Scientific activities

Soft Matter

The interaction between the neutron community and the soft matter theoretical community – using tools from statistical mechanics (including Monte-Carlo and molecular dynamics simulations) has a long history. In the field of fluids at solid interfaces, neutron science has long been a tool of choice for studying both the structural properties, vibrational dynamics and self/collective transport of molecules confined in nanoporous materials. Within the theory group, our work aims at determining the impact of nanometric confinement and surface forces on adsorption, phase transition and transport phenomena. In fine, the objective of this theoretical activity is twofold: (1) Identification of new mechanisms related to confinement and transport in nanoporous materials and (2) Optimization/functionalization of nanoporous solids for energy and environmental applications.

Our research on fluids in nanoporous materials is at the crossroad of physics, physical chemistry, and materials science. As such, almost without exception, our current activities on the confinement/transport of fluids within these materials are an ideal playground for strong interactions with experimentalists – especially from the ILL community. In addition to such ongoing activities, based on our expertise, we develop within the Theory Group at ILL an original program dealing with the impact of solid/fluid coupling on the dynamics and phase transitions in nanoconfined fluids. In more details, despite significant research efforts devoted to nanoconfined fluids, many complex phenomena keep being unraveled. In particular, by considering fluid adsorption/transport in nanoporous materials, several studies have identified a rich yet poorly understood coupling between the physics of the host porous solid (e.g. phonon/acoustic response, electrostatic screening, mechanical deformation) and the fluid behavior within its porosity. To carry out this project, we conduct the two following studies: (1) Acoustic stimulation of fluids in nanoporous solids and (2) Confinement in nanoporous metallic materials.

Neutrons scattering has always been one of the most valuable tool for the study of the electronic structures. This is even more the case when the system contains unpaired (magnetic) electrons, since both the ground state (DIF) and the excitations can be studied (TOF, TAS). The interaction between the neutron experimental community and the ab initio electronic structure theoreticians is characterized by the fact that in the ab-initio calculations the whole complexity of the real system (chemical as well as structural aspects) are taken into account, allowing not only a direct confrontation between the theoretical predictions and the experimental observations, but also the possibility to extract from the calculations the degrees of freedom that are pertinent for the system low energy physics and their interactions. For instance, in magnetic systems, wave-function based ab-initio calculations allow the determination of the different direct as well as super-(or double-) exchange paths, their relative importance/amplitude and thus the pertinent magnetic Hamiltonian to be used for the experimental interpretation. Similarly, in multiferroic systems,  it is possible to obtain information on the microscopic origin of the electro-magnon excitations, since the variation of the different terms entering the exchange integrals can be obtained as a function of the atomic displacements associated with a phonon mode.

Understanding the properties of a system often means building up a simple representation of this system, where a few pertinent degrees of freedom are related through dominant interactions. It also means understanding how external parameters (pressure, chemical doping, applied fields, changes in composition or structure, etc...) act on these degrees of freedom and modify them. Ab initio theoretical calculations help to build up such a picture, bridging the gap between real systems (specific composition, specific geometry, etc...) and the models of the theoretical physics. It also allows to tackle a large variety of systems (from atoms and molecules to crystals and non periodic infinite systems), of problems (from chemistry and reactivity to magnetism and strongly correlated fermions), etc. It does also allows the hope of answering more technical problems such as the importance of the non sphericity (dependance on the orbitals shape) of the atomic/magnetic form factor in neutron scattering.