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

TL;DR

This paper explores the electronic, magnetic, and topological properties of a novel two-dimensional biphenylene network, revealing unique Dirac states, Lifshitz transitions, and topological boundary phenomena under strain.

Contribution

It introduces a new 2D carbon structure with distinctive electronic states, topological features, and strain-induced phase transitions not previously reported.

Findings

Presence of zone-center saddle point and type-II Dirac fermions.

Strain-induced merging of Dirac points with saddle point, causing Lifshitz transitions.

Existence of topological grain boundary states with quantized Zak phases.

Abstract

We study 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 with 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 pseudo-spin texture around the Dirac point, quantized Zak phases, and topological grain boundary states.