  Recently, higher-order topological matter and 3D quantum Hall effects have
attracted great attention. The Fermi-arc mechanism of the 3D quantum Hall
effect proposed in Weyl semimetals is characterized by the one-sided hinge
states, which do not exist in all the previous quantum Hall systems and more
importantly pose a realistic example of the higher-order topological matter.
The experimental effort so far is in the Dirac semimetal Cd$_3$As$_2$, where
however time-reversal symmetry leads to hinge states on both sides of the
top/bottom surfaces, instead of the aspired one-sided hinge states. We propose
that under a tilted magnetic field, the hinge states in Cd$_3$As$_2$-like Dirac
semimetals can be one-sided, highly tunable by field direction and Fermi
energy, and robust against weak disorder. Furthermore, we propose a scanning
tunneling Hall measurement to detect the one-sided hinge states. Our results
will be insightful for exploring not only the quantum Hall effects beyond two
dimensions, but also other higher-order topological insulators in the future.
