Jun-Ichi Yamaura

Superconductivity at 1 K in Cd2Re2O7

We report the first pyrochlore oxide superconductor Cd2Re2O7. Resistivity,magnetic susceptibility,and specific heat measurements on single crystals evidence a bulk superconductivity at 1 K. Another phase transition found at 200 K suggests that a peculiar electronic structure lies behind the superconductivity.

Rattling-Induced Superconductiviy in the β-Pyrochlore Oxides AOs2O6

The superconducting properties of two β-pyrochlore oxides, CsOs 2 O 6 and RbOs 2 O 6 , are studied by thermodynamic and transport measurements using high-quality single crystals. It is shown that the character of superconductivity changes systematically from weak coupling for CsOs 2 O 6 to moderately strong coupling for RbOs 2 O 6 , and finally to extremely strong coupling with BCS-type superconductivity for KOs 2 O 6 , with increasing T c . Strong-coupling correction analyses of the superconducting properties reveal that a low-energy rattling mode of the alkali metal ions is responsible for the mechanism of the superconductivity in each compound. The large enhancement of T c from Cs to K is attributed to the increase in the electron–rattler coupling with decreasing characteristic energy of the rattling and with increasing anharmonicity. The existence of weak anisotropy in the superconducting gap or in the electron–rattler interactions is found for the Cs and Rb compounds.

Cov2O6 Single Crystals Grown in a Closed Crucible: Unusual Magnetic Behaviors with Large Anisotropy and 1/3 Magnetization Plateau

Single crystals of CoV(2)O(6) were obtained in a closed crucible using a flux method. Magnetic measurements showed that this material displays a large magnetic anisotropy and a 1/3 magnetization plateau under a magnetic field applied along the c axis.

Crystal structure of the pyrochlore oxide superconductor KOs2O6

Abstract We report the single-crystal X-ray analysis of the structure of the pyrochlore oxide superconductor KOs2O6. The structure was identified as the β -pyrochlore structure with space group Fd 3 ¯ m and lattice constant a = 10.089 ( 2 ) A at 300 K : the K atom is located at the 8 b site, not at the 16 d site as in conventional pyrochlore oxides. We found an anomalously large atomic displacement parameter U iso = 0.0735 ( 8 ) A 2 at 300 K for the K cation, which suggests that the K cation weakly bound to an oversized Os12O18 cage exhibits intensive rattling, as recently observed for clathrate compounds. The rattling of A cations is a common feature in the series of β -pyrochlore oxide superconductors AOs2O6 ( A = Cs , Rb and K), and is greatest for the smallest K cation.

Second Anomaly in the Specific Heat of β-Pyrochlore Oxide Superconductor KOs2O6

Resistivity and specific heat have been measured on a single-crystalline sample of a β-pyrochlore oxide superconductor, KOs 2 O 6 . It is found that a second peak in specific heat, which may evidence an unknown phase transition, appears at around T p ∼7.5 K below the superconducting transition temperature T c =9.53 K. Applying magnetic fields up to 14 T, T c is reduced gradually down to 7.1 K, while T p is slightly raised and becomes even higher than T c at 14 T, which implies that the second anomaly is not associated directly with the superconductivity. It is demonstrated, however, that there is significant communication between the two anomalies, suggesting that they come from the same electrons. It is also reported that the Sommerfeld coefficient γ in KOs 2 O 6 is possibly much larger than those in other members of β-pyrochlore oxide superconductors, RbOs 2 O 6 ( T c =6.3 K) and CsOs 2 O 6 ( T c =3.3 K).

Structural phase transition in the superconducting pyrochlore oxide Cd2Re2O7

Abstract We report a structural phase transition found at T s =200 K in a pyrochlore oxide Cd2Re2O7 which shows superconductivity at T c =1.0 K . X-ray diffraction experiments indicate that the phase transition is of the second-order, from a high-temperature phase with the ideal cubic pyrochlore structure (space group Fd 3 m ) to a low-temperature phase with another cubic structure (space group F 4 3m ). It is accompanied by a dramatic change in the resistivity and magnetic susceptibility and thus must induce a significant change in the electronic structure of Cd2Re2O7.

Tetrahedral magnetic order and the metal-insulator transition in the pyrochlore lattice of Cd2Os2O7.

Cd2Os2O7 shows a peculiar metal-insulator transition at 227 K with magnetic ordering in a frustrated pyrochlore lattice, but its magnetic structure in the ordered state and the transition origin are yet uncovered. We observed a commensurate magnetic peak by resonant x-ray scattering in a high-quality single crystal. X-ray diffraction and Raman scattering experiments confirmed that the transition is not accompanied with any spatial symmetry breaking. We propose a noncollinear all-in-all-out spin arrangement on the tetrahedral network made of Os atoms. Based on this we suggest that the transition is not caused by the Slater mechanism as believed earlier but by an alternative mechanism related to the formation of the specific tetrahedral magnetic order on the pyrochlore lattice in the presence of strong spin-orbit interactions.

Photogenerated hole carrier injection to YBa2Cu3O7−x in an oxide heterostructure

We have fabricated a YBa2Cu3O7−x∕SrTiO3:Nb heterostructure and measured the current–voltage and photovoltaic properties under ultraviolet light irradiation at room temperature. A large photovoltage of 0.8V is observed and is positive to the film. The photovoltage appears under illumination of light with photon energy larger than 3.2eV. These results indicate that photogenerated hole carriers in the SrTiO3:Nb substrate are injected to the film. The maximum surface hole density is attained to be 3.5×1013cm−2 at a light power of 44mW∕cm2. The present photocarrier injection technique could apply to many transition metal oxides to control the hole carrier density externally.

Hydrogen Ordering and New Polymorph of Layered Perovskite Oxyhydrides: Sr2VO4–xHx

Compositionally tunable vanadium oxyhydrides Sr2VO(4-x)H(x) (0 ≤ x ≤ 1.01) without considerable anion vacancy were synthesized by high-pressure solid-state reaction. The crystal structures and their properties were characterized by powder neutron diffraction, synchrotron X-ray diffraction, thermal desorption spectroscopy, and first-principles density functional theory (DFT) calculations. The hydrogen anions selectively replaced equatorial oxygen sites in the VO6 layers via statistical substitution of hydrogen in the low x region (x < 0.2). A new orthorhombic phase (Immm) with an almost entirely hydrogen-ordered structure formed from the K2NiF4-type tetragonal phase with x > 0.7. Based on the DFT calculations, the degree of oxygen/hydrogen anion ordering is strongly correlated with the bonding interaction between vanadium and the ligands.

Charge disproportionation in highly one-dimensional molecular conductor TPP[Co(Pc)(CN)2]2

We investigated the ground state of a highly one-dimensional conductor, TPP[Co(Pc)(CN) 2 ] 2 (TPP = tetraphenylphosphonium and Pc = phthalocyanine), by the measurement of the X-ray diffraction, electron spin resonance, nuclear quadrupole resonance, and magnetoresistance. An increase of the magnetic fluctuations was observed below 20 K, where no structural deformation was detected. In the 59 Co nuclear quadrupole resonance, we found an asymmetric broadening of the spectra owing to the intrinsic inhomogeneity of the molecular charge. We propose that the ground state is characterized by a weak charge disproportionation with antiferromagnetic fluctuations due to the high one dimensionality. A large magnetoresistance was observed under a high magnetic field. Spin effects are dominant at low fields. The anisotropic magnetoresistance suggests a change in the ground-state nature above 10 T.