Materials Research Lecture

 A grand challenge in materials research revolves around controlling structure-property relationships to achieve desired properties, so-called "materials by design." The discovery and design of new superconducting materials has remained particularly enigmatic: not only do we lack universal structure-property relationships, but we also lack the ability to synthesize metastable materials predicted to have desirable properties.  In the iron-based and related superconductors, magnetic interactions are thought to underlie or compete with the emergence of superconductivity, and subtleties of the structure balance these interactions.  While in structurally homologous nickel-based materials (KNi₂S₂ and KNi₂Se₂) we anticipated (and observed) superconductivity, we also discovered unusual charge fluctuations and their hybridization.  These interactions produce a many-body, coherent, heavy-electronic state, thus prolonging the enigmatic relationship between structure and properties. 

The example of KNi₂Se₂ shows that exploring predicted structure/property relationships in superconductivity is surprising and rewarding.  But a great challenge lies in exploring metastable compounds predicted to be higher-T_c superconductors: how can these structural phases be accessed?  We show how kinetic control of the synthetic reaction pathway can produce metastable, high-pressure polymorphs of superconducting metal chalcogenides, but without the use of pressure; this commences a new paradigm of materials by design.