arXiv subjects

Alexis Forestier

Publications and source records attributed to Alexis Forestier.

3 recordsLinked to original sources

arXiv2025

Observation of hexagonal close-packed water ice at conditions in ice giant planetary interiors

Using synchrotron x-ray diffraction in laser-heated diamond anvil cells, we report the observation of an hexagonal close-packed (hcp) phase of water ice at high pressure and temperature conditions. Above 200 GPa and 1800 K, the hcp phase becomes dominant upon entering the superionic regime, as evidenced by anomalous thermal expansion. Observations are consistent with the hcp phase becoming thermodynamically more stable than the face-centered cubic (fcc) phase via a martensitic transition extending across the 130 - 200 GPa pressure range, within the superionic regime. Hcp ice is also observed to emerge from stacking disorder developing within the fcc oxygen lattice upon cooling, during its reversion to the bcc phase. The presence of an fcc-hcp martensitic transition in the superionic regime of warm dense ice may have implications for planetary models of Uranus and Neptune.

cond-mat.mtrl-sci↗

arXiv2025

Strain and doping transfer between suspended and supported bilayer graphene

Due to their unique dimensionality, the physical properties of two-dimensional materials are deeply impacted by their surroundings, calling for a thorough understanding and control of these effects. We investigated the influence of the substrate and the pressure transmitting medium on bilayer graphene in a unique high-pressure environment where the sample is partially suspended and partially supported. By employing Raman spectroscopy with a sub-micron spatial resolution, we explored the evolution of strain and doping, and demonstrated that they are both similarly induced in the suspended and supported regions of the bilayer graphene within the studied pressure range. Almost full strain and doping transfer between the supported and suspended regions is concluded. We observed that charge carrier density saturates quickly at low pressures (2 GPa) while biaxial strain continuously increases with pressure. Additionally, Raman spatial mapping highlights a rather uniform doping and strain distribution, yet with significant local variations revealing a more complex scenario than previously documented by single-point studies at high pressure.

cond-mat.mes-hall↗

arXiv2024

Biaxial strain effects in 2D diamond formation from graphene stacks

Discovering innovative methods to understand phase transitions, modify phase diagrams, and uncover novel synthesis routes poses significant and far-reaching challenges. In this study, we demonstrate the formation of nanodiamond-like sp3 carbon from few-layer graphene (FLG) stacks at room temperature and relatively low transition pressure (~7.0 GPa) due to chemical interaction with water and physical biaxial strain induced by substrate compression. By employing resonance Raman and optical absorption spectroscopies at high-pressure on FLG systems, utilizing van der Waals heterostructures (hBN/FLG) on different substrates (SiO2/Si and diamond), we originally unveiled the key role of biaxial strain. Ab initio molecular dynamics simulations corroborates the pivotal role of both water and biaxial strain in locally stabilizing sp3 carbon structures at the graphene-ice interface. This breakthrough directly enhances nanodiamond technology but also establishes biaxial strain engineering as a promising tool to explore novel phases of 2D nanomaterials.

cond-mat.mtrl-sci↗