posted on 2016-02-20, 02:13authored byMurong Lang, Liang He, Xufeng Kou, Pramey Upadhyaya, Yabin Fan, Hao Chu, Ying Jiang, Jens H. Bardarson, Wanjun Jiang, Eun Sang Choi, Yong Wang, Nai-Chang Yeh, Joel Moore, Kang L. Wang
We demonstrate evidence of a surface gap opening in topological
insulator (TI) thin films of (Bi0.57Sb0.43)2Te3 below six quintuple layers through transport
and scanning tunneling spectroscopy measurements. By effective tuning
the Fermi level via gate-voltage control, we unveil a striking competition
between weak localization and weak antilocalization at low magnetic
fields in nonmagnetic ultrathin films, possibly owing to the change
of the net Berry phase. Furthermore, when the Fermi level is swept
into the surface gap of ultrathin samples, the overall unitary behaviors
are revealed at higher magnetic fields, which are in contrast to the
pure WAL signals obtained in thicker films. Our findings show an exotic
phenomenon characterizing the gapped TI surface states and point to
the future realization of quantum spin Hall effect and dissipationless
TI-based applications.