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Abstract We argue that alternating-layer structures of lattice mismatched or misaligned (twisted) atomically-thin layers should be expected to be more efficient absorbers of the broad-spectrum of solar radiation than the bulk material of each individual layer. In such mismatched layer-structures the conduction and valence bands of the bulk material, split into multiple minibands separated by minigaps confined to a small-size emerging Brillouin zone due to band-folding. We extended the Shockley–Queisser approach to calculate the photovoltaic efficiency for a band split into minibands of bandwidth ΔEand mini-gaps δGto model the case when such structures are used as solar cells. We find a significant efficiency enhancement due to impact ionization processes, especially in the limit of small but non-zero δG, and a dramatic increase when fully concentrated Sun-light is used. 

Beginning from the conventional square-lattice nearest-neighbor antiferromagnetic Heisenberg model, we allow the 𝐽𝑥 and 𝐽𝑦 couplings to be anisotropic, with their values depending on the bond orientation. The emergence of anisotropic, bond-dependent couplings should be expected to occur naturally in most antiferromagnetic compounds which undergo structural transitions that reduce the point-group symmetry at lower temperature. Using the spin-wave approximation, we study the model in several parameter regimes by diagonalizing the reduced Hamiltonian exactly and computing the edge spectrum and Berry connection vector, which show clear evidence of localized topological charges. We discover phases that exhibit Weyl-type spin-wave dispersion, characterized by pairs of degenerate points and edge states, as well as phases supporting lines of degeneracy. We also identify a parameter regime in which there is an exotic state hosting gapless linear spin-wave dispersions with different longitudinal and transverse spin-wave velocities. 

In this paper, we analyze the band structure of two-dimensional (2D) halide perovskites by considering structures related to the simpler case of the series, (BA)2⁢PbI4, in which PbI4 layers are intercalated with butylammonium [BA=CH3(CH2)3⁢NH3] organic ligands. We use density-functional-theory (DFT) based calculations and tight-binding (TB) models aiming to discover a simple description of the bands within 1 eV below the valence-band maximum and 2 eV above the conduction-band minimum, which, including the energy gap, is about a Δ⁢𝐸=5 eV energy range. The bands in this Δ⁢𝐸 range are those expected to contribute to the transport phenomena, photoconductivity, and light emission in the visible spectrum, at room and low temperature. We find that the atomic orbitals of the butylammonium chains have negligible contribution to the Bloch states which form the conduction and valence bands in the above defined Δ⁢𝐸 range. Our calculations reveal a rather universal, i.e., independent of the intercalating BA, rigid-band picture inside the above Δ⁢𝐸 range characteristic of the layered perovskite “matrix” (i.e., PbI4 in our example). Besides demonstrating the above conclusion, the main goal of this paper is to find accurate TB models which capture the essential features of the DFT bands in this Δ⁢𝐸 range. First, we ignore electron hopping along the 𝑐 axis and the octahedral distortions and this increased symmetry (from C2 to C4) halves the Bravais lattice unit cell size and the Brillouin zone unfolds to a 45∘ rotated square and this allows some analytical handling of the 2D TB Hamiltonian. The Pb 6⁢𝑠 and I 5⁢𝑠 orbitals are far away from the above Δ⁢𝐸 range and, thus, we integrate them out to obtain an effective model which only includes hybridized Pb 6⁢𝑝 and I 5⁢𝑝 states. Our TB-based treatment (a) provides a good quantitative description of the DFT band structure, (b) helps us conceptualize the complex electronic structure in the family of these materials in a simple way, and (c) yields the one-body part to be combined with appropriately screened electron interaction to describe many-body effects, such as excitonic bound states.