Coexistence of topological Dirac fermions in the surface and three-dimensional Dirac cone state in the bulk of ZrTe${}_5$ single crystal

Although, the long-standing debate on the resistivity anomaly in ZrTe${}_5$ somewhat comes to an end, the exact topological nature of the electronic band structure remains elusive till today. Theoretical calculations predicted that bulk ZrTe${}_5$ to be either a weak or a strong three-dimensional (3D) topological insulator. However, the angle resolved photoemission spectroscopy and transport measurements clearly demonstrate 3D Dirac cone state with a small mass gap between the valence band and conduction band in the bulk. From the magnetization and magneto-transport measurements on ZrTe${}_5$ single crystal, we have detected both the signature of helical spin texture from topological surface state and chiral anomaly associated with the 3D Dirac cone state in the bulk. This implies that ZrTe${}_5$ is a novel 3D topological insulator having massless Dirac fermionic excitation in its bulk gap state. Whereas, no 3D topological insulator known in material science holds linear band dispersion in its insulating bulk. Apart from the band topology, it is also apparent from the resistivity and Hall measurements that the anomalous peak in the resistivity can be shifted to a much lower temperature ($T$$<$2 K) by controlling impurity and defects.