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We use an ab-initio approach to design and study a novel two-dimensional material -a planar array of carbon nanotubes separated by an optimal distance defined by the van der Waals interaction. We show that the energy spectrum for an array of quasi-metallic nanotubes is described by a strongly anisotropic hyperbolic dispersion and formulate a model low-energy Hamiltonian for its semi-analytical treatment. Periodic-potential-induced lifting of the valley degeneracy for an array of zigzag narrow-gap nanotubes leads to the band gap collapse. In contrast, the band gap is opened in an array of gapless armchair tubes. These unusual spectra, marked by pronounced van Hove singularities in the low-energy density of states, open the opportunity for interesting physical effects and prospective optoelectronic applications. arXiv:1904.06372v4 [cond-mat.mes-hall]

The results of theoretical investigation of the equilibrium spatial configuration, electronic structure, and optical properties of the Zn‐based metal–organic framework [{Zn2(TBAPy)(H2O)2}*3.5DEF]n are reported. The electronic structure is obtained in frameworks of the density‐functional theory (DFT) with BLYP and PBE exchange‐correlation functionals, and optical response is computed using time‐dependent density functional theory (TDDFT). It is shown that the formation of narrow LUMO and HOMO bands is provided by conjugated π‐electrons of the aromatic rings of the ligand. These bands have nontrivial pattern and contain regions of hyperbolic dispersion corresponding to highly anisotropic heavy fermions. Optical transitions between these bands result in the appearance of near‐threshold peaks in photoabsorption spectra. The structure of the bands prevents formation of the bright exciton states in the system, and thus optical response is governed by single particle excitations only.

In a recent experimental paper, it was claimed that pronounced excitonic signatures are observed in optical response of Zn-based metal-organic frameworks (MOFs) at room temperature. Performing ab initio modelling, it is demonstrated that an alternative interpretation based on single-electron optical transitions between narrow π-bands in the system of aromatic rings of the ligand is far more plausible. Although these results do not rule the possibility of exciton formation in MOFs out completely, they show that extreme caution should be taken in attributing the features in photoabsorption spectra alone to excitons, and additional proof, such as data on long-distance energy transfer, is necessary.

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