Etude Theorique de la morphologie et energetique des surfaces vicinales de metaux de transition, ... more Etude Theorique de la morphologie et energetique des surfaces vicinales de metaux de transition, a l'aide de methodes de structure elecrtonique et de potentiels empiriques. Une attention particuliere est dediee a l'etude de la stabilite des surfaces vicinales vis a vis du facettage.
In this contribution I will present an efficient tight-binding (TB) model that we have developed ... more In this contribution I will present an efficient tight-binding (TB) model that we have developed to describe the electronic, magnetic and energetic properties of metallic alloys. It is based on a rather simple procedure that only requires a fit on ab-initio data of band-structures and energy for single element bulk metals. No specific properties of the binary system is explicitly required. The magnetic part of our TB Hamiltonian is described by a straight-forward spin-dependent Stoner term that can be generalized to deal with non-collinear magnetism and include spin-orbit effects. Usual local charge neutrality condition is applied and simple rules are used to deal with bi-metallic systems.
An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic ex... more An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic excitations and magnetic transitions. We propose an effcient approach to address these properties via Monte Carlo simulation, using ab-initio based effective interaction models. The temperature evolution of self- and Cu diffusion coeffcients in alpha-iron are successfully predicted, particularly the diffusion acceleration around the Curie point, which requires a quantum treatment of spins. We point out a dominance of magnetic disorder over chemical effects on diffusion in the very dilute systems.
Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because... more Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because of their ability to switch between high-spin and low-spin states with external stimuli such as voltage and light. In actual devices, the molecules are deposited on a substrate, which can modify their properties. However, surprisingly little is known about such molecule-substrate effects. Here we show for the first time, by grazing incidence X-ray diffraction, that a Fe$^{\mathrm{II}}$ spin-crossover molecular layer displays a well-defined epitaxial relationship with a metal substrate. Then we show, by both density functional calculations and a mechanoelastic model, that the resulting epitaxial strain and the related internal pressure can induce a partial spin conversion at low temperatures, which has indeed been observed experimentally. Our results emphasize the importance of substrate-induced spin state transitions, and raise the possibility of exploiting them.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 29, 2017
We present thorough tight-binding analysis of the band structure of a wide variety of lattices be... more We present thorough tight-binding analysis of the band structure of a wide variety of lattices belonging to the class of honeycomb and Kagome systems including several mixed forms combining both lattices. The band structure of these systems are made of a combination of dispersive and flat bands. The dispersive bands possess Dirac cones (linear dispersion) at the six corners (K points) of the Brillouin zone although in peculiar cases Dirac cones at the center of the zone $(\Gamma$ point) appear. The flat bands can be of different nature. Most of them are tangent to the dispersive bands at the center of the zone but some, for symmetry reasons, do not hybridize with other states. The objective of our work is to provide an analysis of a wide class of so-called ligand-decorated honeycomb Kagome lattices that are observed in 2D metal-organic framework (MOF) where the ligand occupy honeycomb sites and the metallic atoms the Kagome sites. We show that the $p_x$-$p_y$ graphene model is relev...
Spin cross-over molecules show the unique ability to switch between two spin states when submitte... more Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensio...
The structure and growth of thin films of titanium on �−Al2O3 at room temperature were investigat... more The structure and growth of thin films of titanium on �−Al2O3 at room temperature were investigated though in situ RHEED observations. Two different structures coexists at low coverage. One corresponds to the Ti(0001) k Al2O3(0001), Ti(1100) k Al2O3(2110) and Ti(1010) k Al2O3(1100) epitaxy of thephase of titanium reported before for thick films prepared at high temperature. The other structure can be explained by co-existence ofand ! Ti in thin films. It was shown, with the use of tight-binding total energetic calculations that, the ! phase could actually be stabilized by the substrate. In addition, it was demonstrated that the presence of this extra structure has a dramatic effect on the epitaxial growth of the Ag overlayers on the system. This can be the origin of the non-trivial buffer effect of titanium previously observed.
Nous presentons des resultats experimentaux et 'une modelisation d'un procede de deformat... more Nous presentons des resultats experimentaux et 'une modelisation d'un procede de deformation a chaud pour des aciers au carbone ou microallies, dans le domaine biphase. Nous determinous le comportement en compression a chaud (1x10 -3 + 2,5x10 3 s -1 , 650<T<1050°C) de l'acier pre-deforme. Les conditions de deformation a chaud ont egalement ete simulees par une methode a elements finis afin d'integrer l'histoire thermomecanique. Nous examinons particulierement, ici, le cas ou la derniere deformation se produit dans une region biphasee ou ferritique. Nous etudions un materiau ainsi deforme pour determiner l'evolution de la microstructure et son influence sur les proprietes mecaniques du produit final.
A dynamics of the precession of coupled atomic moments in the tight-binding (TB) approximation is... more A dynamics of the precession of coupled atomic moments in the tight-binding (TB) approximation is presented. By implementing an angular penalty functional in the energy that captures the magnetic effective fields self-consistently, the motion of the orientation of the local magnetic moments is observed faster than the variation of their magnitudes. This allows the computation of the effective atomic magnetic fields that are found consistent with the Heisenberg’s exchange interaction, by comparison with classical atomistic spin dynamics on Fe, Co and Ni magnetic clusters.
Etude Theorique de la morphologie et energetique des surfaces vicinales de metaux de transition, ... more Etude Theorique de la morphologie et energetique des surfaces vicinales de metaux de transition, a l'aide de methodes de structure elecrtonique et de potentiels empiriques. Une attention particuliere est dediee a l'etude de la stabilite des surfaces vicinales vis a vis du facettage.
In this contribution I will present an efficient tight-binding (TB) model that we have developed ... more In this contribution I will present an efficient tight-binding (TB) model that we have developed to describe the electronic, magnetic and energetic properties of metallic alloys. It is based on a rather simple procedure that only requires a fit on ab-initio data of band-structures and energy for single element bulk metals. No specific properties of the binary system is explicitly required. The magnetic part of our TB Hamiltonian is described by a straight-forward spin-dependent Stoner term that can be generalized to deal with non-collinear magnetism and include spin-orbit effects. Usual local charge neutrality condition is applied and simple rules are used to deal with bi-metallic systems.
An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic ex... more An accurate prediction of atomic diffusion in Fe alloys is challenging due to thermal magnetic excitations and magnetic transitions. We propose an effcient approach to address these properties via Monte Carlo simulation, using ab-initio based effective interaction models. The temperature evolution of self- and Cu diffusion coeffcients in alpha-iron are successfully predicted, particularly the diffusion acceleration around the Curie point, which requires a quantum treatment of spins. We point out a dominance of magnetic disorder over chemical effects on diffusion in the very dilute systems.
Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because... more Spin-crossover molecules are very appealing for use in multifunctional spintronic devices because of their ability to switch between high-spin and low-spin states with external stimuli such as voltage and light. In actual devices, the molecules are deposited on a substrate, which can modify their properties. However, surprisingly little is known about such molecule-substrate effects. Here we show for the first time, by grazing incidence X-ray diffraction, that a Fe$^{\mathrm{II}}$ spin-crossover molecular layer displays a well-defined epitaxial relationship with a metal substrate. Then we show, by both density functional calculations and a mechanoelastic model, that the resulting epitaxial strain and the related internal pressure can induce a partial spin conversion at low temperatures, which has indeed been observed experimentally. Our results emphasize the importance of substrate-induced spin state transitions, and raise the possibility of exploiting them.
Journal of physics. Condensed matter : an Institute of Physics journal, Jan 29, 2017
We present thorough tight-binding analysis of the band structure of a wide variety of lattices be... more We present thorough tight-binding analysis of the band structure of a wide variety of lattices belonging to the class of honeycomb and Kagome systems including several mixed forms combining both lattices. The band structure of these systems are made of a combination of dispersive and flat bands. The dispersive bands possess Dirac cones (linear dispersion) at the six corners (K points) of the Brillouin zone although in peculiar cases Dirac cones at the center of the zone $(\Gamma$ point) appear. The flat bands can be of different nature. Most of them are tangent to the dispersive bands at the center of the zone but some, for symmetry reasons, do not hybridize with other states. The objective of our work is to provide an analysis of a wide class of so-called ligand-decorated honeycomb Kagome lattices that are observed in 2D metal-organic framework (MOF) where the ligand occupy honeycomb sites and the metallic atoms the Kagome sites. We show that the $p_x$-$p_y$ graphene model is relev...
Spin cross-over molecules show the unique ability to switch between two spin states when submitte... more Spin cross-over molecules show the unique ability to switch between two spin states when submitted to external stimuli such as temperature, light or voltage. If controlled at the molecular scale, such switches would be of great interest for the development of genuine molecular devices in spintronics, sensing and for nanomechanics. Unfortunately, up to now, little is known on the behaviour of spin cross-over molecules organized in two dimensions and their ability to show cooperative transformation. Here we demonstrate that a combination of scanning tunnelling microscopy measurements and ab initio calculations allows discriminating unambiguously between both states by local vibrational spectroscopy. We also show that a single layer of spin cross-over molecules in contact with a metallic surface displays light-induced collective processes between two ordered mixed spin-state phases with two distinct timescale dynamics. These results open a way to molecular scale control of two-dimensio...
The structure and growth of thin films of titanium on �−Al2O3 at room temperature were investigat... more The structure and growth of thin films of titanium on �−Al2O3 at room temperature were investigated though in situ RHEED observations. Two different structures coexists at low coverage. One corresponds to the Ti(0001) k Al2O3(0001), Ti(1100) k Al2O3(2110) and Ti(1010) k Al2O3(1100) epitaxy of thephase of titanium reported before for thick films prepared at high temperature. The other structure can be explained by co-existence ofand ! Ti in thin films. It was shown, with the use of tight-binding total energetic calculations that, the ! phase could actually be stabilized by the substrate. In addition, it was demonstrated that the presence of this extra structure has a dramatic effect on the epitaxial growth of the Ag overlayers on the system. This can be the origin of the non-trivial buffer effect of titanium previously observed.
Nous presentons des resultats experimentaux et 'une modelisation d'un procede de deformat... more Nous presentons des resultats experimentaux et 'une modelisation d'un procede de deformation a chaud pour des aciers au carbone ou microallies, dans le domaine biphase. Nous determinous le comportement en compression a chaud (1x10 -3 + 2,5x10 3 s -1 , 650<T<1050°C) de l'acier pre-deforme. Les conditions de deformation a chaud ont egalement ete simulees par une methode a elements finis afin d'integrer l'histoire thermomecanique. Nous examinons particulierement, ici, le cas ou la derniere deformation se produit dans une region biphasee ou ferritique. Nous etudions un materiau ainsi deforme pour determiner l'evolution de la microstructure et son influence sur les proprietes mecaniques du produit final.
A dynamics of the precession of coupled atomic moments in the tight-binding (TB) approximation is... more A dynamics of the precession of coupled atomic moments in the tight-binding (TB) approximation is presented. By implementing an angular penalty functional in the energy that captures the magnetic effective fields self-consistently, the motion of the orientation of the local magnetic moments is observed faster than the variation of their magnitudes. This allows the computation of the effective atomic magnetic fields that are found consistent with the Heisenberg’s exchange interaction, by comparison with classical atomistic spin dynamics on Fe, Co and Ni magnetic clusters.
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