Description du sujet de thèse

The recent progress in controlling the stacking of atomic sheets in van der Waals heterostructures has opened up new avenues for manipulating electronic properties by moiré superlattices, i.e. by long-wavelength periodic potential landscapes. In two-dimensional (2D) materials, a moiré superlattice can be formed by vertically stacking two layered materials with a twist angle and/or a difference in lattice constant that generally modifies the electronic band structure. In twisted stacks, it has been shown that at so-called “magic angle twist”, low-energy flat subbands can appear, in which electron interactions become the dominant energy scale and lead to emergent electronic phases, such as correlated insulators, superconductors, magnetism and topological electronic structures. Until recently, correlated phases emerging from isolated flat bands were only realized in twisted graphene-based stacking. Their discovery in twisted transition metal dichalcogenides (TMDs) homo- and hetero-bilayers offers the additional fascinating perspective of a solid-state platform in which plenty of correlated states may be realized. Although these last two years studies on semiconductor-based TMD moiré systems have revealed novel phenomena driven by strong Coulomb interactions such as the Mott insulating state, Wigner crystal states, stripe phases, antiferromagnetism and pseudospin ferromagnetism, the twisted TMDs physics is in its infancy and many exotic states of matter remain to be uncovered.

This PhD project aims at exploring and exploiting the electronic band structures of TMDs heterostructures generated either by twist or by in-situ intercalation of alkali atoms that induces lattice mismatches and strain effects. The PhD candidate will be strongly involved in developing samples elaboration strategies on a recently acquired transfer system (hq graphene company), devoted to selectively pick up large sections of TMD flakes, rotate and transfer them with a sub-degree control of twist angles. Since some exfoliated TMDs tend to degrade when exposed to air, this transfer system, which is fully motorized, will be installed within a glovebox and externally controlled using a computer. An important component of the PhD project will also be the use of angle-resolved photoemission spectroscopy (ARPES) for accessing to the electronic band structures of the as-obtained TMDs heterostructures and under different doping with in-situ chemical gating. In that context, the PhD candidate will take part to an exciting experimental development at IPR devoted to low-temperature ARPES using monochromatized micro-focused UV source. In parallel, regular applications for beamtimes at synchrotron facilities will be conducted in order to efficiently target the scientific questions that need facilities only available on synchrotron to be addressed such as tunable polarization and photon energy.

Contexte de travail

This thesis is part of an ANR project whose funding has been acquired. The candidate will benefit from a unique experimental environment combining sample growth environments and photoemission spectroscopies allowing for elaborating and characterizing the materials in the laboratory. Strong interplay with local theoreticians will also offer the possibility of performing ab-initio calculations of electronic structures and spectral functions within the density functional theory (DFT) and multiple scattering framework.

Contraintes et risques

None