Shubnikov-de Haas oscillations in bulk ZrTe5 single crystals: Evidence for a weak topological insulator

The study of ZrTe5 crystals is revived because of the recent theoretical prediction of topological phase in bulk ZrTe5. However, the current conclusions for the topological character of bulk ZrTe5 are quite contradictory. To resolve this puzzle, we here identify the Berry phase on both b- and c planes of high-quality ZrTe5 crystals by the Shubnikov-de-Hass (SdH) oscillation under tilted magnetic field at 2 K. The angle-dependent SdH oscillation frequency, both on b- and c planes of ZrTe5, demonstrates the two-dimensional feature. However, phase analysis of SdH verifies that a nontrivial pi-Berry phase is observed in the c-plane SdH oscillation, but not in the b-plane one. Compared to bulk Fermi surface predicted by the first-principle calculation, the two-dimensional-like behavior of SdH oscillation measured at b plane comes from the bulk electron. Based on these analyses, it is suggested that bulk ZrTe5 at low temperature (similar to 2 K) belongs to a weak topological insulator, rather than Dirac semimetal or strong topological insulator as reported previously.