  Weyl semimetals are novel topological conductors that host Weyl fermions as
emergent quasiparticles. While the Weyl fermions in high-energy physics are
strictly defined as the massless solution of the Dirac equation and uniquely
fixed by Lorentz symmetry, there is no such constraint for a topological metal
in general. Specifically, the Weyl quasiparticles can arise by breaking either
the space-inversion ($\mathcal{I}$) or time-reversal ($\mathcal{T}$) symmetry.
They can either respect Lorentz symmetry (type-I) or strongly violate it
(type-II). To date, different types of Weyl fermions have been predicted to
occur only in different classes of materials. In this paper, we present a
significant materials breakthrough by identifying a large class of Weyl
materials in the RAlX (R=Rare earth, Al, X=Ge, Si) family that can realize all
different types of emergent Weyl fermions ($\mathcal{I}$-breaking,
$\mathcal{T}$-breaking, type-I or type-II), depending on a suitable choice of
the rare earth elements. Specifically, RAlX can be ferromagnetic, nonmagnetic
or antiferromagnetic and the electronic band topology and topological nature of
the Weyl fermions can be tuned. The unparalleled tunability and the large
number of compounds make the RAlX family of compounds a unique Weyl semimetal
class for exploring the wide-ranging topological phenomena associated with
different types of emergent Weyl fermions in transport, spectroscopic and
device-based experiments.
