NSRRC Activity Report 2023

T his year’s activities on Physics and Materials Science present important results on a broad range of topics in basic and applied research. We report on studies of a Mott insulator to metal transformation at an interface, Kondo effect in a transition metal compound, the well-known spin state transition, a deep look at the synthesis of high-entropy alloy nanocrystals, magnetic proximity effects in composite films, and the relation between charge density wave order and superconductivity in high temperature copper-oxide based superconductors. Angle-resolved photoemission was used to show the emergence of Kondo lattice behavior arising from d -electron character heavy fermions in the transition metal compound FeTe. A long-standing question on the relationship between spin- state transition and the semiconductor-metal transition in LaCoO 3 was answered using temperature-dependent hard X-ray photoemission and X-ray absorption spectroscopy. The transformation of a Mott insulator to a metal at the interface of an ultrathin film of SrCuO 2 grown on a TiO 2 -terminated SrTiO 3 substrate was investigated. In a very challenging study, aimed at finding a rational method for designing stable high-entropy alloy (HEA) nanocrystals, experiments on the reduction kinetics and entropy of mixing provided a quantitative understanding of the fundamental requirements for synthesis of HEA nanocrystals. In a careful study on composite thin films, it was shown that proximity effects enhance long range ordering in adjacent antiferromagnetic layers and also modulate its spin structure, which in turn induced perpendicular magnetic anisotropy in adjacent ferromagnetic films. Finally, a detailed study of charge density wave order and its relationship with superconductivity in high temperature copper-oxide based superconductors showed that the charge ordering region in the phase diagram extends beyond the superconducting dome. These studies identify important aspects of understanding the reduction kinetics and entropy of mixing in alloy formation, magnetic proximity effects, and spin-charge coupling phenomena in strongly-correlated materials, and are expected to direct synthesis of complex materials with improved properties. (by Ashish Chainani) Physics and Materials Science