Superconductivity, Magnetism and Physical Properties of New Types of Low-dimensional Materials

This dissertation focuses on the physical property studies of a broad range of new low dimensional compounds discovered in our research lab. These new materials structurally have two-dimensional (2D) or quasi-2D features with Van der Waals interaction between layers, and display various interesting physical properties such as superconductivity, magnetic order, anomalous Hall effect, transport anomalies, and linear temperature dependent resistivity behavior. Chapter 1 & 2 we introduce physics background and various experimental techniques involved in this dissertation, including the different synthetic methods, X-ray diffraction, magnetic, electrical transport studies. Chapter 3 we develop a unique soft chemical intercalation route where both alkali metals (Li or Na) and polar organic solvents could be cointercalated into the layered materials and significantly change their electronic properties. Through this new method and using transition metal dichalcogenide SnSe2 as prototype, we discovered a series of new superconductors with Tc up to ~8 K. These Tcs are significantly higher than those with only alkali metal doping or electrostatic gating, suggesting these polar organic species are playing an important role for the enhanced superconductivity in this system. Chapter 4 we report several new layered compounds in the Nb-Fe-Te system. Two distinct α- NbFeTe2 and β-NbFeTe2 are synthesized through different temperature route, with completely different electrical and magnetic properties. In addition, some new phases are discovered with different structures under different stoichiometry and growth conditions. Their structures and physical properties are discussed in the dissertation. Chapter 5 we present three new phases in the Ba-Cu-Pn (Pn = P, As) system. Through carefully controlled growth conditions, we discover a new polymorphic β-BaCu2As2 phase as intergrowth prototype of ThCr2Si2-type and CaBe2Ge2-type motifs, and two BaCu6Sn2Pn4-x (Pn = P, As) phase with layered feature. The β-BaCu2As2 phase becomes superconducting under pressure with a maximum Tc up to 7.2 K at 27.6 GPa, representing as the first Cu-based pnictide superconductor under high pressure. Chapter 6 we show structural diversity in the Ba-Ag-Pn (Pn = As, Sb, Bi) system where more than six new phases are discovered with different stacking orders of ThCr2Si2 and CaBe2Ge2-type structure motif. Most of these new phases display linear temperature dependent resistivity across a large temperature range, and their exact physics origin are still under investigation. We have also extended this study to the A-Ag-Sb (A = Alkali metal) system and find a few new compounds as well. Chapter 7 we report two new Pt-based chalcogenide compounds, one non-stoichiometric Zr6.5Pt6Se19 and another layered BaPt4Se6. The Zr6.5Pt6Se19 is a new structure type (oC68) with quasi-1D and quasi-2D structure feature, which is the first Pt-based ternary chalcogenide with group 4 elements. The BaPt4Se6 adopts sesqui-selenide Pt2Se3 layer with mix-valence Pt oxidation states, and has two distinct transport anomalies: a resistivity crossover, mimic to the metal- insulator (M-I) transition at ~150 K, and a resistivity plateau at temperatures below 10 K. Chapter 8 we report some several miscellaneous projects where crystal growth and bulk synthesis of several materials are discussed while the full characterization of the materials are to be investigated due to the time and instrument breakdown limitation at the end of this dissertation work.