After the discovery of graphene and its many fascinating properties, there
has been a growing interest for the study of "artificial graphenes". These are
totally different and novel systems which bear exciting similarities with
graphene. Among them are lattices of ultracold atoms, microwave or photonic
lattices, "molecular graphene" or new compounds like phosphorene. The advantage
of these structures is that they serve as new playgrounds for measuring and
testing physical phenomena which may not be reachable in graphene, in
particular: the possibility of controlling the existence of Dirac points (or
Dirac cones) existing in the electronic spectrum of graphene, of performing
interference experiments in reciprocal space, of probing geometrical properties
of the wave functions, of manipulating edge states, etc. These cones, which
describe the band structure in the vicinity of the two connected energy bands,
are characterized by a topological "charge". They can be moved in reciprocal
space by appropriate modification of external parameters (pressure, twist,
sliding, stress, etc.). They can be manipulated, created or suppressed under
the condition that the total topological charge be conserved. In this short
review, I discuss several aspects of the scenarios of merging or emergence of
Dirac points as well as the experimental investigations of these scenarios in
condensed matter and beyond.Comment: 16 pages, 26 figures. To appear in Comptes-rendus de l'Acad\'emie des
Sciences, Franc
