Unusual superexchange pathways in a Ni triangular lattice of NiGa2_2S4_4 with negative charge-transfer energy

We have studied the electronic structure of the Ni triangular lattice in NiGa$_2$S$_4$ using photoemission spectroscopy and subsequent model calculations. The cluster-model analysis of the Ni 2$p$ core-level spectrum shows that the S 3$p$ to Ni 3$d$ charge-transfer energy is $\sim$ -1 eV and the ground state is dominated by the $d^9L$ configuration ($L$ is a S 3$p$ hole). Cell perturbation analysis for the NiS$_2$ triangular lattice indicates that the strong S 3$p$ hole character of the ground state provides the enhanced superexchange interaction between the third nearest neighbor sites.Comment: 10 pages, 5 figures, accepted to PR

Signature of Carrier-Induced Ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta}: Exchange Interaction Between High-Spin Co 2+ and the Ti 3d Conduction Band

X-ray photoemission spectroscopy measurements were performed on thin-film samples of rutile Ti_{1-x}Co_{x}O_{2-delta} to reveal the electronic structure. The Co 2p core level spectra indicate that the Co ions take the high-spin Co 2+ configuration, consistent with substitution on the Ti site. The high spin state and the shift due to the exchange splitting of the conduction band suggest strong hybridization between carriers in the Ti 3d t2g band and the t2g states of the high-spin Co 2+. These observations support the argument that room temperature ferromagnetism in Ti_{1-x}Co_{x}O_{2-delta} is intrinsic.Comment: 4 pages, 5 figures. Accepted for publication in Physical Review Letter

Orbital Ordering Structures in (Nd,Pr)0.5Sr0.5MnO3 Manganite Thin Films on Perovskite (011) Substrates

Structural study of orbital-ordered manganite thin films has been conducted using synchrotron radiation, and a ground state electronic phase diagram is made. The lattice parameters of four manganite thin films, Nd0.5Sr0.5MnO3 (NSMO) or Pr0.5Sr0.5MnO3 (PSMO) on (011) surfaces of SrTiO3 (STO) or [(LaAlO3){0.3}(SrAl0.5Ta0.5O3){0.7}] (LSAT), were measured as a function of temperature. The result shows, as expected based on previous knowledge of bulk materials, that the films' resistivity is closely related to their structures. Observed superlattice reflections indicate that NSMO thin films have an antiferro-orbital-ordered phase as their low-temperature phase while PSMO film on LSAT has a ferro-orbital-ordered phase, and that on STO has no orbital-ordered phase. A metallic ground state was observed only in films having a narrow region of A-site ion radius, while larger ions favor ferro-orbital-ordered structure and smaller ions stabilize antiferro-orbital-ordered structure. The key to the orbital-ordering transition in (011) film is found to be the in-plane displacement along [0-1 1] direction.Comment: 19pages, 11 figure

High-energy spectroscopic study of the III-V nitride-based diluted magnetic semiconductor Ga1−x_{1-x}Mnx_{x}N

We have studied the electronic structure of the diluted magnetic semiconductor Ga$_{1-x}$Mn$_{x}$N ($x$ = 0.0, 0.02 and 0.042) grown on Sn-doped $n$-type GaN using photoemission and soft x-ray absorption spectroscopy. Mn $L$-edge x-ray absorption have indicated that the Mn ions are in the tetrahedral crystal field and that their valence is divalent. Upon Mn doping into GaN, new state were found to form within the band gap of GaN, and the Fermi level was shifted downward. Satellite structures in the Mn 2$p$ core level and the Mn 3$d$ partial density of states were analyzed using configuration-interaction calculation on a MnN$_{4}$ cluster model. The deduced electronic structure parameters reveal that the $p$-$d$ exchange coupling in Ga$_{1-x}$Mn$_{x}$N is stronger than that in Ga$_{1-x}$Mn$_{x}$As.Comment: 6pages, 10figures. To be published to Phys. Rev.