Preparation And Electron Transport Properties Of Topological Insulator/Antiferromagnetic Insulator Heterostructures

Topological insulators（TIs）possess a lot of exotic quantum properties due to their unique electron structures and thus have become a forefront of condensed matter physics research in the past decade.Magnetic proximity effect,which can be realized by constructing topological insulator and magnetic insulator（MI）heterostructures,was proposed as a promising mechanism to induce long-range magnetic order on the surface of topological insulator without introducing additional defects or impurities.Based on this approach,the TI surface state can in principle be gapped without spatial uniformity.This may allow for observation of novel topological quantum effects（e.g.,the quantum anomalous Hall effect（QAHE）,and topological magnetoelectric effect）at high temperature.However,the experimental investigation of TI/MI heterostructure has been hindered by the scarcity of the ferromagnetic insulators with out-of-plane magnetization and complex interface interactions.Recent studies have shown that the antiferromagnetic insulator Mn Se with ferromagnetic basal plane can also open a large Dirac gap in the surface states of the TIs of Bi2Se₃-family due to the good compatibility in their crystalline structures and the growth conditions.This alternative approach seems to be promising to obtain strong magnetic proximity effect on the TI surface states.Therefore,the work on Mn Se/TI heterostructures might eventually provide a new avenue to realization of the QAHE and manipulation of the Majorana zero modes.In this dissertation,we have prepared films of antiferromagnetic insulator（Mn Se）and topological insulator（（Bi,Sb）2Te₃,BST）heterostructure by molecular beam epitaxy（MBE）,and investigated their electron transport properties systematically.The main results are summarized as follows:1.Mn Se/BST heterostructures with atomically sharp interface were grown on Sr Ti O₃（111）substrates.We have observed pronounced anomalous Hall effect in Mn Se/BST heterostructures.The detailed results are as follows:a.The anomalous Hall effect curves show significant hysteresis characteristic at low temperature.The anomalous Hall conductivity at high magnetic field increases monotonically with increasing gate voltage,and its maximum value can reach 0.12 e2/h.b.We observed the weak anti-localization effect of single channel from the bottom surface（not in contact with Mn Se）of BST layer.As the gate voltage increases,the top and bottom surfaces of BST gradually decouple,and the weak anti-localization effect becomes more significant.c.Based on analysis of the magnetoresistance and the anomalous Hall effect,we found that charge transfer occurs at the Mn Se/BST interface,resulting in a holedoping in the surface state of the top surface.Based on this,we proposed a band evolution picture of BST with increasing gate voltage in Mn Se/BST heterostructures.d.Linear magnetoresistance was observed in high magnetic field region.It becomes more pronounced when the bulk carriers are involved in the transport.Therefore,the linear magnetoresistance cannot be attributed to Abrikosov’s linear dispersion model.2.We have systematically studied the in-plane anisotropic magnetoresistance（AMR）and planar Hall effect（PHE）in Mn Se/BST heterostructures,and discussed their possible origin based on the experimental data.The detailed results are as follows: a.In the Mn Se/BST heterostructures,AMR persists up to 250 K and exhibits an obvious hysteresis at temperatures below 10 K.The coercive field is independent on the gate voltage.The requirement of interface quality for the observation of AMR in Mn Se/BST heterostructures is not as strict as the anomalous Hall effect.b.The temperature dependence of the PHE amplitude in Mn Se/BST is nonmonotonic,in contrast to the monotonic decrease with increasing temperature in the pure BST thin film.Therefore,the AMR/PHE in the Mn Se/BST heterostructures can probably be attributed to the interfacial magnetic proximity effect.c.Within the temperature range from 10 K to 50 K,the angular dependence of AMR shows a beat feature,which appears to be a superposition of the oscillations with π and 2π periods.In summary,we have experimentally demonstrated strong magnetic proximity effect in the Mn Se/BST heterostructures.Our work deepened the understanding of the interfacial coupling between topological insulators and antiferromagnetic insulators,and paved a way for further exploration of spin-related quantum properties of topological insulators as well as their potential spintronic applications.