Abstract:We have prepared polycrystalline samples of the trimer Ir oxide BaIrO 3 with face-shared Ir 3 O 12 trimers, and have investigated the origin of the phase transition at 182 K by measuring resistivity, thermopower, magnetization and synchrotron X-ray diffraction. We propose a possible electronic model and transition mechanism, starting from a localized electron picture on the basis of the Rietveld refinement. Within this model, BaIrO 3 can be basically regarded as a Mott insulator, when the Ir 3 O 12 trimer is identified to one pseudo-atom or one lattice site. The transition can be viewed as a transition from the Mott insulator phase to a kind of charge ordered insulator phase.

We have discovered a novel candidate for a spin liquid state in a ruthenium oxide composed of dimers of S = 3/2 spins of Ru 5+ , Ba 3 ZnRu 2 O 9 . This compound lacks a long range order down to 37 mK, which is a temperature 5000-times lower than the magnetic interaction scale of around 200 K. Partial substitution for Zn can continuously vary the magnetic ground state from an antiferromagnetic order to a spin-gapped state through the liquid state. This indicates that the spin-liquid state emerges from a delicate balance of inter-and intra-dimer interactions, and the spin state of the dimer plays a vital role. This unique feature should realize a new type of quantum magnetism.Since Anderson proposed the idea of a quantum spin liquid as a possible ground state for a spin-half (S = 1/2) antiferromagnetic triangular lattice 1 with a suppressed longrange magnetic ordering due to geometrical frustration and quantum fluctuations of interacting spins, researchers have sought this state of quantum matter. 2 A quantum spin liquid should possess a ground state consisting of highly entangled singlet-spin pairs and exotic excited states called spinons. 3,4 Although several candidates have been reported experimentally, none has been confirmed. Organic candidates consist of ill-defined localized magnetic moments where the magnetic exchange interaction is comparable to the charge gap. [5][6][7] On the other hand, inorganic candidates suffer from unwanted disorder/impurity/nonstoichiometry. Na 4 Ir 3 O 8 8 shows a spin-glass-like transition near 6-7 K, 9, 10 whereas ZnCu 3 (OH) 6 Cl 2 11 and Ba 3 CuSb 2 O 9 12 include a considerable intermixture of cations. BaCu 3 V 2 O 8 (OH) 2 13 and 6H-B Ba 3 NiSb 2 O 9 14 have a substantial low-temperature Curie tail due to unwanted impurities. In the case of BaCu 3 V 2 O 8 (OH) 2 , an inhomogeneous magnetic order has been detected through NMR measurements around 9 K, below which the unwanted Curie tail grows rapidly. 15,16 We have discovered the absense of magnetic long range order in a hexagonal lattice of Ru 5+ dimers in Ba 3 ZnRu 2 O 9 down to 37 mK, where neither Curie tail nor glassy behavior is detected. The magnetic specific heat shows no anomaly below 80 K, and is found to be linear in temperature below around 5 K. These thermodynamic measurements suggest a spin-liquid like ground state in this oxide. The Ru 5+ ion acts as a well-localized S = 3/2 spin, and the spin liquid is totally unprecedented in such a large spin.

We present the single-crystalline x-ray diffraction study on the Ba4Ru3O10 consisting of the corner-shared Ru3O12 trimers. The crystal structure is re-determined from 78 to 300 K across an antiferromagnetic transition at 105 K. The orthorhombic symmetry (Cmca, space group No. 64) is preserved at all temperatures measured. This structure presents exceptionally long Ru-O distances characterized by a significant distribution within the Ru3O12 trimer. A bond valence sum calculation suggests that the charge disproportionation within the Ru3O12 trimer emerges even at room temperature, which we ascribe to molecular orbital formation in the Ru3O12 trimer, as supported by recent theoretical calculations. Based on the analyzed crystal structure, the electronic states and the nature of the phase transition at 105 K are discussed.PACS numbers:

We have measured the Seebeck coefficient and the resistance under light illumination of 365 nm in a ceramic sample of ZnO at 100, 150, and 200 K, and have analyzed the photo-Seebeck coefficient and the photoconductivity using a two-layer model. We have evaluated the photo-doped carrier concentration from the magnitude of the photo-Seebeck coefficient to be of the order of 1019 cm−3, as is similar to the case of thin-film and single-crystal samples. The photo-doping effects are compared among ceramic, single crystal, and thin-film samples.

We have prepared polycrystalline samples of LaSrRh1−xGaxO4 and LaSr1−xCaxRhO4, and have measured the x-ray diffraction, resistivity, Seebeck coefficient, magnetization and electron spin resonance in order to evaluate their electronic states. The energy gap evaluated from the resistivity and the Seebeck coefficient systematically changes with the Ga concentration, and suggests that the system changes from a small polaron insulator to a band insulator. We find that all the samples show Curie-Weiss-like susceptibility with a small Weiss temperature of the order of 1 K, which is seriously incompatible with the collective wisdom that a trivalent rhodium ion is nonmagnetic. We have determined the g factor to be g=2.3 from the electron spin resonance, and the spin number to be S=1 from the magnetization-field curves by fitting with a modified Brillouin function. The fraction of the S=1 spins is 2-5%, which depends on the degree of disorder in the La/Sr/Ca-site, which implies that disorder near the apical oxygen is related to the magnetism of this system. A possible origin for the magnetic Rh 3+ ions is discussed.