tip dissipation should in principle be possible by tuning tip-surface distance.Comment: 4 pages, 4 figure

    Theory of the Metal-Paramagnetic Mott-Jahn-Teller Insulator Transition in A_4C_{60}

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    We study the unconventional insulating state in A_4C_{60} with a variety of approaches, including density functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (paramagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intra-molecular. Experimental gaps of 0.5 eV and 0.1 eV respectively compare well with molecular ion values, in agreement with this picture.Comment: 4 pages, 2 postscript figure

    Electron-phonon interaction at the Be(0001) surface

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    We present a first principle study of the electron-phonon (e-p) interaction at the Be(0001) surface. The real and imaginary part of the e-p self energy are calculated for the surface state in the binding energy range from the őďňČ\bar{\Gamma} point to the Fermi level. Our calculation shows an overall good agreement with several photoemission data measured at high and low temperatures. Additionally, we show that the energy derivative of real part of the self-energy presents a strong temperature and energy variation close to EFE_{F}, making it difficult to measure its value just at EFE_{F}.Comment: Accepted in Phys. Rev. Lett., 5 figure

    Realistic simulations of Au(100): Grand Canonical Monte Carlo and Molecular Dynamics

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    The large surface density changes associated with the (100) noble metals surface hex-reconstruction suggest the use of non-particle conserving simulation methods. We present an example of a surface Grand Canonical Monte Carlo applied to the transformation of a square non reconstructed surface to the hexagonally covered low temperature stable Au(100). On the other hand, classical Molecular Dynamics allows to investigate microscopic details of the reconstruction dynamics, and we show, as an example, retraction of a step and its interplay with the surface reconstruction/deconstruction mechanism.Comment: 9 pages, 5 figures, accepted for publication in Surf. Rev. and Letters (ICSOS-6

    The effect of surface roughness on the adhesion of elastic solids

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    We study the influence of surface roughness on the adhesion of elastic solids. Most real surfaces have roughness on many different length scales, and this fact is taken into account in our analysis. We consider in detail the case when the surface roughness can be described as a self affine fractal, and show that when the fractal dimension D_f >2.5, the adhesion force may vanish, or be at least strongly reduced. We consider the block-substrate pull-off force as a function of roughness, and find a partial detachment transition preceding a full detachment one. The theory is in good qualitative agreement with experimental data.Comment: 15 pages, 16 figures. Submitted to J. Chem. Phy

    Dynamical chiral symmetry breaking in sliding nanotubes

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    We discovered in simulations of sliding coaxial nanotubes an unanticipated example of dynamical symmetry breaking taking place at the nanoscale. While both nanotubes are perfectly left-right symmetric and nonchiral, a nonzero angular momentum of phonon origin appears spontaneously at a series of critical sliding velocities, in correspondence with large peaks of the sliding friction. The non-linear equations governing this phenomenon resemble the rotational instability of a forced string. However, several new elements, exquisitely "nano" appear here, with the crucial involvement of Umklapp and of sliding nanofriction.Comment: To appear in PR
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