van der Waals heterostructures provide many novel applications due to a combination of properties. However, their experimental construction and theoretical simulation suffer from the incommensurability of 2D crystals with respect to their symmetry and their lattice constants. In this work, we present a simplified method to predict favorable combinations of 2D crystals based on the coincidence lattice method. We present a huge set of possible heterostructures made from transition-metal dichalcogenides, group IV dichalcogenides, graphene, and hexagonal boron nitride. The method is then validated for theoretically and experimentally studied 2D crystals and van der Waals-bonded heterostructures. The power of the approach is demonstrated by comparison of resulting supercell sizes, strain, and relative orientation with experimental and theoretical data available. To display the prospects of this approach, we simulate three heterostructures and analyze the resulting structural and electronic properties, finding favorable stackings and small changes in band alignments in the weakly interacting heterojunction.

Stacks of two-dimensional crystals in van der Waals heterostructures pave the way to novel applications in electronics and optoelectronics. Based on first-principles calculations, we study heterobilayers constructed with phosphorene on MoSe2 and WSe2. Both combinations are stable upon contact, while van der Waals interaction leads to a long-range structural bending, affecting electronic properties from phosphorene. Including quasiparticle effects, strong orbital overlaps are observed in the heterobilayers, influencing band offsets and, hence, emphasizing the importance of quasiparticle calculations over standard density functional theory ones. Interface effects also change the heterostructure type, modify local band gaps, and favor indirect transitions. To tailor the electronic properties of the heterostructures, interface interactions and external perturbations are taken into account through vertical pressures and electric fields. Uniaxial pressure strongly affects local direct gaps, and small electric fields can sweep band lineups and even change the heterostructure type. The studied features demonstrate the potential of bilayer systems for field-effect transistors, optoelectronic devices, and sensitive sensors.

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Growth of X-enes, such as silicene, germanene and stanene, requires passivated substrates to ensure the survival of their exotic properties. Using first-principles methods, we study as-grown graphene on polar SiC surfaces as suitable substrates. Trilayer combinations with coincidence lattices with large hexagonal unit cells allow for strain-free group-IV monolayers. In contrast to the Si-terminated SiC surface, van der Waals-bonded honeycomb X-ene/graphene bilayers on top of the C-terminated SiC substrate are stable. Folded band structures show Dirac cones of the overlayers with small gaps of about 0.1 eV in between. The topological invariants of the peeled-off X-ene/graphene bilayers indicate the presence of topological character and the existence of a quantum spin Hall phase.

The use of spatial quantum superpositions of electron states in a gated vdW heterostructure as a charge qubit is presented. We theoretically demonstrate the concept for the ZrSe2/SnSe2 vdW heterostructure using rigorous ab initio calculations. In the proposed scheme, the quantum state is prepared by applying a vertical electric field, is manipulated by short field pulses, and is measured via electric currents. The qubit is robust, operational at high temperature, and compatible with the current 2D technology. The results open up new avenues for the field of physical implementation of qubits. :1904.10785v1 [cond-mat.mes-hall] arXiv