CAREER:Designing Hund's Metals from Transition Metal Sulfides

在固态和材料化学项目的支持下，首席研究员的NSF职业计划侧重于开发一类具有功能磁性和电子特性的新型材料。这些结晶固体被称为洪德金属，它们独特的电学行为源于铁、铜等丰富的金属与硫的结合。此外，这些元素聚集在一起形成晶体的方式对调整它们的材料特性非常重要。为了制备这些新化合物，PI开发了新的化学方法，可供更广泛的全国材料科学家使用。该项目为研究生和本科生提供新固体合成方面的培训，并利用先进仪器表征其原子结构和物理性质。学生的部分培训还包括参加外联活动，在这些活动中，他们与马里兰大学附近的高中生进行接触。PI和他的大学生团队通过化学辅导和参观附近的国家实验室等活动指导高中化学学生，这些学生主要是西班牙裔和非裔美国人，在科学领域的代表性不足。该研究小组正在进行假设驱动的新硫化物合成，定义为洪德金属。这些材料由晚期过渡金属硫化物组成，具有金属的电子特性，同时通过洪德最大化自旋规则规定的相互作用表达局部磁性行为。这些亨德金属的独特性源于d区元素的d轨道行为，包括铁、钴、镍和铜。当五个d轨道中的一个子集形成在费米能级上分裂的电子带时，就像在绝缘体中一样，其他的形成流动带。该研究项目专门针对含有某些晶体结构基序的硫化物，包括八核簇或d-块元素的二维薄片。在这两种情况下，过渡金属总是与硫化物阴离子呈四面体配位，以尽量减少晶体场分裂，因此有利于高自旋构型，从而导致所需的洪德金属的磁性行为。研究目标包括通过固体合成和晶体生长实验证实预测的相，然后测量新相的磁化、电输运和比热性能。利用中子衍射和非弹性中子散射测量了这些硫化物的结构和动力学。对于二维层状硫化物，采用软化学方法，如在液氨中插入锂，以形成亚稳相，亚稳相容易表现出非凡的电子行为，如超导性。

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.