SEARCH FOR NATURALLY OCCURRING SUPERCONDUCTIVITY AND NOVEL PHENOMENA: MAGNETIC TRANSITIONS IN NATURAL TRANSITION METAL COMPOUNDS
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Transition metal chalcogenides and transition metal arsenides are important families of natural mineral compounds widely distributed in the natural world. With similar structural and electronic properties of transition metal oxides, natural transition metal compounds are expected to have similar novel phenomena. With an ongoing project for searching natural superconductors in collaboration with Department of Mineral Science, Smithsonian National Museum of Natural History, we had a chance to investigate several natural minerals from the Smithsonian Museum in order to study previously unexpected naturally occurring mineral compounds for interesting ground states. We found several interesting magnetic transitions in these natural occurring mineral samples. Some of the magnetic transitions are not reported, some of the transitions are associated with other unreported novel quantum phenomena. In this thesis, I will discuss Bornite (Cu5FeS4), Berthierite (FeSb2S4), Nagyagite (Pb5Au(Te,Sb)4S58), Maucherite (Ni11As8) and related experiments in detail. Bornite (Cu5FeS4) has a semiconductor-insulator transition accompanied with an antiferromagnetic transition. As shown by our ability to tune the transition temperature and low-temperature metallicity by applying external pressure, Bornite may be a good candidate for Mott system and searching new superconductors. Berthierite (FeSb2S4) is a quasi-1-dimensional antiferromagnet. With strong anisotropic physical properties, berthierite may provide a very good system for understanding the low dimensional magnetic material. A Ferromagnetic order was found in natural Nagyagite (Pb5Au(Te,Sb)4S58) samples. The magnetic order, the weak anti-localization property with strong spin-orbital coupling and the 2-dimensional structure of this compound makes it a very interesting system for realizing topological properties in a natural compound. The magnetic order and transitions in both natural and synthetic Maucherite (Ni11As8) samples show interesting finite-size scale effect. It gives us a different approach to understand the differences in some physical properties between natural and synthetic compounds. Also, we will present a summary of other magnetic transitions and magnetic properties of more than 40 distinct minerals for this study and show the relation and similarities between strongly correlated transition metal oxide materials and other quantum materials. We will also make a list of other transition metal minerals that are worthy of investigation based on our research experience.