Superconductivity, Magnetism and Topological Properties of Several Quasi-1D Materials




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This dissertation focuses on the investigation of several material systems with distinct quasi-onedimensional (quasi-1D) or pseudo-quasi-1D structural features and hosting rather interesting magnetic, superconducting, and topological properties. High quality of powders or single crystals have been synthesized through various methods and carefully characterized through X-ray diffraction, magnetic, electrical transport and thermal transport studies at low temperature and under high magnetic fields. Their unique magnetic, superconducting, and possible topological properties are presented with their future implications further discussed. Firstly, we investigated the magnetic property and superconductivity in two iron-based chalcogenides with quasi-1D structure: BaFe2Se4 and K3Fe2Se4. Both compounds consist of 1D chains of iron chalcogenide with FeSe4 tetrahedra, similar to the fundamental building block FeAs4 in the iron-based superconductors. However, these FeSe4 tetrahedra is edge-shared along one specific crystallographic axis and forms 1D chain structure, rather than the two-dimensional layered Fe2As2 structure seen in the common iron-based superconductors. X-ray powder pure phases of both compounds are synthesized and found to be semiconducting and magnetic. Intuitively doping studies based on the nominal iron valence charges are also carried out, however, no superconductivity is detected yet at this stage in the doped samples. Secondly, we carried out systematical doping studies aiming to induce superconductivity in the Zr5Ge3 system with Mn5Si3-type structure and pseudo-quasi-1D Zr3Ge3 chain along the c axis. Different transition metal doping at different crystallographic sites in this Zr5Ge3 system have been thoroughly investigated. Interestingly, superconductivity was successfully induced through Ru and Pt doping, but only at the Ge site. Superconductivity is found to be absent with same amount of carrier doping at the Zr site or interstitial site. The highest superconducting Tc is found to be at 5.7 K for Ru doping, and 2.8 K for Pt doping. Detailed transport and magnetic studies have suggested bulk superconductivity, high upper critical field, enhanced electron correlation, and extremely small electron-phonon coupling, indicating possible unconventional superconductivity in this system. In addition, we also investigated the superconductivity properties of the A2Mo3As3 (A = K, Rb), which has a more 1D-like Mo3As3 chain structure but structurally very close to the Zr5Ge3 compound. The superconducting Tc is ~ 10.6 K, and the electrical transport studies suggested a much higher upper critical field ~ 27 T, exceeding the Pauli limit based on conventional criteria. Thirdly, we present a study of synthesis and crystal growth of α, β-Bi4X4 (X = I, Br) materials with different stacking of 1D BiX chains and various interesting topological properties. Different types of synthetic methods have been tested for the high quality crystal growth, and crystals as large as 8 mm size of Bi4X4 was successfully synthesized. A hypothetical growth mechanism was proposed for Bi4Br4. The transport measurements have revealed a topological phase transition from β-Bi4I4 as a strong topological insulator to α-Bi4I4 with high ordered hinge states around room temperature at 300 K. The exact origin of several transport anomaly in the α-Bi4I4 and α’-Bi4Br4 remain elusive at this stage. Single crystals of mixed Bi4I4-xBrx (0.1 ≤ x ≤ 3) were also synthesized and a rather interesting structural transformations upon Br doping have been observed, and further studies are needed to fully understand the topological phase transitions in this system. Last but not least, chemical intercalation studies were also carried out, and preliminary data suggests superconductivity is successfully induced in these materials. The last part of this dissertation focus on a two-dimensional material Black phosphorus (Black-P). We carried out thorough experimental studies to investigate the growth mechanism for Black-P aiming to facilitate its future layer-by-layer thin film growth. A new synthetic strategy to grow large size of Black-P crystals through a ternary clathrate Sn24P22-xI8 under lower temperature and pressure was reported. The chemical vapor transport mechanism was found not play a critical role for the growth of Black-P, but rather the vapor-solid-solid (VSS)-like growth mechanism is found to be crucial for the quality growth. The ternary clathrate Sn24P22-xI8 acts as the solid catalyst and the P vacancies in Sn24P22-xI8 was found plays an important role in this mechanism.



Superconductivity, Magnetism, Topological insulators, Dimension theory (Topology)