4D and 5D Transition Metal Oxides: A New Frontier of Materials with Exotic Phenomena

Project Summary The extensive and intensive research on complex quantum systems in the last decade has made it increasingly clear that the behavior of an assembly of interacting electrons will routinely surprise us with entirely unexpected phenomena that call for further experimental and theoretical exploration. The 4d and 5d transition metal oxides as a class of new correlated electron systems have recently revealed a large array of exotic electronic, magnetic and orbital phases and even some paradoxes that challenge the very fundamentals of conventional physics such as Fermi liquid theory. The objective of this proposed research is to continue our experimental investigation of these novel materials to further address issues that are currently among the most important and interesting in the field of Condensed Matter Physics. Intellectual Merit: These 4d and 5d transition metal oxides, particularly, the ruthenates and the iridates, are characterized by the strong interplay of charge, spin and orbital degrees of freedom and the high sensitivity to small perturbations, such as magnetic field, pressure and chemical doping. These materials not only cover almost every state in Condensed Matter Physics but also exhibit a wide array of extraordinary phenomena not or rarely found in other materials. As revealed in our work, these phenomena include an orbitally-driven colossal magnetoresistance achieved by avoiding a ferromagnetic state, bulk spin filter behavior, orbital order of the t2g orbitals, coexistence of a Mott transition and quantum oscillations periodic in both Band liB (B is magnetic field), strong competitions between ferromagnetism and anti ferromagnetism, itinerant metamagnetism, "diamagnetism" in non-superconducting states, weak ferromagnetism with high Curie temperatures, coexistence of weak ferromagnetism and a COW, and astonishing dimensionality-dependence of properties, etc. These phenomena reflect new physics and present profound challenges. It is these challenges that provide more and strong motivation for us to further and thoroughly pursue these studies, which will extend the already broad impact of our work on the field of strongly correlated systems. Our research encompasses a systematic effort to elucidate the underlying physics of these materials as well as a search for new compounds with interesting properties. Research activities range from single crystal growth to characterization of structural, transport and thermodynamic properties as a function of temperature, magnetic field, pressure and chemical composition. The PI's lab is well-equipped with capabilities of single crystal growth using floating zone and flux techniques and transport and thermodynamic measurements in a wide temperature range of 0.3- 900 K and in magnetic fields up to 15 T. Broad Impact: The US leadership in Condensed Matter Physics has been eroded in recent years due in part to the lack of materials physicists who specialize in both synthesizing and characterizing new materials. As an effort to help fill this gap, we have practiced graduate training that emphasizes both materials synthesis and characterization in the same lab. Our research program will continue to provide graduate students with unique training that covers a broad spectrum of materials research from synthesis to characterization using a large variety of techniques and probes. The students will continue to interact closely with our collaborators at UK and other institutions and frequently visit national labs so that they not only gain broad research experience but are also exposed to different viewpoints. We believe that this effort, together with others', will help eventually fill the hiatus in our graduate education that has already stalled our advancement in condensed matter and materials physics in the US. The Department of Physics and Astronomy at UK is the only physics Ph.D. granting department in the entire Kentucky university system. The impact of this research program on our graduate program is thus particularly significant and far-reaching. The PI is also committed to drawing young minds to physical sciences by providing hands-on experience and introducing modem techniques and equipment. This is particularly urgent and important as we are entering an era of high technologies with steadily declining numbers of physical sciences undergraduates in the US, who are the future human capital in technologies that drive the economy.