Designable exciton mixing through layer alignment in WS2_2-graphene heterostructures

Optical properties of heterostructures composed of layered 2D materials, such as transition metal dichalcogenides (TMDs) and graphene, are broadly explored. Of particular interest are light-induced energy transfer mechanisms in these materials and their structural roots. Here, we use state-of-the-art first-principles calculations to study the excitonic composition and the absorption properties of WS$_2$-graphene heterostructures as a function of interlayer alignment and the local strain resulting from it. We find that Brillouin zone mismatch and the associated energy level alignment between the graphene Dirac cone and the TMD bands dictate an interplay between interlayer and intralayer excitons, mixing together in the many-body representation upon the strain-induced symmetry breaking in the interacting layers. Examining the representative cases of the 0$^\circ$ and 30$^\circ$ interlayer twist angles, we find that this exciton mixing strongly varies as a function of the relative alignment. We quantify the effect of these structural modifications on exciton charge separation between the layers and the associated graphene-induced homogeneous broadening of the absorption resonances. Our findings provide guidelines for controllable optical excitations upon interface design and shed light on the importance of many-body effects in the understanding of optical phenomena in complex heterostructures.Comment: 8 pages, 4 figure

Spin-orbit torque in single-molecule junctions from ab initio

The use of electric fields applied across magnetic heterojunctions that lack spatial inversion symmetry has been previously proposed as a non-magnetic mean of controlling localized magnetic moments through spin-orbit torques (SOT). The implementation of this concept at the single-molecule level has remained a challenge, however. Here, we present first-principle calculations of SOT in a single-molecule junction under bias and beyond linear response. Employing a self-consistency scheme invoking density functional theory and non-equilibrium Green's function theory, we compute the current-induced SOT. Responding to this torque, a localized magnetic moment can tilt. Within the linear regime our quantitative estimates for the SOT in single-molecule junctions yield values similar to those known for magnetic interfaces. Our findings contribute to an improved microscopic understanding of SOT in single molecules.Comment: 7 pages + 5 figures; Supporting Information (16 pages + 3 figures

Charge quenching at defect states in transition metal dichalcogenide-graphene van der Waals heterobilayers

We study the dynamical properties of point-like defects, represented by monoatomic chalcogen vacancies, in WS$_2$-graphene and MoS$_2$-graphene heterobilayers. Employing a multidisciplinary approach based on the combination of ab initio, model Hamiltonian and density matrix techniques, we propose a minimal interacting model that allows for the calculation of electronic transition times associated to population and depopulation of the vacancy by an additional electron. We obtain the "coarse-grained" semiclassical dynamics by means of a master equation approach and discuss the potential role of virtual charge fluctuations in the internal dynamics of impurity quasi-degenerate states. The interplay between the symmetry of the lattice and the spin degree of freedom through the spin-orbit interaction and its impact on charge quenching is studied in detail.Comment: 17 pages + 9 figures; Supplemental Material (10 pages + 4 figures

Reduced absorption due to defect-localized interlayer excitons in transition metal dichalcogenide-graphene heterostructures

Associating the presence of atomic vacancies to excited-state transport phenomena in two dimensional semiconductors is of emerging interest, and demands detailed understanding of the involved exciton transitions. Here we study the effect of such defects on the electronic and optical properties of WS$_2$-graphene and MoS$_2$-graphene van der Waals heterobilayers by employing many-body perturbation theory. We find that the combination of chalcogen defects and graphene adsorption onto the transition metal dichalcogenide layer can radically alter the optical properties of the heterobilayer, due to a combination of dielectric screening, the impact of the missing chalcogen atoms in the intralayer and interlayer optical transitions, and the different nature of each layer. By analyzing the intrinsic radiative rates of the most stable subgap excitonic features, we find that while the presence of defects introduces low-lying optical transitions, resulting in excitons with larger oscillator strength, it also decreases the optical response associated to the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.Comment: 7 pages + 3 figures; Supporting Information (20 pages + 13 figures

Low-scaling GW algorithm applied to twisted transition-metal dichalcogenide heterobilayers

The $GW$ method is widely used for calculating the electronic band structure of materials. The high computational cost of $GW$ algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling and highly efficient $GW$ algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables $G_0W_0$ calculations on a MoSe$_2$/WS$_2$ bilayer with 984 atoms per unit cell, in 42 hours using 1536 cores. This is four orders of magnitude faster than a plane-wave $G_0W_0$ algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale

Solitonics with Polyacetylenes

Polyacetylene molecular wires have attracted a long-standing interest for the past 40 years. From a fundamental perspective, there are two main reasons for the interest. First, polyacetylenes are a prime realization of a one-dimensional topological insulator. Second, long molecules support freely propagating topological domain-wall states, so-called "solitons," which provide an early paradigm for spin-charge separation. Because of recent experimental developments, individual poly- acetylene chains can now be synthesized on substrates. Motivated by this breakthrough, we here propose a novel way for chemically supported soliton design in these systems. We demonstrate how to control the soliton position and how to read it out via external means. Also, we show how extra soliton-antisoliton pairs arise when applying a moderate static electric field. We thus make a step toward functionality of electronic devices based on soliton manipulation, that is, "solitonics"

Mixed excitonic nature in water-oxidized BiVO4_4 surfaces with defects

BiVO$_4$ is a promising photocatalyst for efficient water oxidation, with surface reactivity determined by the structure of active catalytic sites. Surface oxidation in the presence of oxygen vacancies induces electron localization, suggesting an atomistic route to improve the charge transfer efficiency within the catalytic cycle. In this work, we study the effect of oxygen vacancies on the electronic and optical properties at BiVO$_4$ surfaces upon water oxidation. We use density functional theory and many-body perturbation theory to explore the change in the electronic and quasiparticle energy levels and to evaluate the electron-hole coupling as a function of the underlying structure. We show that while the presence of defects alters the atomic structure and largely modifies the wavefunction nature, leading to defect-localized states at the quasipatricle gap region, the optical excitations remain largely unchanged due to substantial hybridization of defect and non-defect electron-hole transitions. Our findings suggest that defect-induced surface oxidation supports improved electron transport, both through bound and tunable electronic states and via a mixed nature of the optical transitions, expected to reduce electron-hole defect trapping

Low-Scaling GW Algorithm Applied to Twisted Transition-Metal Dichalcogenide Heterobilayers

The GW method is widely used for calculating the electronic band structure of materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling, and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G0W0 calculations on a MoSe2/WS2 bilayer with 984 atoms per unit cell, in 42 h using 1536 cores. This is 4 orders of magnitude faster than a plane-wave G0W0 algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale

Low-Scaling GW Algorithm Applied to Twisted Transition-Metal Dichalcogenide Heterobilayers

The GW method is widely used for calculating the electronic band structure of materials. The high computational cost of GW algorithms prohibits their application to many systems of interest. We present a periodic, low-scaling, and highly efficient GW algorithm that benefits from the locality of the Gaussian basis and the polarizability. The algorithm enables G0W0 calculations on a MoSe2/WS2 bilayer with 984 atoms per unit cell, in 42 h using 1536 cores. This is 4 orders of magnitude faster than a plane-wave G0W0 algorithm, allowing for unprecedented computational studies of electronic excitations at the nanoscale