Spectroscopic evidence for the gapless electronic structure in bulk ZrTe5

Recently, transition metal pentatellurides MTe5 (M = Zr, Hf) have inspired intensive research effort. Being predicted to be quantum spin Hall insulators (QSHI) with the bulk gap up to hundreds of meVs, it could lead to promising applications at unprecedented high temperature compared with previously discovered QSHI (e.g. HgTe/CdTe or InAs/GaSb quantum wells). However, the experimental works soon followed illustrated considerable discrepancies regarding to whether MTe5 compounds possess a full bulk gap, making their topological nature (topological insulators or Dirac semimetals) illusive. In this work, combining investigations of angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy (STM), we systematically studied the electronic properties of ZrTe5. In intrinsic samples, we observed little evidence for the existence of topological surface states or large bulk gap. With bulk and surface doping to adjust the position of the Fermi-level, ARPES spectra indicate gapless and highly linear dispersions at the valance band top, in consistence with the STM measurements that show a V-shaped total density of states near the Fermi-level (i.e. suggesting a gapless nature of the electronic structure of ZrTe5). Moreover, near the terrace edge on the surface, we observed non-zero DOS, indicating the existence of edge states.