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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 1,621 records · Page 90
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DOE OSTI2015

Electronic structure and the origin of the high ordering temperature in SrRu 2 O 6

SrRu 2 O 6 is a layered honeycomb-lattice material with an extraordinarily high magnetic ordering temperature. We investigated this material using density functional calculations. We find that the energy scales for moment formation and ordering are similar and high. Additionally, we find that the magnetic anisotropy is high and favors moments oriented along the c axis. This provides an explanation for the exceptionally high ordering temperature. Finally, the compound is found to be semiconducting at the bare density functional level, even without magnetic order. Lastly, we discuss experimental consequences of this scenario for the high ordering temperature.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Controlling the electronic structure of graphene using surface-adsorbate interactions

Hybridization of atomic orbitals in graphene on Ni(111) opens up a large energy gap of ≈2.8eV between nonhybridized states at the K point. Here we use alkali-metal adsorbate to reduce and even eliminate this energy gap, and also identify a new mechanism responsible for decoupling graphene from the Ni substrate without intercalation of atomic species underneath. Using angle-resolved photoemission spectroscopy and density functional theory calculations, we show that the energy gap is reduced to 1.3 eV due to moderate decoupling after adsorption of Na on top of graphene. Calculations confirm that after adsorption of Na, graphene bonding to Ni is much weaker due to a reduced overlap of atomic orbitals, which results from n doping of graphene. Finally, we show that the energy gap is eliminated by strong decoupling resulting in a quasifreestanding graphene, which is achieved by subsequent intercalation of the Na underneath graphene. Furthermore, the ability to partially decouple graphene from a Ni substrate via n doping, with or without intercalation, suggests that the graphene-to-substrate interaction could be controlled dynamically.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Electronic structure, magnetism, and antisite disorder in CoFeCrGe and CoMnCrAl quaternary Heusler alloys

In this study, we present a combined theoretical and experimental study of two quaternary Heusler alloys CoFeCrGe (CFCG) and CoMnCrAl (CMCA), promising candidates for spintronics applications. Magnetization measurement shows the saturation magnetization and transition temperature to be 3 μ B , 866 K and 0.9 μ B , 358 K for CFCG and CMCA respectively. The magnetization values agree fairly well with our theoretical results and also obey the Slater-Pauling rule, a prerequisite for half metallicity. A striking difference between the two systems is their structure; CFCG crystallizes in fully ordered Y-type structure while CMCA has L2 1 disordered structure. The antisite disorder adds a somewhat unique property to the second compound, which arises due to the probabilistic mutual exchange of Al positions with Cr/Mn and such an effect is possibly expected due to comparable electronegativities of Al and Cr/Mn. Ab initio simulation predicted a unique transition from half metallic ferromagnet to metallic antiferromagnet beyond a critical excess concentration of Al in the alloy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗