Electron-vibration coupling induced renormalization in the photoemission spectrum of diamondoids.

The development of theories and methods devoted to the accurate calculation of the electronic quasi-particle states and levels of molecules, clusters and solids is of prime importance to interpret the experimental data. These quantum systems are often modelled by using the Born-Oppenheimer approximation where the coupling between the electrons and vibrational modes is not fully taken into account, and the electrons are treated as pure quasi-particles. Here, we show that in small diamond cages, called diamondoids, the electron-vibration coupling leads to the breakdown of the electron quasi-particle picture. More importantly, we demonstrate that the strong electron-vibration coupling is essential to properly describe the overall lineshape of the experimental photoemission spectrum. This cannot be obtained by methods within Born-Oppenheimer approximation. Moreover, we deduce a link between the vibronic states found by our many-body perturbation theory approach and the well-known Jahn-Teller effect.

Orbital-ordering-driven multiferroicity and magnetoelectric coupling in GeV4S8.

We report here the discovery of multiferroicity and large magnetoelectric coupling in the type I orbital order system GeV4S8. Our study demonstrates that this clustered compound displays a para-ferroelectric transition at 32 K. This transition originates from an orbital ordering which reorganizes the charge within the transition metal clusters. Below the antiferromagnetic transition at 17 K, the application of a magnetic field significantly affects the ferroelectric polarization, revealing thus a large magnetoelectric coupling. Our study suggests that the application of a magnetic field induces a metamagnetic transition which significantly affects the ferroelectric polarization thanks to an exchange striction phenomenon.

Effect of the quantum zero-point atomic motion on the optical and electronic properties of diamond and trans-polyacetylene.

The quantum zero-point motion of the carbon atoms is shown to induce strong effects on the optical and electronic properties of diamond and trans-polyacetylene, a conjugated polymer. By using an ab initio approach, we interpret the subgap states experimentally observed in diamond in terms of entangled electron-phonon states. These states also appear in trans-polyacetylene causing the formation of strong structures in the band structure that even call into question the accuracy of the band theory. This imposes a critical revision of the results obtained for carbon-based nanostructures by assuming the atoms frozen in their equilibrium positions.