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At least 703 records · Page 39

Restoring Symmetry and Enhancing Exchange via Chiral Molecular-Magnetic Hexagons

Modified behaviors of molecules, designed for device or energy applications, can occur due to lattice-molecule incompatibilities in point-group symmetries (PGS), long-range interactions between neighboring molecules, or latticemolecule charge transfer. Such sensitivities are prerequisite for the use of quantum molecules as devices, but removing certain broken symmetries is desirable from the standpoint of simplifying their behaviors. Here, we demonstrate symmetry restoration to molecular-magnetic lattices and show that it leads to strengthened exchange-coupling due to dipolar electrostatic interactions between neighboring molecules and due to manifestations of an earlier identification of a spin-sensitive slightly mobile electron by Hooshmand et al. Starting with a broken symmetry molecular magnet of current interest, we demonstrate a more robust structure composed of six molecular magnets on a h-BN-like surface. The broken point-group symmetry is healed by constructing interpenetrating equilateral triangular structures that are appropriately ratcheted to preserve three-fold rotational symmetry and inversion symmetry.

Pederson, Mark R.↗

CO Adsorption on Au Nanoparticles Grown on Hexagonal Boron Nitride/Rh(111)

CO adsorption on size-controlled Au nanoparticles grown on an $h$-BN/Rh(111) nanomesh surface has been examined to probe their potential catalytic properties. A combination of high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), and density functional theory (DFT) calculations demonstrate that the CO adsorption strength depends heavily on the Au deposition coverage and particle morphology. Particles resulting from low Au coverages deposited at the liquid nitrogen temperature exhibit significantly enhanced CO binding relative to bulk crystalline Au. The resulting CO TPD spectra, and the significantly red-shifted C–O stretching frequency and negative charging of the Au nanoparticles as evidenced by HREELS and DFT, all correspond to those reported for catalytically active Au nanoparticles grown on reactive metal oxides, even though the h-BN/Rh(111) surface is free of carbon, oxygen, or defects. DFT modeling further suggests that the enhanced CO adsorption occurs at highly undercoordinated Au atoms on the perimeters of the nanoparticles. As the Au coverage is increased, the CO adsorption energy and C–O stretching frequency converge toward values associated with bulk Au. Annealing to 600 K results in bulk-like CO adsorption characteristics at any Au coverage. These results suggest that the $h$-BN/Rh(111) surface represents a potential platform for evaluating the role of Au vs support in low-temperature CO oxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

Coexistence and Coupling of Multiple Charge Orderings and Spin States in Hexagonal Ferrite

The coupling between charge and spin orderings in strongly correlated systems plays a crucial role in fundamental physics and device applications. As a candidate of multiferroic materials, LuFe 2 O 4 with a nominal Fe 2.5+ valence state has the potential for strong charge–spin interactions; however, these interactions have not been fully understood until now. Here, combining complementary characterization methods with theoretical calculations, two types of charge orderings with distinct magnetic properties are revealed. The ground states of LuFe 2 O 4 are decided by the parallel/antiparallel coupling of both charge and spin orderings in the adjacent FeO double layers. Whereas the ferroelectric charge ordering remains ferrimagnetic below 230 K, the antiferroelectric ordering undergoes antiferromagnetic–ferrimagnetic–paramagnetic transitions from 2 K to room temperature. This study demonstrates the unique aspects of strong spin-charge coupling within LuFe 2 O 4 . Our results shed light on the coexistence and competing nature of orderings in quantum materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗