  The planar Hall effect in 3D systems is an effective probe for their Berry
curvature, topology, and electronic properties. However, the Berry
curvature-induced conventional planar Hall effect is forbidden in 2D systems as
the out-of-plane Berry curvature cannot couple to the band velocity of the
electrons moving in the 2D plane. Here, we demonstrate a unique 2D planar Hall
effect (2DPHE) originating from the hidden planar components of the Berry
curvature and orbital magnetic moment in quasi-2D materials. We identify all
planar band geometric contributions to 2DPHE and classify their crystalline
symmetry restrictions. Using gated bilayer graphene as an example, we show that
in addition to capturing the hidden band geometric effects, 2DPHE is also
sensitive to the Lifshitz transitions. Our work motivates further exploration
of hidden planar band geometry-induced 2DPHE and related transport phenomena
for innovative applications.
