  A single atomic layer of black phosphorus, phosphorene, was experimentally
realized in 2014. It has a puckered honeycomb lattice structure and a
semiconducting electronic structure. In the first part of this paper, we use a
simple LCAO model, and discuss qualitatively the electronic structure of
phosphorene systems under electric and magnetic fields, especially, noting
their midgap edge states. The next part is spent for the review of the research
progress on phosphorene in the past one year since its appearance in 2014.
Phosphorene has been a typical object to study the semiconductor physics in
atomic layers.
