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It has been suggested that Ba3In2O6 might be a high-Tcsuperconductor. Experimental investigation of the properties of Ba3In2O6 was long inhibited by its instability in air. Recently epitaxial Ba3In2O6 with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6 is determined to be 2.99 eV in-the (001) plane and 2.83 eV along the c-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. Neither A- nor B-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6 was successfully grown epitaxially on an epitaxial SrRuO3 bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6, leaving the answer to the initial question of whether Ba3In2O6 is a high-Tcsuperconductor to be “no” thus far.

Epitaxial untwinned SrRuO3 thin films were grown on (110)-oriented DyScO3 substrates by molecular-beam epitaxy. We report an exceptional sample with a residual resistivity ratio (RRR), ρ [300 K]/ρ [4 K] of 205 and a ferromagnetic Curie temperature, TC, of 168.3 K. We compare the properties of this sample to other SrRuO3 films grown on DyScO3(110) with RRRs ranging from 8.8 to 205, and also compare it to the best reported bulk single crystal of SrRuO3. We determine that SrRuO3 thin films grown on DyScO3(110) have an enhanced TC as long as the RRR of the thin film is above a minimum electrical quality threshold. This RRR threshold is about 20 for SrRuO3. Films with lower RRR exhibit TCs that are significantly depressed from the intrinsic strain-enhanced value.

High‐entropy materials defy historical materials design paradigms by leveraging chemical disorder to kinetically stabilize novel crystalline solid solutions comprised of many end‐members. Formulational diversity results in local crystal structures that are seldom found in conventional materials and can strongly influence macroscopic physical properties. Thermodynamically prescribed chemical flexibility provides a means to tune such properties. Additionally, kinetic metastability results in many possible atomic arrangements, including both solid‐solution configurations and heterogeneous phase assemblies, depending on synthesis conditions. Local disorder induced by metastability, and extensive cation solubilities allowed by thermodynamics combine to give many high‐entropy oxide systems utility as electrochemical, magnetic, thermal, dielectric, and optical materials. Though high‐entropy materials research is maturing rapidly, much remains to be understood and many compositions still await discovery, exploration, and implementation.

Manipulating a crystalline material's configurational entropy through the introduction of unique atomic species can produce novel materials with desirable mechanical and electrical properties. From a thermal transport perspective, large differences between elemental properties such as mass and interatomic force can reduce the rate at which phonons carry heat and thus reduce the thermal conductivity. Recent advances in materials synthesis are enabling the fabrication of entropy‐stabilized ceramics, opening the door for understanding the implications of extreme disorder on thermal transport. Measuring the structural, mechanical, and thermal properties of single‐crystal entropy‐stabilized oxides, it is shown that local ionic charge disorder can effectively reduce thermal conductivity without compromising mechanical stiffness. These materials demonstrate similar thermal conductivities to their amorphous counterparts, in agreement with the theoretical minimum limit, resulting in this class of material possessing the highest ratio of elastic modulus to thermal conductivity of any isotropic crystal.