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Single-photon emitters play a key role in present and emerging quantum technologies. Several recent measurements have established monolayer WSe2 as a promising candidate for a reliable single photon source. The origin and underlying microscopic processes have remained, however, largely elusive. We present a multi-scale tight-binding simulation for the optical spectra of WSe2 under nonuniform strain and in the presence of point defects employing the Bethe-Salpeter equation. Strain locally shifts excitonic energy levels into the band gap where they overlap with localized intra-gap defect states. The resulting hybridization allows for efficient filing and subsequent radiative decay of the defect states. We identify inter-valley defect excitonic states as the likely candidate for antibunched single-photon emission. This proposed scenario is shown to account for a large variety of experimental observations including brightness, radiative transition rates, the variation of the excitonic energy with applied magnetic and electric fields as well as the variation of the polarization of the emitted photon with the magnetic field.Transition Metal Dichalcogenides (TMDs) have attracted considerable interest over the last decade. A direct band gap in the mono layer case [1,2], extremely large excitonic binding energies in the order of 300-500 meV [3,5,11] and valley as well as spin selective optical transitions due to the D 3h symmetry make these materials very promising candidates for optical devices [6,7]. Single photon emitters (SPEs) in WSe 2 are among the most intriguing candidates for such future optical applications attracting considerable attention in the field of two-dimensional materials [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28]. Single-photon emitters promising photon emission "on demand" are key building blocks for optoelectronic and photonic-based quantum-technological devices, e.g., for generating entangled photons [26].SPEs in WSe 2 emit antibunched light from highly localized spots in suspended WSe 2 flakes featuring a narrow linewidth (down to 100 µeV ) and an intricate fine structure (for a review see [27]). A large number of experimental investigations have provided key insight to help unraveling the puzzle of the microscopic origin of SPEs. The prominent observation of SPEs in regions of enhanced strain, for example close to pillars suspending the WSe 2 membrane [19][20][21]25], points to the crucial role of locally non-uniform strain. The large defect density in WSe 2 also seems to play a role in the formation of SPEs [21]. The appearance of doublets in the optical spectra -i.e., single photon emission lines with energy spacing up to 1 meV -has been attributed to the exchange interaction between excitons but the underlying mechanism has remained an open question. While in some early studies few SPEs were found to be only weakly dependent on the magnetic field, in most measurements an unexpectedly large effective g-factor ranging from 8 to 13 was observed [13-15, 17, 23, 24, 28]. S...

We present measurements of quantized conductance in electrostatically induced quantum point contacts in bilayer graphene. The application of a perpendicular magnetic field leads to an intricate pattern of lifted and restored degeneracies with increasing field: at zero magnetic field the degeneracy of quantized one-dimensional subbands is four, because of a twofold spin and a twofold valley degeneracy. By switching on the magnetic field, the valley degeneracy is lifted. Due to the Berry curvature states from different valleys split linearly in magnetic field. In the quantum Hall regime fourfold degenerate conductance plateaus reemerge. During the adiabatic transition to the quantum Hall regime, levels from one valley shift by two in quantum number with respect to the other valley, forming an interweaving pattern that can be reproduced by numerical calculations. arXiv:1809.01920v1 [cond-mat.mes-hall]

Band nesting occurs when conduction and valence bands are approximately equispaced over regions in the Brillouin zone. In two-dimensional materials, band nesting results in singularities of the joint density of states and thus in a strongly enhanced optical response at resonant frequencies. We exploit the high sensitivity of such resonances to small changes in the band structure to sensitively probe strain in semiconducting transition metal dichalcogenides (TMDs). We measure and calculate the polarization-resolved optical second harmonic generation (SHG) at the band nesting energies and present the first measurements of the energy-dependent nonlinear photoelastic effect in atomically thin TMDs (MoS 2 , MoSe 2 , WS 2 , and WSe 2 ) combined with a theoretical analysis of the underlying processes. Experiment and theory are found to be in good qualitative agreement displaying a strong energy dependence of the SHG, which can be exploited to achieve exceptionally strong modulation of the SHG under strain. We attribute this sensitivity to a redistribution of the joint density of states for the optical response in the band nesting region. We predict that this exceptional strain sensitivity is a general property of all 2D materials with band nesting.

We construct a continuum model of twisted trilayer graphene devices using information from ab-initio density-functional theory calculations, and apply it to address electronic structure and transport properties. When the middle layer of an ABA Bernal stacked graphene trilayer is twisted, its important mirror symmetry is retained. The mirror symmetry, which decouples states with even and odd parity under reflection in the plane of the middle-layer, is however lost when the top layer is then shifted laterally relative to the bottom layer, and when a gate electric field is applied. We study the influence of top-layer lateral shifts, which break both mirror and C3v symmetry, on electronic structure, using the Drude weight and the weak-field Hall conductivity as experimentally relevant electronic structure characteristics.

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We present an approach for embedding defect structures modeled by density functional theory into large-scale tight-binding simulations. We extract local tight-binding parameters for the vicinity of the defect site using Wannier functions. In the transition region between the bulk lattice and the defect the tight-binding parameters are continuously adjusted to approach the bulk limit far away from the defect. This embedding approach allows for an accurate high-level treatment of the defect orbitals using as many as ten nearest neighbors while keeping a small number of nearest neighbors in the bulk to render the overall computational cost reasonable. As an example of our approach, we consider an extended graphene lattice decorated with Stone-Wales defects, flower defects, double vacancies, or silicon substitutes. We predict distinct scattering patterns mirroring the defect symmetries and magnitude that should be experimentally accessible.

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