CAREER:Designing Hund's Metals from Transition Metal Sulfides

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Non-Technical SummaryWith support from the Solid State and Materials Chemistry program, the Principal Investigator's NSF CAREER plan focuses on the development of a new class of materials with functional magnetic and electronic properties. These crystalline solids are termed Hund's metals and their unique electrical behavior arises from the combination of abundant metals such as iron and copper with sulfur. Furthermore, the manner in which these elements come together to form crystals is highly important in tailoring their materials properties. To prepare these new compounds, the PI develops new chemical methods available to the broader, national community of materials scientists. The project provides training for both graduate and undergraduate students in the synthesis of new solids and in characterizing their atomic structure and physical properties with advanced instrumentation. Part of the students' training also includes participation in outreach activities where they engage high school students in the immediate vicinity of the University of Maryland. The PI and his group of university students mentor the high school chemistry students, whom are predominately Hispanic, African American and underrepresented in the sciences, through activities that include chemistry tutoring and guided tours of nearby national laboratories. Technical SummaryThe research group is engaged in hypothesis-driven synthesis of new sulfides defined as Hund's metals. These materials are composed of late transition metal sulfides and possess the electronic properties characteristic of a metal while expressing localized magnetic behavior through interactions dictated by Hund's rule for maximizing spin. The uniqueness of these Hund's metals arises from the d-orbital behavior of the d-block elements including iron, cobalt, nickel, and copper. While one subset of the five d-orbitals form electronic bands that split across the Fermi level, as in an insulator, the others form itinerant bands. The research projects specifically target sulfides that contain certain crystal structure motifs including either octanuclear clusters or two-dimensional sheets of the d-block elements. In both cases, the transition metal is always tetrahedrally coordinated to sulfide anions in order to minimize the crystal field splitting and therefore favor a high spin configuration, which leads to the desired magnetic behavior of the Hund's metals. The research goals include experimentally confirming the predicted phases through solid state synthesis and crystal growth followed by measurements of the new phases' magnetization, electrical transport, and specific heat properties. Neutron diffraction and inelastic neutron scattering are utilized to measure both the structure and dynamics in these sulfides. For the two-dimensional layered sulfides, soft chemical methods such as intercalation of lithium in liquid ammonia are pursued in order to form metastable phases, which are prone to display extraordinary electronic behavior such as superconductivity.