Magnetic Coupling and Superconductivity in Topological Crystalline Insulator Heterostructures

The discovery of Z₂ topological insulators (TI), which have an insulating bulk but possess metallic surface states, has been an important fundamental advancement in physics and has impacted spintronic and quantum device applications. A crystalline symmetry-protected topological phase has been identified in the IV-VI Pb_1-xSn_xSe and Pb_1-xSn_xTe materials class, known as topological crystalline insulators (TCIs). Crystalline symmetry and valley degeneracy in TCIs were proposed to yield novel quantum phenomena, which are yet unexplored. This thesis encompassed a diverse array of compound materials and heterostructures within the realm of IV-VI materials, focusing on TCIs.

The first part of the thesis is focused on Pb_1-xSn_xSe quantum well with non-magnetic barrier Pb_1-yEu_ySe (y=0.05-0.15).High-quality quantum wells with mobility reaching 11300cm²/Vs are grown by MBE. The weak anti-localization beyond the diffusion regime is explained by a new constructed model. Magneto-transport and magneto-optical measurement are used to explain the aperiodic quantum oscillations. g-factor of the topological interface state is extracted.

Following the work above, the second part of the thesis explores the magnetic proximity effect by changing the barrier of the quantum well to a magnetic insulator EuSe. Through infrared magneto-optical measurement, Landau level fan-chart is obtained to extract the surface state gap. By comparing the experimental gap with the theoretical calculation, the upper bound of the Eu-based magnetic-proximity induced gap is determined to be 10meV. The value is too small for the quantum anomalous Hall effect, but can be useful for spintronics.

The last part of the thesis systematically studies the Sn_1-xIn_xTe (x=0-0.3) thin film as a candidate topological superconductor (TSC). 100nm Films are grown by MBE and characterized by transport and spectroscopic measurements. The coexistence of weak anti-localization and superconducting fluctuations is for the first time observed in TSC class. Indium is shown to form an impurity band below the valence band edge, which may be critical for the formation of superconductivity. Ultra-thin films (<10nm) are also explored, which have shown a possible breaking of Pauli limit.

This thesis has developed systematic experimental techniques that can be used to probe electronic properties of topological materials, and built a solid basis for band engineering of TCI materials. The quantum well structure can be used to study various topological phases by tuning well parameters or changing the barrier type. Sn_1-xIn_xTe is also a perfect candidate for heterostructure growth with other TCIs due to the lattice match, which can be used for the study of superconducting proximity effect. Lastly, the knowledge obtained for IV-VI TCIs can be essential for future real device applications.