  Magnetoresistance (MR), the change of a material's electrical resistance in
response to an applied magnetic field, is a technologically important property
that has been the topic of intense study for more than a quarter century. Here
we report the observation of an unusual "butterfly" shaped titanic angular
magnetoresistance (AMR) in the non-magnetic, Dirac material, ZrSiS. The MR is
large and positive, reaching nearly 1.8 x 10^5 percent at 9 T and 2 K at an
angle of 45o between the applied current (along the a-axis) and the applied
field (90o is H parallel to the c-axis). Approaching 90o, a "dip" is seen in
the AMR which can be traced to an angle dependent deviation from the H^2 law.
By analyzing the SdH oscillations at different angles, we find that ZrSiS has a
combination of 2D and 3D Dirac pockets comprising its Fermi surface and that
the anomalous transport behavior coincides with a topological phase transition
whose robust signature is evident despite transport contributions from other
parts of the Fermi surface. We also find that as a function of angle, the
temperature dependent resistivity in high field displays a broad peak-like
behavior, unlike any known Dirac/Weyl material. The combination of very high
mobility carriers and multiple Fermi surfaces in ZrSiS allow for large bulk
property changes to occur as a function of angle between applied fields makes
it a promising platform to study the physics stemming from the coexistence of
2D and 3D Dirac electrons.
