Thickness driven topological quantum phase transition and experimental evidence of Dirac semimetal state in Cr-doped Bi2Se3

In this report, we demonstrate that the energy gap in Cr-doped Bi2Se3 closes and reopens in oscillatory fashion with increasing sample thickness indicating the topological quantum phase transition driven by the quantum finite size effect. The pattern of the oscillation provides an evidence that Cr-doped Bi2Se3 can have a Dirac semimetal state depending on the sample thickness and Cr-content. This resolves the puzzle of non-magnetic gap opening at the Dirac point of topological insulators. An energy gap can be opened due to the quantum finite size confinement of the bands in Dirac semimetal state. In addition, our room temperature ARPES measurements revealed a topological quantum phase transition between x = 0.06 and x = 0.08 in 7 quintuple layers Bi2-xCrxSe3 without ferromagnetism.