Structural and Correlation Effects in the Itinerant Insulating Antiferromagnetic Perovskite NaOsO3

The orthorhombic perovskite NaOsO3 undergoes a continuous metal-insulator transition (MIT), accompanied by antiferromagnetic (AFM) order at T_N=410 K, suggested to be an example of the rare Slater (itinerant) MIT. We study this system using ab initio and related methods, focusing on the origin and nature of magnetic ordering and the MIT. The rotation and tilting of OsO6 octahedra in the GdFeO3 structure result in moderate narrowing the band width of the t_{2g} manifold, but sufficient to induce flattening of bands and AFM order within the local spin density approximation (LSDA), where it remains metallic but with a deep pseudogap. Including on-site Coulomb repulsion U, at U_c ~2 eV a MIT occurs only in the AFM state. Effects of spin-orbit coupling (SOC) on the band structure seem minor as expected for a half-filled $t_{2g}^{3}$ shell, but SOC doubles the critical value U_c necessary to open a gap and also leads to large magnetocrystalline energy differences in spite of normal orbital moments no greater than 0.1$\mu_B$. Our results are consistent with a Slater MIT driven by magnetic order, induced by a combination of structurally-induced band narrowing and moderate Coulomb repulsion, with SOC necessary for a full picture. Strong p-d hybridization reduces the moment, and when bootstrapped by the reduced Hund's rule coupling (proportional to the moment) gives a calculated moment of ~1 $\mu_B$, consistent with the observed moment and only a third of the formal $d^3$ value. We raise and discuss one important question: since this AFM ordering is at q=0 (in the 20 atom cell) where nesting is a moot issue, what is the microscopic driving force for ordering and the accompanying MIT?Comment: 9 page

Stellar Contents and Globular Cluster Candidates in the Sculptor Group Galaxy NGC 300

We present UBVI CCD photometry of the stellar contents and globular cluster(GC) candidates in the spiral galaxy NGC 300 in the Sculptor group. Color-magnitude diagrams for 18 OB associations having more than 30 member stars are presented. The slope of the initial mass function for the bright stars in NGC 300 is estimated to be Gamma = -2.6 +/- 0.3. Assuming the distance to NGC 300 of (m-M)_0 = 26.53 +/- 0.07, the mean absolute magnitude of three brightest blue stars is obtained to be = -8.95 mag. We have performed search for GCs in NGC 300 and have found 17 GC candidates in this galaxy. Some characteristics of these GC candidates are discussed.Comment: 24 pages(AASLaTeX), 14 Postscript figures, Accepted for publication in the Journal of the Korean Astronomical Society, 2002 March issue, Typos Correcte