Large phosphorene in-plane contraction induced by interlayer interactions in graphene-phosphorene heterostructures

Intralayer deformation in van der Waals (vdW) heterostructures is generally assumed to be negligible due to the weak nature of the interactions between the layers, especially when the interfaces are found incoherent. In the present work, graphene-phosphorene vdW-heterostructures are investigated with the Density Functional Theory (DFT). The challenge of treating nearly incommensurate (very large) supercell in DFT is bypassed by considering different energetic quantities in the grand canonical ensemble, alternative to the formation energy, in order to take into account the mismatch elastic contribution of the different layers. In the investigated heterostructures, it is found that phosphorene contracts by ~4% in the armchair direction when compared to its free-standing form. This large contraction leads to important changes in term of electronic properties, with the direct electronic optical transition of phosphorene becoming indirect in specific vdW-heterostructures. More generally, such a contraction indicates strong substrate effects in supported or encapsulated phosphorene -neglected hitherto- and paves the way to substrate-controlled stress- tronic in such 2D crystal. In addition, the stability of these vdW-heterostructures are investigated as a function of the rotation angle between the layers and as a function of the stacking composition. The alignment of the specific crystalline directions of graphene and phosphorene is found energetically favored. In parallel, several several models based on DFT-estimated quantities are presented; they allow notably a better understanding of the global mutual accommodation of 2D materials in their corresponding interfaces, that is predicted to be non-negligible even in the case of incommensurate interfaces.Comment: 33 pages, 6 figure

First-principles prediction of lattice coherency in van der Waals heterostructures

The emergence of superconductivity in slightly-misaligned graphene bilayer [1] and moir\'e excitons in MoSe$_2$-WSe$_2$ van der Waals (vdW) heterostructures [2] is intimately related to the formation of a 2D superlattice in those systems. At variance, perfect primitive lattice matching of the constituent layers has also been reported in some vdW-heterostructures [3-5], highlighting the richness of interfaces in the 2D world. In this work, the determination of the nature of such interface, from first principles, is demonstrated. To do so, an extension of the Frenkel-Kontorova (FK) model [6] is presented, linked to first-principles calculations, and used to predict lattice coherency for a set of 56 vdW-heterostructures. Computational predictions agree with experiments, when available. New superlattices as well as perfectly-matching interfaces are predicted.Comment: 16 pages, 3 figure

Optimizing Dirac fermions quasi-confinement by potential smoothness engineering

With the advent of high mobility encapsulated graphene devices, new electronic components ruled by Dirac fermions optics have been envisioned and realized. The main building blocks of electron-optics devices are gate-defined p-n junctions, which guide, transmit and refract graphene charge carriers, just like prisms and lenses in optics. The reflection and transmission are governed by the p-n junction smoothness, a parameter difficult to tune in conventional devices. Here we create p-n junctions in graphene, using the polarized tip of a scanning gate microscope, yielding Fabry-P\'erot interference fringes in the device resistance. We control the p-n junctions smoothness using the tip-to-graphene distance, and show increased interference contrast using smoother potential barriers. Extensive tight-binding simulation reveal that smooth potential barriers induce a pronounced quasi-confinement of Dirac fermions below the tip, yielding enhanced interference contrast. On the opposite, sharp barriers are excellent Dirac fermions transmitters and lead to poorly contrasted interferences. Our work emphasizes the importance of junction smoothness for relativistic electron optics devices engineering

Lattice dynamics localization in low-angle twisted bilayer graphene

A low twist angle between the two stacked crystal networks in bilayer graphene enables self-organized lattice reconstruction with the formation of a periodic domain. This superlattice modulates the vibrational and electronic structures, imposing new rules for electron-phonon coupling and the eventual observation of strong correlation and superconductivity. Direct optical images of the crystal superlattice in reconstructed twisted bilayer graphene are reported here, generated by the inelastic scattering of light in a nano-Raman spectroscope. The observation of the crystallographic structure with visible light is made possible due to lattice dynamics localization, the images resembling spectral variations caused by the presence of strain solitons and topological points. The results are rationalized by a nearly-free-phonon model and electronic calculations that highlight the relevance of solitons and topological points, particularly pronounced for structures with small twist angles. We anticipate our discovery to play a role in understanding Jahn-Teller effects and electronic Cooper pairing, among many other important phonon-related effects, and it may be useful for characterizing devices in the most prominent platform for the field of twistronics.Comment: 9 pages, 8 figure

Analyse fonctionnelle de FRD3 chez deux espèces d'Arabidopsis

Transcriptomic studies identified genes which are constitutively over-expressed in A. halleri compared to A. thaliana and which may have a role in metal tolerance or accumulation (1-3). A candidate gene encodes FRD3, a member of the MATE family of membrane transporters (56 members in A. thaliana). It is a citrate transporter involved in iron homeostasis (4-6) and playing a role in zinc tolerance in A. thaliana (7). We are aiming to analyse the FRD3 high expression in A. halleri and the FRD3 function in zinc and iron homeostasis in A. thaliana