| Topological insulator（TI）has a nontrivial bulk energy band and gapless surface states protected by time reversion.The surface state is composed of massless Dirac fermion,and spin and momentum are locked together vertically due to the strong spin-orbit coupling.Due to the unique band structure of the surface states,TIs present many novel physical effects.Since the development for more than ten years,there are still some basic problems yet to be solved.First,the weak localization effect in ultra-thin TIs.According to the theory,the change of Berry’s phase could lead to the crossover from the weak antilocalization（WAL）to the weak localization（WL）in the quantum diffusion region.But it remains to be understood why the crossover to the WL is so difficult to realize experimentally.The second is the origin of the planar Hall effect（PHE）in TIs.The PHE is common in the magnetic systems,but it has also been observed in the pure TIs with no magnetism.The mechanism has been inconclusive.For these two issues,we have grown（Bi,Sb）2Te3（BST）films with different thicknesses by molecular beam epitaxy,fabricated back-gate Hall bar devices,and carried out systematic transport studies.The main results and conclusions are summarized as follows:1)We systematically studied the transport properties of BST thin films with well-controlled thickness,doping level,and chemical potential.We found that the sign of magnetoconductance in perpendicular magnetic field remains negative in all circumstances,suggesting absence of global WL in the ultrathin BST films.This is attributed to long-range disorder,as well as conductance corrections by quantum interference and electron-electron interaction.Our work shows that the electron transport in ultrathin TIs is distinctively different from the WAL regime in the systems with symplectic symmetry,and the WL or weak insulator regime associated with the orthogonal symmetry class.2)We observed the PHE withπperiod in BST films with different thicknesses,and determined its origin from the surface states based on the dependences of PHE on the gate voltage and temperature.Although the PHE in the gapped 3 quintuple layer（QL）ultrathin film shows the same form with that in the gapless 10 QL film,their dependences on the chemical potential are different.When the chemical potential is tuned across the Dirac point,the sign of PHE in the former is reversed from positive to negative while it remains positive in the latter.None of the theories apply to our experiments,and further studies are needed.In summary,we have demonstrated the absence of global WL could be attributed to the long-range electrostatic potential fluctuations and quantum corrections,and pointed out the unique characteristics of the transport in ultrathin TIs.Besides,we have observed that the PHE in ultrathin TI films shows completely different behavior on the dependence of chemical potential from that of the gapless thick films,which provides a new idea for theoretical study of PHE in TIs. |
