Dear Editor

The search for exotic Dirac fermions in functional materials has been a central theme in condensed
matter physics during the past decade. Most of the observed Dirac fermions materialize in weakly-correlated
systems; identification of Dirac fermions in strongly-correlated materials would offer
insight into their intricate physics. To this end, we turn to the strongly-correlated cerium
monopnictides (CeX), and find an unprecedented type of Dirac fermions which is uniquely distinct
from graphene and previously-discovered topological insulators. The experimental robustness of
these fermions challenge our understanding of correlated magnetic phases, as well as promise
applications in low-power electronics and spintronics devices.

The charge carriers of graphene mimic massless, relativistic electrons with an isotropic speed of
light. In CeX, we find two flavors of massless, relativistic electrons with distinct velocities that
strongly depend on the direction of propagation – their energy-momentum dispersion comprises two
graceful, elliptical cones which surprisingly interpenetrate each other without mixing. While these
strange fermions live only on a two-dimensional surface, each surface flavor originates from a
distinct Moebius-like twist in the quantum wavefunctions of three-dimensional CeX. This precise
correspondence between surface and bulk fermions presents a fascinating case study of holography
in condensed matter physics.