Overlapping electron density and the global delocalization of π-aromatic fragments as the reason of conductivity of the biphenylene network

Recently fabricated 2D biphenylene network is an astonishing solid-state material, which possesses unique metal-like conductive properties. At the same time, two-dimensional boron nitride network (2D-BN)-an isoelectronic and structural analogue of biphenylene network, is an insulator with a wide direct bandgap. This study investigates the relationship between the electronic properties and chemical bonding patterns for these species. It is shown that the insulating 2D-BN network possesses a strong localization of electron density on the nitrogen atoms. In turn, for a carbon-containing sheet, we found a highly delocalized electron density and an appreciable overlap of p_z orbitals of neighboring C₆ rings, which might be a reason for the conductive properties of the material.

Magnetic Ordering, Anomalous Lifshitz Transition, and Topological Grain Boundaries in Two-Dimensional Biphenylene Network

We study the electronic properties of a new planar carbon crystal formed through networking biphenylene molecules. Novel electronic features among carbon materials such as zone-center saddle point and peculiar type-II Dirac fermionic states are shown to exist in the low-energy electronic spectrum. The type-II state here has a nearly flat branch and is close to a transition to type I. Possible magnetic instabilities related to low-energy bands are discussed. Furthermore, with a moderate uniaxial strain, a pair of Dirac points merge with the zone-center saddle point, realizing concurrent Lifshitz transitions of van Hove singularity as well as pair annihilation of the Dirac fermions. A new effective Hamiltonian encompassing all distinctive low-energy states is constructed, revealing a finite winding number of the pseudospin texture around the Dirac point, quantized Zak phases, and topological grain boundary states.