Preparation And Electron Transport Properties Of Low-Dimensional Magnetic Topological Insulator

Because of their unique electronic structures and physical properties,threedimensional topological insulators(referred to as topological insulators,or TIs below)have been a forefront of the condensed matter physics research since their discovery a decade ago.When magnetic order is introduced into TIs,the time-reversal symmetry(TRS)is broken,and the TI surface states are driven into a massive Dirac fermion system.Many novel quantum phenomena resulting from the broken TRS have been proposed.Some of them have shown great potential in device applications,for instance,the dissipationless edge states of quantum anomalous Hall effect(QAHE)may serve a basis for low power-consumption electronics,chiral Majorana zero modes may be building blocks for fault-tolerant topological quantum computation.A lot of attention has therefore been paid to the study of magnetic topological insulators.Despite that a series of exciting discoveries have been made recently,the research on magnetic TIs are still far from being satisfactory.For instance,it remains very challenging to realize a strong,weak disordred,long-range magnetic order in TI-based low dimensional structures.There are still many puzzling and controversial issues regarding the nature of magnetism in magnetic TIs at the microscopic level.In light of the above questions,two types of magnetic TIs are studied systematically in this work.One is the Mn-doped Bi2Se3.Although some groups have obtained strong evidences for ferromagnetic order in both the bulk and the surface of Mn-doped Bi2Se3 thin films,the anomalous Hall effect(AHE)has never been reported in this system.Moreover,a recent study has suggested a non-magnetic orgin for the surface energy gap observed in the Mn-doped Bi2Se3,in contradiction to earlier conclusions of the hedgehog-like spin structure due to magnetic interactions.The other system we have studied is heterostructures of antiferromagnetic insulator Mn Se and TIs.Using the interfacial magnetic interaction of antiferromagnets to open a gap in the TI surface states can avoid some complications in their ferromagnetic counterparts.Moreover,Mn Se can be epitaxially grown at low temperatures,paving a way to fabricate Mn Se/TI/Mn Se sandwich structures that might be promising for realizing QAHE.In this dissertation,we have used a home-made molecular beam epitaxy(MBE)system to prepare low-dimensional structures of the magnetic TIs and studied their electron transport properties systematically.The main experimental results are summarized into the following two aspects:1.A series of(Bi1-x Mnx)2Se3 thin films have been grown by MBE method with different Mn concentrations(x = 0-0.088).The AHE was observed in(Bi1-x Mnx)2Se3 thin films for the first time and the main results are described as follows.a.By changing the Mn doping level and gate voltage,we find that the AH resistance has two components with oppoisite signs.These two components can coexist in a wide range of experimental parameters,and they exhibit qualitatively different dependences on the applied magnetic field and gate voltage.The positive and negative AH components originate from the bulk and surface states,respectively.And such a two-component AH effect has never been observed in any magnetically doped TIs or other magnetic materials.b.The surface-related AH component has a non-monotonic dependence on the chemical potential,inconsistent with the predictions based on the massive Dirac fermion model with the mean field treatment of the magnetic doping effect.The suppression of the AH effect at low chemical potential is believed to be associated with the non-magnetic potential scatterings of the magnetic impurities in the Mn-doped Bi2Se3.c.By electrically gating heavily doped(Bi1-x Mnx)2Se3 samples,the magnetoconductance can be tuned from negative to positive.The former is related to the weak antilocalization,whereas the latter may result from a joint effect of strong disorder,non-magnetic scattering,and the ferromagnetic order.2.We have grown a series of different kinds of Mn Se/Bi2Se3 and Mn Se/(Bi,Sb)2Te3 heterostructures with MBE.By varying temperature,back-gate voltage and magnetic field,we have studied the transport properties of Mn Se/TI heterostructures systematically.Signatures for strong interfacial magnetic interactions have been oberserved in Mn Se/(Bi,Sb)2Te3 heterostructures,such as negative magnetoresistance surviving at temperatures as high as 25 K in parallel magnetic fields,anomalous Hall resistances up to tens of ohms in perpendicular magnetic fields.