In-plane anisotropy on the transport properties in the modulated Bi_2O_2-based conductors Bi-2212 and Bi-Sr-Co-O

We investigated the in-plane anisotropy on the resistivity and thermopower of the Bi_2Sr_2CaCu_2O_{8+\delta} (Bi-2212) and Bi-Sr-Co-O (BiCo) single crystals. In Bi-2212, the b-axis resistivity is higher than the a-axis resistivity, and is expressed as a sum of the a-axis resistivity and an additional residual resistivity. A downward deviation due to pseudogap is observed below a characteristic temperature T^*, which is isotropic in the form of conductivity. These results suggest that the modulation structure along the b-axis works as an anisotropic scattering center, but does not affect the pseudogap formation. On the other hand, the anisotropy of the resistivity and the thermopower in Pb-doped BiCo is substantial, probably owing to the misfit structure between the hexagonal CoO_2 layer and the rock salt Bi_2O_2 layer. However, the anisotropy in the resistivity in Pb-free BiCo is very small, suggesting that the in-plane anisotropy is averaged by the modulation structure, whose direction is tilted by 45 deg from the a- and b-axes.Comment: 4pages 5 figures, Proceedings of ISS2001, Physica C (in press

Physical properties of misfit-layered (Bi,Pb)-Sr-Co-O system: Effect of hole doping into triangular lattice formed by low-spin Co ions

Pb-doping effect on physical properties of misfit-layered (Bi,Pb)-Sr-Co-O system, in which Co ions form a two-dimensional triangular lattice, was investigated in detail by electronic transport, magnetization and specific-heat measurements. Pb doping enhances the metallic behavior, suggesting that carriers are doped. Pb doping also enhances the magnetic correlation in this system and increases the magnetic transition temperature. We found the existence of the short-range magnetic correlation far above the transition temperature, which seems to induce the spin-glass state coexisting with the ferromagnetic long-range order at low temperatures. Specific-heat measurement suggests that the effective mass of the carrier in (Bi,Pb)-Sr-Co-O is not enhanced so much as reported in NaCo${}_2$O${}_4$. Based on these experimental results, we propose a two-bands model which consists of narrow $a_{1g}$ and rather broad $e'{}_g$ bands. The observed magnetic property and magnetotransport phenomena are explained well by this model

Dual electronic states in thermoelectric cobalt oxide

We investigate the low temperature magnetic field dependence of the resistivity in the thermoelectric misfit cobalt oxide [Bi1.7Ca2O4]0.59CoO2 from 60 K down to 3 K. The scaling of the negative magnetoresistance demonstrates a spin dependent transport mechanism due to a strong Hund's coupling. The inferred microscopic description implies dual electronic states which explain the coexistence between localized and itinerant electrons both contributing to the thermopower. By shedding a new light on the electronic states which lead to a high thermopower, this result likely provides a new potential way to optimize the thermoelectric properties

Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound

Ferromagnetism and accompanying large negative magnetoresistance in Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail. Recent structural analysis of (Bi,Pb)${}_2$Sr${}_{3}$Co${}_2$O${}_9$, which has been believed to be a Co analogue of Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+\delta}$, revealed that it has a more complex structure including a CoO${}_2$ hexagonal layer [T. Yamamoto {\it et al.}, Jpn. J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only introduces holes into the conducting CoO${}_2$ layers but also creates a certain amount of localized spins. Ferromagnetic transition appears at $T$ = 3.2 K with small spontaneous magnetization along the $c$ axis, and around the transition temperature large and anisotropic negative magnetoresistance was observed. This compound is the first example which shows ferromagnetic long-range order in a two-dimensional metallic hexagnonal CoO${}_2$ layer.Comment: 8 pages including eps figures. To be published in J. Phys. Soc. Jp

Transport properties and magnetic field induced localization in the misfit cobaltite [Bi2_2Ba1.3_{1.3}K0.6_{0.6}Co0.1_{0.1}]RS^{RS}[CoO2_2]1.97_{1.97} single crystal

Resistivity under magnetic field, thermopower and Hall coefficient are systematically studied for [Bi$_2$Ba$_{1.3}$K$_{0.6}$Co$_{0.1}$]$^{RS}$[CoO$_2$]$_{1.97}$ single crystal. In-plane resistivity ($\rho_{ab}$(T)) shows metallic behavior down to 2 K with a $T^2$ dependence below 30 K; while out-of-plane resistivity ($\rho_{c}(T)$) shows metallic behavior at high temperature and a thermal activation semiconducting behavior below about 12 K. Striking feature is that magnetic field induces a ln(1/$T$) diverging behavior in both $\rho_{ab}$ and $\rho_{c}(T)$ at low temperature. The positive magnetoresistance (MR) could be well fitted by the formula based on multi-band electronic structure. The ln(1/$T$) diverging behavior in $\rho_{ab}$ and $\rho_{c}(T)$ could arise from the magnetic-field-induced 2D weak localization or spin density wave.Comment: 7 pages, 8 figure

Novel electronic structure induced by a highly strained oxide interface with incommensurate crystal fields

The misfit oxide, Bi$_{2}$Ba$_{1.3}$K$_{0.6}$Co$_{2.1}$O$_{y}$, made of alternating rocksalt-structured [BiO/BaO] layers and hexagonal CoO$_{2}$ layers, was studied by angle-resolved photoemission spectroscopy. Detailed electronic structure of such a highly strained oxide interfaces is revealed for the first time. We found that under the two incommensurate crystal fields, electrons are confined within individual sides of the interface, and scattered by umklapp scattering of the crystal field from the other side. In addition, the high strain on the rocksalt layer raises its chemical potential and induces large charge transfer to the CoO$_{2}$ layer. Furthermore, a novel interface effects, the interfacial enhancement of electron-phonon interactions, is discovered. Our findings of these electronic properties lay a foundation for designing future functional oxide interfaces.Comment: 5 pages, 4 figure

Structure cristalline du solvate Hg Br2• C4H8O

The crystal structure of solvate HgBr₂ • C₄H₈O has been determined from the intensities of 590 reflexions (hk0 to hk3). They had been registered with a Weissenberg camera. The final R factor is 0.096. The structure consists of groups in which HgBr₂ chains parallel to c are linked to tetrahydrofuran rings by Hg…O = 2.67 Å. HgBr₂ molecules are slightly bent (Hg-Br = 2.475 Å, Br-Hg-Br = 174°6). Mercury atoms have octahedral surroundings. The chains of strongly bonded HgBr₂ have the same structure as in pure HgBr₂.La structure cristalline a été déterminée à partir des intensités de 590 taches intégrées (hk0 à hk3) recueillies à l'aide d'une chambre de Weissenberg. Le facteur R final est égal à 0,096. La structure est formée de groupes de deux chaînes HgBr₂ parallèles à c et reliées aux cycles tétrahydrofuranne par Hg…O = 2,67 Å. Les molécules HgBr₂ sont légèrement coudées (Hg-Br = 2,475 Å, Br-Hg-Br = 174°6). L'entourage des atomes de mercure est octaédrique. Les chaînes de liaisons fortes HgBr₂ ont la même structure que celles de HgBr₂ pur.Frey Michel, Leligny Henri, Ledésert Maryannick. Structure cristalline du solvate Hg Br2• C4H8O. In: Bulletin de la Société française de Minéralogie et de Cristallographie, volume 94, 5-6, 1971. pp. 467-470