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We present a precision analysis of the 136 Xe two-neutrino ββ electron spectrum above 0.8 MeV, based on highstatistics data obtained with the KamLAND-Zen experiment. An improved formalism for the two-neutrino ββ rate allows us to measure the ratio of the leading and subleading 2νββ nuclear matrix elements (NMEs), ξ 2ν 31 ¼ −0.26 þ0.31 −0.25. Theoretical predictions from the nuclear shell model and the majority of the quasiparticle random-phase approximation (QRPA) calculations are consistent with the experimental limit. However, part of the ξ 2ν 31 range allowed by the QRPA is excluded by the present measurement at the 90% confidence level. Our analysis reveals that predicted ξ 2ν 31 values are sensitive to the quenching of NMEs and the competing contributions from low-and high-energy states in the intermediate nucleus. Because these aspects are also at play in neutrinoless ββ decay, ξ 2ν 31 provides new insights toward reliable neutrinoless ββ NMEs.

We report on a search for electron antineutrinos ( ν ¯ e ) from astrophysical sources in the neutrino energy range 8.3–30.8 MeV with the KamLAND detector. In an exposure of 6.72 kton-year of the liquid scintillator, we observe 18 candidate events via the inverse beta decay reaction. Although there is a large background uncertainty from neutral current atmospheric neutrino interactions, we find no significant excess over background model predictions. Assuming several supernova relic neutrino spectra, we give upper flux limits of 60–110 cm−2 s−1 (90% confidence level, CL) in the analysis range and present a model-independent flux. We also set limits on the annihilation rates for light dark matter pairs to neutrino pairs. These data improve on the upper probability limit of 8B solar neutrinos converting into ν ¯ e , P ν e → ν ¯ e < 3.5 × 10 − 5 (90% CL) assuming an undistorted ν ¯ e shape. This corresponds to a solar ν ¯ e flux of 60 cm−2 s−1 (90% CL) in the analysis energy range.

We have studied the electronic structure of the spinel-type compound CuIr 2 S 4 using x-ray photoemission spectroscopy (XPS). CuIr 2 S 4 undergoes a metal-insulator transition (MIT) at 226 K. In going from the metallic to insulating states, the valence-band photoemission spectrum shows a gap opening at the Fermi level and a rigid-band shift of 0:15 eV. In addition, the Ir 4f core-level spectrum is dramatically changed by the MIT. The Ir 4f line shape of the insulating state can be decomposed into two contributions, consistent with the charge disproportionation of Ir 3 :Ir 4 1:1. XPS measurements under laser irradiation indicate that the charge disproportionation of CuIr 2 S 4 is very robust against photoexcitation in contrast to Cs 2 Au 2 Br 6 which shows photo-induced valence transition.The spinel-type compound CuIr 2 S 4 has been attracting much interest because of its first-order metal-insulator transition (MIT) at T MI 226 K accompanied by the loss of localized magnetic moments [1][2][3][4][5][6]. Since the valence state of the Cu ion is Cu , an ionic configuration of Cu Ir 3 Ir 4 S 2ÿ 4 is expected in the insulating phase [7][8][9][10][11][12]. A recent structural study [13] indicates that the cubic spinel structure of CuIr 2 S 4 becomes tetragonally elongated along the c axis and that octamers of Ir 3 (S 0) and Ir 4 (S 1=2) are formed below T MI . In the Ir 4 octamer, the Ir 4 ions are dimerized in two directions (see Fig. 1). Croft et al. have found a dramatic electronic structural change above the Fermi level (E F ) across the MIT using x-ray absorption spectroscopy [4]. They have proposed that the low-temperature structure can be decomposed into onedimensional chains and that the dimerization due to charge and orbital ordering, i.e., the charge density wave formation along the chain direction, is responsible for the electronic structural change [4]. Very recently, it has been proposed that the dimerization in CuIr 2 S 4 and MgTi 2 O 4 [14] can be understood as an orbitally driven Peierls transition [15]. The experimental and theoretical studies indicate that the electronic structure of CuIr 2 S 4 itself is very exotic and interesting. Another interesting point is that the resistivity of CuIr 2 S 4 is dramatically reduced by x-ray or visible light irradiation in the insulating phase [16 -18]. It has been proposed that the photoexcitations break the Ir 3 =Ir 4 charge ordering and induce metallic conductivity.The nature of charge ordering and the effect of light irradiation are still controversial, partly because of the difficulty in the photoemission measurement. Photoemission spectroscopy is a powerful technique to investigate electronic states below E F although it is a surface sensitive method and clean surface must be prepared to obtain precise information. In previous photoemission studies of polycrystalline CuIr 2 S 4 , the Ir 4f core-level spectrum was reported to show no spectral change across the MIT [11,12] and no evidence of charge ordering was obtained. The Ir 4f photoemission data can provide ...

We report on an electronic structure study of a quasi-two-dimensional Co oxide Ca 3 Co 4 O 9 with Ca 2 CoO 3 rocksalt layers and CoO 2 triangular lattice layers by means of x-ray photoemission spectroscopy ͑XPS͒, ultraviolet photoemission spectroscopy ͑UPS͒, configuration-interaction calculation on a CoO 6 cluster model, and unrestricted Hartree-Fock calculation on a multiband d-p model. The Co 2p XPS spectrum shows that the Co valence of the rocksalt layer is similar to that of the triangular lattice layer. The cluster-model analysis of the Co 2p XPS spectrum indicates that the Co sites of the rocksalt and triangular lattice layers commonly have charge-transfer energy ⌬ of ϳ1.0 eV, d-d Coulomb interaction U of ϳ6.5 eV, and transfer integral ͑pd͒ of ϳ−2.3 eV. The Co 3d t 2g peak in the valence-band XPS spectrum remains sharp even above the spin-state transition temperature at ϳ380 K, indicating that the spin-state transition is different from the low-spin to intermediate-spin or high-spin transitions in perovskite LaCoO 3 . The line shape of the UPS spectrum near the Fermi level can be reproduced by the combination of unrestricted Hartree-Fock results for the charge-ordered insulating ͑COI͒ and paramagnetic metallic ͑PM͒ states. The analysis shows that the phase separation between the COI and PM states plays important roles in Ca 3 Co 4 O 9 .