We report measurements of the optical gap in a GdN film at temperatures from 300 to 6K, covering both the paramagnetic and ferromagnetic phases. The gap is 1.31eV in the paramagnetic phase and red-shifts to 0.9eV in the spin-split bands below the Curie temperature. The paramagnetic gap is larger than was suggested by very early experiments, and has permitted us to refine a (LSDA+U)-computed band structure. The band structure was computed in the full translation symmetry of the ferromagnetic ground state, assigning the paramagnetic-state gap as the average of the majority- and minority-spin gaps in the ferromagnetic state. That procedure has been further tested by a band structure in a 32-atom supercell with randomly-oriented spins. After fitting only the paramagnetic gap the refined band structure then reproduces our measured gaps in both phases by direct transitions at the X point.Comment: 5 pages, 4 figure

The electronic structure of graphdiyne exposed to air is investigated utilizing X-ray absorption spectroscopy and scanning transmission X-ray microscopy. It is found that carbon−carbon triple bonds at defect sites in graphdiyne have been changed to double bonds after 3 months in air. The experimental results reveal the existence of oxygen and nitrogen functional groups and indicate that the oxidation takes place throughout the aged graphdiyne while the nitrogen contamination is mainly on its surface. Buckling of the aged graphdiyne is observed, resulting from bond length change due to the opening of triple bonds. It is also shown that annealing at a high temperature such as 800 °C may remove most of the functional groups in the aged graphdiyne.

The magnetic behavior of SmN has been investigated in stoichiometric polycrystalline films. All samples show ferromagnetic order with Curie temperature ͑T C ͒ of 27Ϯ 3 K, evidenced by the occurrence of hysteresis below T C . The ferromagnetic state is characterized by a very small moment and a large coercive field, exceeding even the maximum applied field of 6 T below about 15 K. The residual magnetization at 2 K, measured after cooling in the maximum field, is 0.035 B per Sm. Such a remarkably small moment results from a near cancellation of the spin and orbital contributions for Sm +3 in SmN. Coupling to an applied field is therefore weak, explaining the huge coercive field. The susceptibility in the paramagnetic phase shows temperatureindependent Van Vleck and Curie-Weiss contributions. The Van Vleck contribution is in quantitative agreement with the field-induced admixture of the J = 7 2 excited state and the 5 2 ground state. The Curie-Weiss contribution returns a Curie temperature that agrees with the onset of ferromagnetic hysteresis, and a conventional paramagnetic moment with an effective moment of 0.4 B per Sm ion, in agreement with expectations for the crystal-field modified effective moment on the Sm +3 ions.

We have investigated the electronic states of highly oriented pyrolitic graphite and single-walled carbon nanotubes using x-ray absorption spectroscopy (XAS) before and after annealing treatment in ultrahigh vacuum, and observed that the small peak between π* and σ* features, which has been previously assigned to free-electron-like interlayer states, disappears after in situ annealing treatment, suggesting that the signal may be assigned to a surface contamination, especially oxygen contamination introduced by chemical processing or gas adsorption. Additional experiments by photoelectron spectroscopy as well as XAS methods, performed after aging in air, fully support this interpretation.

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