We conduct basic research in condensed matter theoretical physics from an ab initio point of view, i.e. working on the full degrees-of-freedom microscopical hamiltonian and without parameters to be adjusted on the experiment.
The methodology relies in particular on ab initio many-body quantum field theory (GW approximation, Bethe- Salpeter equation), but also on density-functional theory (DFT) and time-dependent density-functional theory (TDDFT). Since the integration in 2013 of a theoretician from CRISMAT in the group, a many-body wavefunction activity around post-Hartree-Fock techniques is further developed with applications to strongly correlated sys- tems of interest for magnetic, multiferroic or magnéto-electric properties.
The global aim is from one side to analitically develop theory and approximations, and from another to imple- ment them into codes that are later used in numerical calculations on real systems and finally compared with the experiment. In particular, two ambitious and inovating activities, from analytic theories to code developments and applications to real-materials, have been initiated these last few years around (a) the writing of a massively- parallel Gaussian-basis GW and Bethe-Salpeter package for application to organic systems (the Fiesta initiative) and (b) the development of a “time-dependent Bethe-Salpeter” formalism for non-linear optics.
We focus in particular on electronic excited-state properties and spectroscopy (electronic structure, (super)con- ductivity, optical, X-ray and dielectric spectroscopy including excitons, plasmons, etc.), but also on ground-state structural and dynamical properties (atomic structure, phonons).
The systems range from molecules to nanotubes, from 2D systems (e.g. graphene et al.) to bulk solids of interest for electronic correlations, superconductivity, (non)linear optics or photovoltaics.
Important lines developed and opened in the last 5 years include :
superconductivity (BCS) in highly-doped diamond and silicon and 2D materials, in collaboration with the experi- mental “Grand Gap” and “MagSup” teams at NEEL ;
many-body theory for nonlinear optics, world first ab initio calculations of excitons in non-linear optics ;
many-body theory for organic and hybrid photovoltaics, pioneering role (FIESTA initiative) in adapting physics many-body quantum-field methodology to organic systems, in collaboration with the L_Sim-INAC@CEA. This ativity is detailed in a subsequent Scientific achievement ;
many-body theory for the momentum distribution, with focus on electronic correlation and in collaboration with the QMC group at LPMMC and the experimental IXS ID16 line at ESRF ;
post-Hartree Fock coupled-cluster studies of strongly correlated oxydes.