研究目的
To introduce van der Waals (vdW) heterostructures in the form of topological insulator/insulator/graphite to effectively control chemical potential of the topological surface states (TSS) and to utilize two types of gate dielectrics, normal insulator hexagonal boron nitride (hBN) and ferromagnetic insulator Cr2Ge2Te6 (CGT), to tune charge density of TSS in the quaternary TI BiSbTeSe2.
研究成果
The study demonstrates the effectiveness of van der Waals heterostructures in controlling the topological surface states of 3D topological insulators. The use of hBN/Gr and CGT/Gr gates allows for improved quantization of the quantum Hall effect and access to gapped TSS, respectively. The observation of half-quantized Hall conductance steps provides a clear manifestation of massive Dirac fermion physics on the magnetized TI surface, paving the way for advanced TI-based devices.
研究不足
The study is limited by the quality of the interfaces in the heterostructures and the ability to control the chemical potential near the Dirac point. The observation of half-quantized Hall conductance steps is not a true plateau due to the presence of substantial longitudinal conductivity, indicating the importance of edge transport or participation of localized 3D bulk states.
1：Experimental Design and Method Selection:
The study involves the fabrication of BSTS/hBN/Gr and BSTS/CGT/Gr devices using mechanical transfer techniques to achieve clean interfaces and effective gate tuning of the topological surface states (TSS).
2：Sample Selection and Data Sources:
Single crystals of BSTS and CGT were grown using vertical Bridgman and flux methods, respectively. Devices were fabricated by transferring exfoliated flakes onto pre-patterned electrodes.
3：List of Experimental Equipment and Materials:
Equipment includes a Leica optical microscopy for characterization, Bruker Dimension Icon atomic force microscopy for thickness measurement, JEOL JEM-2800 scanning transmission electron microscopy (STEM) for microstructure and elemental analyses, and a FEI Helios NanoLabTM 650 focused ion beam dual beam microscope for TEM cross-section specimen preparation.
4：Experimental Procedures and Operational Workflow:
The fabrication process involves electron beam lithography for electrode patterning, Cr/Au deposition using an e-beam evaporator, and transfer of BSTS, hBN, and Gr flakes using a home-made transfer stage. Electrical transport measurements were performed at cryogenic temperatures with a superconducting magnet up to 9 Tesla.
5：Data Analysis Methods:
The study includes analysis of longitudinal and Hall resistances, conductivity maps as functions of gate voltages, and renormalization group flow plots to analyze the quantization of Hall conductivity.
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