Search

Recent theoretical and experimental studies suggest that van der Waals heterostructures with n- and p-doped bilayers of transition metal dichalcogenides are promising facilitators of exciton superfluidity. Exciton superfluidity in such bilayer systems is often modelled by solving a mean-field gap equation defined for only the conduction and valence band of the electron and hole material respectively. A key quantity entering the gap equation is the effective Coulomb potential acting as the bare interaction in the subspace of the model. Since the model only includes a few bands around the Fermi energy the effective model interaction is partially screened. Although the screening is a material dependent quantity it has in previous works been accounted for in an ad hoc manner, by assuming a static dielectric constant of 2 for a wide range of different materials. In this work we show that the effective model interaction can be derived from first principles using open source code frameworks. Using this novel ab initio downfolding procedure we show that the material dependent screening is essential to predict both magnitude and trends of exciton binding energies and superfluid properties. Furthermore, we suggest new material platforms of both transition metal oxides and dichalcogenides with superior properties compared to the standard devices with two transition metal dichalcogenide layers., 16 pages, 8 figures

We explore a new platform for realising excitonic insulators, namely van der Waals (vdW) bilayers comprising 2D Janus materials. In previous studies, Type-II heterobilayers have been brought to the excitonic insulating regime by tuning the band alignment using external gates. In contrast, the Janus bilayers of the current work represent intrinsic excitonic insulators. We first conduct ab initio calculations to obtain the quasiparticle band structures, screened Coulomb interaction, and interlayer exciton binding energies of the bilayers. These ab initio-derived quantities are then used to construct a BCS-like Hamiltonian of the exciton condensate. By solving the mean-field gap equation, we identify 16 vdW Janus bilayers with insulating ground states and superfluid properties. Our calculations expose a new class of advanced materials that are likely to exhibit novel excitonic phases at low temperatures, and highlight the subtle competition between interlayer hybridization, spin-orbit coupling, and dielectric screening that govern their properties., Comment: 5 figures, 11 pages

A mismatch of atomic registries between single-layer transition metal dichalcogenides (TMDs) in a two dimensional van der Waals heterostructure produces a moir\'e superlattice with a periodic potential, which can be fine-tuned by introducing a twist angle between the materials. This approach is promising both for controlling the interactions between the TMDs and for engineering their electronic band structures, yet direct observation of the changes to the electronic structure introduced with varying twist angle has so far been missing. Here, we probe heterobilayers comprised of single-layer MoS$_2$ and WS$_2$ with twist angles of $(2.0 \pm 0.5)^{\circ}$, $(13.0 \pm 0.5)^{\circ}$, and $(20.0 \pm 0.5)^{\circ}$ and investigate the differences in their electronic band structure using micro-focused angle-resolved photoemission spectroscopy. We find strong interlayer hybridization between MoS$_2$ and WS$_2$ electronic states at the $\bar{\mathrm{\Gamma}}$-point of the Brillouin zone, leading to a transition from a direct bandgap in the single-layer to an indirect gap in the heterostructure. Replicas of the hybridized states are observed at the centre of twist angle-dependent moir\'e mini Brillouin zones. We confirm that these replica features arise from the inherent moir\'e potential by comparing our experimental observations with density functional theory calculations of the superlattice dispersion. Our direct visualization of these features underscores the potential of using twisted heterobilayer semiconductors to engineer hybrid electronic states and superlattices that alter the electronic and optical properties of 2D heterostructures., Comment: 31 pages, 6 figures in the main text and 5 figures in the supporting information