Study Of Electron Transport Properties Of Three-dimentional Topological Insulators

Three-dimensional?3D?topological insulators are a new type of quantum materials with insulating bulk and conducting surface states that comprise two-dimensional massless Dirac fermions.Due to the protection of time-reversal symmetry,the surface states are not easily destroyed by non-magnetic impurities.In this dissertation,we present a study of the following three subjects that are related to 3D topological insulators:1)preparation and measurement of basic transport properties of ultra-thin topological insulator(Bi_xSb_1-x)₂Te₃ nanoflakes;2)fabrication and characterization of nanoscale topological insulator Hall magnetometers;3)quantum transport properties of dual-gated?Bi,Sb?₂?Te,Se?₃ microflakes.In addition,we also studied quantum corrections to the anomalous Hall effect in Mn Sb₂Te₄,a layered compound which is a candidate for ferrimagnetic Weyl semimetal.The main experimental results are summarized as follows:1. Micrometer-sized topological insulator thin flakes were grown by a home-made chemical vapor deposition?CVD?system,with thicknesses down to about 3 nm.In a 4nm thick(Bi_xSb_1-x)₂Te₃ nanoflake,we observed positive magnetoresistance in low magnetic fields and negative magnetoresistance in high magnetic fields.Taking other measurement results into account,such as the Hall effect,we propose that the high field negative magnetoresistance originates from a semiclassical effect associated with large Hall conductance.2.?Bi,Sb?₂?Te,Se?₃ Hall effect detectors with channel widths as low as=70nm were fabricated.The Hall devices exhibit nearly linear Hall response in a wide range of temperatures and magnetic fields.Based on noise spectroscopy measurements,magnetic moment resolution of the 70 nm device can in principle reach the order of10⁵?_B/Hz^1/2 at 1 Hz at 4.2 K.In comparison with a micrometer-sized Hall device,miniaturization down to sub-100 nm scale does not significantly enhance the noise level(proportional to_RH²,the product of Hall resistance power spectral density and effective area of the Hall cross).This suggests that Hall magnetometers based on topological insulators have room for further improvement,and it is possible to achieve a magnetic moment resolution superior to that of semiconductor Hall detectors.3. Dual-gated devices based on topological insulator?Bi,Sb?₂?Te,Se?₃were fabricated by using a homemade dry transfer system with high precision.Quantum transport properties of the surface states have been measured systematically by tuning chemical potential in both the top and bottom surface states with gate voltages.In the diffusive transport regime with high chemical potential,the observed negative magnetoconductance manifests typical weak antilocalization effect in the surface states,and the electron dephasing rate has a linear dependence on the temperature,consistent with the dephasing properties of conventional two-dimensional electron systems.When the chemical potential is tuned close to the Dirac point,negative magnetoconductance persists.However,the magenetoconductance can no longer be described by the Hikami-Larkin-Nagaoka equation.Moreover,the zero-field conductance exhibits a metallic behavior near the Dirac point.This may be viewed as evidence for the predicted topological protection against localization in presence of electron-electron interaction.4. Quantum corrections to the anomalous Hall effect in Mn Sb₂Te₄,a layeredferrimagnetic compound,has been studied in detail by making use of its nearly square-shaped hysteresis loop and low magnetization saturation field.Both the anomalous Hall resistance and the longitudinal resistance exhibit lntypes of temperature dependence below 4 K,whereas the low temperature correction of the anomalous Hall conductance is negligible.Because the weak localization effect in Mn Sb₂Te₄ can be excluded,the low temperature corrections to the longitudinal and the anomalous Hall resistances can be attributed to the electron-electron interaction effects.These experimental results are in go od agreement with the theory of W?lfle and coworkers.