Anomalous Magneto-Transport Behavior in Transition Metal Pentatelluride HfTe_5

There is a long-standing confusion concerning the physical origin of the anomalous resistivity peak in transition metal pentatelluride HfTe_5. Several mechanisms, such as the formation of charge density wave or polaron, have been proposed, but so far no conclusive evidence has been presented. In this work, we investigate the unusual temperature dependence of magneto-transport properties in HfTe_5. It is found that a three-dimensional topological Dirac semimetal state emerges only at around T_\rm p (at which the resistivity shows a pronounced peak), as manifested by a large negative magnetoresistance. This accidental Dirac semimetal state mediates the topological quantum phase transition between the two distinct weak and strong topological insulator phases in HfTe_5. Our work not only provides the first evidence of a temperature-induced critical topological phase transition in HfTe_5 but also gives a reasonable explanation on the long-lasting question.