  The interplay of electron correlations and topological phases gives rise to
various exotic phenomena including fractionalization, excitonic instability,
and axionic excitation. Recently-discovered transition-metal pentatellurides
can reach the ultra-quantum limit in low magnetic fields and serve as good
candidates for achieving such a combination. Here, we report evidences of
density wave and metal-insulator transition in HfTe5 induced by intense
magnetic fields. Using the nonlinear transport technique, we detect a distinct
nonlinear conduction behavior in the longitudinal resistivity within the a-c
plane, corresponding to the formation of a density wave induced by magnetic
fields. In high fields, the onset of the nonlinear conduction in the Hall
resistivity indicates an impurity-pinned magnetic freeze-out as the possible
origin of the insulating behavior. These frozen electrons can be gradually
re-activated into mobile states above a threshold electric field. These
experimental evidences call for further investigations into the underlying
mechanism for the bulk quantum Hall effect and field-induced phase transtions
in pentatellurides.
