Highly tunable berry phase and ambipolar field effect in topological crystalline insulator Pb_1-xSn_xSe

Recently, rock-salt IV-VI semiconductors, such as Pb_1-xSn_xSe(Te) and SnTe, have been observed to host topological crystalline insulator (TCI) states.1-7 The nontrivial states have long been believed to exhibit ambipolar field effects and possess massive Dirac Fermions in two-dimension (2D) limit due to the surface hybridization.8,9 However, these exciting attributes of TCI remain previously inaccessible owing to the complicated control over composition and thickness.10,11 Here, we systematically investigate doping and thickness-induced topological phase transitions by electrical transport. We demonstrate the first evidence of the ambipolar properties in Pb_1-xSn_xSe thin films. Surface gap opening is observed in 10 nm TCI originated from the strong finite-size effect. Importantly, magnetoconductance hosts a competition between weak antilocalization and weak localization, suggesting a strikingly tunable Berry phase evolution and strong electron-electron interaction. Our findings serve as a new probe to study electron behavior and pave the way for further exploring and manipulating this novel 2D TCI phase.