Unconventional metals in carrier-tuned spin-orbit Mott materials

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Nontechnical Abstract: This project explores the fascinating and complex electronic behaviors endemic to a class of metals born from insulators with both strong interactions between electrons and strong coupling between the electron's orbital motion and its inherent magnetic moment or spin. These 'spin-orbit assisted Mott insulators' can be turned into metals by adding charge carriers, where many exciting states such as high temperature superconductivity and electronic states for quantum information are predicted. Ultrahigh purity crystals of these metals are grown in the project and experiments exploring the structural, magnetic, and electronic properties of their unusual metallic states performed. In particular, research activities target a series of doped iridium oxide materials whose parent insulating states have recently been shown to collapse into metals with anomalous charge and spin properties. This builds the knowledgebase needed for assessing the viability of these materials for potential future superconducting and quantum information applications. The project trains graduate students in advanced crystal growth techniques as well as in advanced scattering techniques at national user facilities, addressing the national need for trained personnel in both arenas. Additionally, the project executes a multitier education and outreach effort targeted at promoting diverse participation in STEM disciplines and within materials science in particular. This is achieved through undergraduate research opportunities targeted at attracting and retaining underrepresented demographics within materials science. Technical Abstract: This project experimentally explores two classes of iridium oxides where canonical spin-orbit Mott states appear--the Ruddlesden-Popper series of Srn+1IrnO3n+1 and the rare-earth series of pyrochlore Al2Ir2O7. Key variants of these compounds are explored with the goals of understanding the origins of unconventional spin and charge density wave formation in the Ruddlesden-Popper iridates, their relation to strange-metal behavior and potential superconductivity in these systems, and the coupling between magnetism and the metal-insulator transitions of the pyrochlore iridates. All of these phenomena are of considerable interest for identifying exotic electronic states predicted to manifest in these materials, with examples ranging from high-temperature superconductivity to magnetic Weyl semimetal states. Experimental approaches include synchrotron x-ray and neutron scattering measurements, bulk electronic properties (such as charge transport and magnetization) measurements, and complementary electronic probes performed via a network of collaborators. A key enabling tool in the research is the use high-pressure floating zone crystal growth techniques to create ultrahigh purity specimens for study. By leveraging this tool, the project creates new pathways for discovering predicted and other unforeseen unconventional electronic states in carrier-doped spin-orbit assisted Mott materials. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.