Andres Saul

Multiphase equation of state for carbon addressing high pressures and temperatures

We present a 5-phase equation of state for elemental carbon which addresses a wide range of density and temperature conditions: 3g/cc 100000K(bothfor ρ between3and12g/cc,withselecthigher-ρ DFTcalculationsas well). The liquid free energy model includes an atom-in-jellium approach to account for the effects of ionization due to temperature and pressure in the plasma state, and an ion-thermal model which includes the approach to the ideal gas limit. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest-temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results.

Grain interaction effect in electronic properties of silicon nanosize films

Electronic properties of both nanometer thickness (111) monocrystalline and nanocrystalline free standing silicon films were calculated within a self-consistent linear combination of atomic orbitals method. Grained nature of the nanocrystalline films is found to induce both a direct band gap and its reduction (down to about 2 eV) with respect to an isolated grain of same size.

Cement As a Waste Form for Nuclear Fission Products: The Case of (90)Sr and Its Daughters.

One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of (90)Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that (90)Sr is stable when it substitutes the Ca(2+) ions in C-S-H, and so is its daughter nucleus (90)Y after β-decay. Interestingly, (90)Zr, daughter of (90)Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for (90)Sr storage.

Simple views on surface stress and surface energy concepts

Some aspects of the thermodynamics and mechanics of solid surfaces, in particular with respect to surface stress and surface energy, are reviewed. The purpose is to enlighten the deep differences between these two physical quantities. We consider successively the case of atomic flat surfaces and the case of vicinal surfaces characterized by surface stress discontinuities. Finally, experimental examples, concerning Si surfaces, are described.

Magnetic nanopantograph in the SrCu2(BO3)2 Shastry–Sutherland lattice

Significance The spins of the unpaired electrons in a solid tend to align along an applied magnetic field. In the case of antiferromagnetic materials having competing interactions (frustration) it is common to observe that the magnetization increases, exhibiting complicated structures with discrete jumps and plateaus. SrCu2(BO3)2 is one of these materials, for which we find experimentally that its macroscopic physical dimensions also change with the magnetic field, mimicking the behavior in the magnetization. Using quantum mechanics, we show quantitatively that due to the orthogonal arrangement of the magnetic Cu2+ dimers acting as pantographs, minute deformations allow significant reduction in the effective interactions responsible for the antiferromagnetism. This drop is sufficient to compensate the elastic energy loss in the lattice deformation. Magnetic materials having competing, i.e., frustrated, interactions can display magnetism prolific in intricate structures, discrete jumps, plateaus, and exotic spin states with increasing applied magnetic fields. When the associated elastic energy cost is not too expensive, this high potential can be enhanced by the existence of an omnipresent magnetoelastic coupling. Here we report experimental and theoretical evidence of a nonnegligible magnetoelastic coupling in one of these fascinating materials, SrCu2(BO3)2 (SCBO). First, using pulsed-field transversal and longitudinal magnetostriction measurements we show that its physical dimensions, indeed, mimic closely its unusually rich field-induced magnetism. Second, using density functional-based calculations we find that the driving force behind the magnetoelastic coupling is the CuOCu^ superexchange angle that, due to the orthogonal Cu2+ dimers acting as pantographs, can shrink significantly (0.44%) with minute (0.01%) variations in the lattice parameters. With this original approach we also find a reduction of ∼10% in the intradimer exchange integral J, enough to make predictions for the highly magnetized states and the effects of applied pressure on SCBO.

Structure and properties of nanoscale materials: theory and atomistic computer simulation

We present a review of a few research topics developed within the Theory and Atomistic Computer Simulation Department at CINaM. The bottom line of the scientific activity is to use up–to–date theoretical and computer simulation techniques to address physics and materials science problems, often at the nanometric scale, in close contact with experimental groups. It ranges from the study of the structure and properties of molecular systems for organic electronics to metallic clusters and alloys, magnetic oxides, nuclear fuels and carbon–based nanostructures. These studies are motivated by fundamental research questions as well as more applied goals including environmental and energy issues, or information technologies. This broad spectrum of activities requires a large range of techniques, from theory and ab initio calculations to semi–empirical models incorporated in Monte Carlo or molecular dynamics simulations.

Alloy model for high-temperature superconductors

Abstract An alloy model is proposed for the electronic structure of high-temperature superconductors. It is based on the assumption that holes and extra electrons are localized in small copper-oxygen clusters, that would be the components of such an alloy. This model, when used together with quantum chemical calculations on small clusters, can explain the structure observed in the experimental densities of states of both hole and electron superconductors close to the Fermi energy. The main point is the strong dependence of the energy level distribution and composition on the number of electrons in a cluster. The alloy model also suggests a way to correlate T c with the number of holes, or extra electrons, and the number of adequate clusters to locate them.

Unconventional field induced phases in a quantum magnet formed by free radical tetramers

We report experimental and theoretical studies on the magnetic and thermodynamic properties of NIT-2Py, a free radical based organic magnet. From magnetization and specific-heat measurements we establish the temperature versus magnetic field phase diagram which includes two Bose-Einstein condensates (BEC) and an infrequent half-magnetization plateau. Calculations based on density functional theory demonstrate that magnetically this system can be mapped to a quasi-two-dimensional structure of weakly coupled tetramers. Density matrix renormalization group calculations show the unusual characteristics of the BECs where the spins forming the low-field condensate are different than those participating in the high-field one.

Magnetic Nanopantograph in the SrCu₂(BO₃)₂ Shastry-Sutherland Lattice

Measurements of the ZZ and WW final states in the mass range above the