  Topological materials with broken inversion symmetry can give rise to
nonreciprocal responses, such as the current rectification controlled by
magnetic fields via magnetochiral anisotropy. Bulk nonreciprocal responses
usually stem from relativistic corrections and are always very small. Here we
report our discovery that ZrTe5 crystals in proximity to a topological quantum
phase transition present gigantic magnetochiral anisotropy, which is the
largest ever observed to date. We argue that a very low carrier density,
inhomogeneities, and a torus-shaped Fermi surface induced by breaking of
inversion symmetry in a Dirac material are central to explain this
extraordinary property.
