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The sp2 hybridized carbon allotropes such as fullerenes and graphene are scientifically and technologically significant because of their unique elastic and electronic properties. These properties make them useful in a wide variety of applications. Recently, experimentalists have synthesized sp-sp2 hybridized carbon tubular arrays of two-dimensional carbon films, referred to as graphdiyne. To explore the possible existence of an sp-sp2 hybridized one-dimensional carbon allotrope, we investigate graphdiyne nanotubes' structural and electronic properties using dispersion-corrected density functional theory calculations. Graphdiyne nanotubes display unique porous characteristics and remarkable stability, which may promote them as a novel class of carbon materials. 

Abstract The superconducting critical temperature T c of intercalated iron-selenide superconductor (Li,Fe)OHFeSe (FeSe11111) can be increased to 42 from 8 K of bulk FeSe. It shows remarkably similar electronic properties as the high- T c monolayer FeSe and provides a bulk counterpart to investigate the origin of enhanced superconductivity. Unraveling the nature of excitations is crucial for understanding the pairing mechanism in high- T c iron selenides. Here we use resonant inelastic x-ray scattering (RIXS) to investigate the excitations in FeSe11111. Our high-quality data exhibit several Raman-like excitations, which are dispersionless and isotropic in momentum transfer in both superconducting 28 K and 42 K samples. Using atomic multiplet calculations, we assign the low-energy ~0.3 and 0.7 eV Raman peaks as local e g  −  e g and e g  −  t 2 g orbital excitations. The intensity of these two features decreases with increasing temperature, suggesting a dominating contribution of the orbital fluctuations. Our results highlight the importance of the orbital degree of freedom for high- T c iron selenides.