1,245 research outputs found
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 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. 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 , consistent with the observed moment and only a third of the formal
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
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