Bonding, Moment Formation, and Magnetic Interactions in Ca14MnBi11 and Ba14MnBi11

The ``14-1-11'' phase compounds based on magnetic Mn ions and typified by Ca14MnBi11 and Ba14MnBi11 show unusual magnetic behavior, but the large number (104) of atoms in the primitive cell has precluded any previous full electronic structure study. Using an efficient, local orbital based method within the local spin density approximation to study the electronic structure, we find a gap between a bonding valence band complex and an antibonding conduction band continuum. The bonding bands lack one electron per formula unit of being filled, making them low carrier density p-type metals. The hole resides in the MnBi4 tetrahedral unit and partially compensates the high spin d^5 Mn moment, leaving a net spin near 4 \mu_B that is consistent with experiment. These manganites are composed of two disjoint but interpenetrating `jungle gym' networks of spin 4/2 MnBi4^{9-} units with ferromagnetic interactions within the same network, and weaker couplings between the networks whose sign and magnitude is sensitive to materials parameters. Ca14MnBi11 is calculated to be ferromagnetic as observed, while for Ba14MnBi11 (which is antiferromagnetic) the ferro- and antiferromagnetic states are calculated to be essentially degenerate. The band structure of the ferromagnetic states is very close to half metallic.Comment: 17 pages, containing 10 postscript figures and 5 tables. Two additional figures (Fig.8 and 11 of the paper) are provided in JPG format in separate files. Submitted to Phys. Rev. B on September 20th 200

RESISTIVITY OF TERNARY CHEVREL-PHASE SUPERCONDUCTORS

Nous avons avons mesure la résistivité entre 4.2 K e t 400 K d'une série de phases de Chevrel qui sont des composés supraconducteurs. Tous montrent une courbure très négative (d2ρ/dT2) au-dessus d'environ 60 K, semblable à ce qu'on observe dans les composés A-15. Les résistivités à la température ambiante sont é1evées, de l'ordre de 10-3 Ω cm.We have measured the resistivities of a series of superconducting Chevrel-phase compounds from 4.2 K to 400 K. All show strong negative curvature d2ρ/dT2 above about 60 K, similar to that observed i n the A-15 compounds. Room temperature resistivities are high, in the 10-3 Ω cm range