A many-body interatomic potential for ionic systems: application to MgO

An analytic representation of the short-range repulsion energy in ionic systems is described that allows for the fact that ions may change their size and shape depending on their environment. This function is extremely efficient to evaluate relative to previous methods of modeling the same physical effects. Using a well-defined parametrization procedure we have obtained parameter sets for this energy function that reproduce closely the density functional theory potential energy surface of bulk MgO. We show how excellent agreement can be obtained with experimental measurements of phonon frequencies and temperature and pressure dependences of the density by using this effective potential in conjunction with ab initio parametrization.Comment: To appear in Journal of Chemical Physics (Oct 15th 2003

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

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

Operational Thermodynamics from Purity (extended abstract)

This is an extended abstract based on the preprint arXiv:1608.04459. We propose four information-theoretic axioms for the foundations of statistical mechanics in general physical theories. The axioms 'Causality, Purity Preservation, Pure Sharpness, and Purification' identify purity as a fundamental ingredient for every sensible theory of thermodynamics. Indeed, in physical theories satisfying these axioms, called sharp theories with purification, every mixed state can be modelled as the marginal of a pure entangled state, and every unsharp measurement can be modelled as a sharp measurement on a composite system. We show that these theories support a well-behaved notion of entropy and of Gibbs states, by which one can derive Landauer's principle. We show that in sharp theories with purification some bipartite states can have negative conditional entropy, and we construct an operational protocol exploiting this feature to overcome Landauer's principlepublished_or_final_versio

The mechanism for the 3 x 3 distortion of Sn/ge (111)

We show that two distinct $3 \times 3$ ground states, one nonmagnetic, metallic, and distorted, the other magnetic, semimetallic (or insulating) and undistorted, compete in $\alpha$-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for stability of this state is analysed, and is traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a Ge-Ge bond directly underneath

Disproportionation Phenomena on Free and Strained Sn/Ge(111) and Sn/Si(111) Surfaces

Distortions of the $\sqrt3\times\sqrt3$ Sn/Ge(111) and Sn/Si(111) surfaces are shown to reflect a disproportionation of an integer pseudocharge, $Q$, related to the surface band occupancy. A novel understanding of the $(3\times3)$-1U (``1 up, 2 down'') and 2U (``2 up, 1 down'') distortions of Sn/Ge(111) is obtained by a theoretical study of the phase diagram under strain. Positive strain keeps the unstrained value Q=3 but removes distorsions. Negative strain attracts pseudocharge from the valence band causing first a $(3\times3)$-2U distortion (Q=4) on both Sn/Ge and Sn/Si, and eventually a $(\sqrt3\times\sqrt3)$-3U (``all up'') state with Q=6. The possibility of a fluctuating phase in unstrained Sn/Si(111) is discussed.Comment: Revtex, 5 pages, 3 figure

SiC(0001): a surface Mott-Hubbard insulator

We present ab-initio electronic structure calculations for the Si-terminated SiC(0001)$\sqrt{3}\times\sqrt{3}$ surface. While local density approximation (LDA) calculations predict a metallic ground state with a half-filled narrow band, Coulomb effects, included by the spin-polarized LDA+U method, result in a magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the experimental value of 2.0 eV. The calculated value of the inter-site exchange parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except at very low temperatures. The observed Si 2p surface core level doublet can naturally be explained as an on-site exchange splitting.Comment: RevTex, 4 pages, 4 eps-figure

First Principles Calculations of Charge and Spin Density Waves of sqr3-Adsorbates on Semiconductors

We present ab-initio electronic structure results on the surface of sqr3 adsorbates. In particular, we address the issue of metal-insulator instabilities, charge-density-waves (CDWs) or spin-density-waves (SDWs), driven by partly filled surface states and their 2D Fermi surface, and/or by the onset of magnetic instabilities. The focus is both on the newly discovered commensurate CDW transitions in the Pb/Ge(111) and Sn/Ge(111) structures, and on the puzzling semiconducting behavior of the Pb/Ge(111), K/Si(111):B and SiC(0001) surfaces. In all cases, the main factor driving the instability appears to be an extremely narrow surface state band. We have carried out so far preliminary calculations for the Si/Si(111) surface, chosen as our model system, within the gradient corrected local density (LDA+GC) and local spin density (LSD+GC) approximations, with the aim of understanding the possible interplay between 2D Fermi surface and electron correlations in the surface + adsorbate system. Our spin- unrestricted results show that the sqr3 paramagnetic surface is unstable towards a commensurate SDW with periodicity 3x3 and magnetization 1/3.Comment: 9 pages, 4 Postscript figures, to be published in Surf. Sc

OPTIMAL PORTFOLIO ALLOCATION WITH CVAR: A ROBUSTAPPROACH

The paper discuss the sensitivity to the presence of outliers of the portfolio optimization procedure based on the expected shortfall as a measure of risk. A robust approach based on the forward search is then suggested which seems to give quite good results

Squeezing Oil into Water under Pressure: Inverting the Hydrophobic Effect

The molecular structure of dense homogeneous fluid water-methane mixtures has been determined for the first time using high-pressure neutron-scattering techniques at 1.7 and 2.2 GPa. A mixed state with a fully H-bonded water network is revealed. The hydration shell of the methane molecules is, however, revealed to be pressure-dependent with an increase in the water coordination between 1.7 and 2.2 GPa. In parallel, ab initio molecular dynamics simulations have been performed to provide insight into the microscopic mechanisms associated with the phenomenon of mixing. These calculations reproduce the observed phase change from phase separation to mixing with increasing pressure. The calculations also reproduce the experimentally observed structural properties. Unexpectedly, the simulations show mixing is accompanied by a subtle enhancement of the polarization of methane. Our results highlight the key role played by fine electronic effects on miscibility and the need to readjust our fundamental understanding of hydrophobicity to account for these

Design of a low band gap oxide ferroelectric: Bi6_6Ti4_4O17_{17}

A strategy for obtaining low band gap oxide ferroelectrics based on charge imbalance is described and illustrated by first principles studies of the hypothetical compound Bi$_6$Ti$_4$O$_{17}$, which is an alternate stacking of the ferroelectric Bi$_4$Ti$_3$O$_{12}$. We find that this compound is ferroelectric, similar to Bi$_4$Ti$_3$O$_{12}$ although with a reduced polarization. Importantly, calculations of the electronic structure with the recently developed functional of Tran and Blaha yield a much reduced band gap of 1.83 eV for this material compared to Bi$_4$Ti$_3$O$_{12}$. Therefore, Bi$_6$Ti$_4$O$_{17}$ is predicted to be a low band gap ferroelectric material