Takeshi Yajima

An oxyhydride of BaTiO3 exhibiting hydride exchange and electronic conductivity

In oxides, the substitution of non-oxide anions (F(-),S(2-),N(3-) and so on) for oxide introduces many properties, but the least commonly encountered substitution is where the hydride anion (H(-)) replaces oxygen to form an oxyhydride. Only a handful of oxyhydrides have been reported, mainly with electropositive main group elements or as layered cobalt oxides with unusually low oxidation states. Here, we present an oxyhydride of the perhaps most well-known perovskite, BaTiO(3), as an O(2-)/H(-) solid solution with hydride concentrations up to 20% of the anion sites. BaTiO(3-x)H(x) is electronically conducting, and stable in air and water at ambient conditions. Furthermore, the hydride species is exchangeable with hydrogen gas at 400 °C. Such an exchange implies diffusion of hydride, and interesting diffusion mechanisms specific to hydrogen may be at play. Moreover, such a labile anion in an oxide framework should be useful in further expanding the mixed-anion chemistry of the solid state.

Oxyhydrides of (Ca,Sr,Ba)TiO3 perovskite solid solutions.

The oxyhydride solid solutions (Ca,Sr)TiO(3-x)H(x) and (Sr,Ba)TiO(3-x)H(x) have been prepared by reducing the corresponding ATiO(3) oxides with calcium hydride. Under the reaction conditions examined, a hydride content of x = 0.1-0.3 was obtained for all compositions. Compared to our previous result with BaTiO(3-x)H(x), the larger particle size in this study (20-30 μm vs 170 nm) resulted in a somewhat lower hydride amount despite prolonged reaction times. We examined changes in cell volume, octahedral tilt angle, and site occupancy of different anion sites after conversion to oxyhydrides; it appears that these oxyhydrides fit the geometrical descriptions typical for regular ABO(3) perovskites quite well. The hydrogen release temperature, previously shown to be indicative of the hydride exchange temperature, however, does not scale linearly with the A-site composition, indicating a potential effect of chemical randomness.

Synthesis and Physical Properties of the New Oxybismuthides BaTi2Bi2O and (SrF)2Ti2Bi2O with a d^{1} Square Net

We have recently reported the \(d^{1}\) square-lattice compound BaTi2Sb2O, which shows superconductivity at \(T_{\text{c}} = 1.2\) K coexisting with a charge- or spin-density wave (CDW/SDW) state. Here, we successfully prepared two new oxybismuthides, BaTi2Bi2O and (SrF)2Ti2Bi2O, as the first Pn = Bi compounds in the \(A\)Ti2 Pn 2O family. The CDW/SDW state disappeared for both compounds, presumably owing to the considerable interaction between the Ti-\(3d\) and Bi-\(6s\) orbitals. The complete suppression of the CDW/SDW instability resulted in an enhanced \(T_{\text{c}}\) of 4.6 K for BaTi2Bi2O. However, (SrF)2Ti2Bi2O exhibits no superconductivity, suggesting the importance of the interlayer interaction for superconductivity.

Molecular Rotors of Coronene in Charge-Transfer Solids

Ten types of neutral charge transfer (CT) complexes of coronene (electron donor; D) were obtained with various electron acceptors (A). In addition to the reported 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex of 1:1 stoichiometry with a DA-type alternating π column, TCNQ also afforded a 3:1 complex, in which a face-to-face dimer of parallel coronenes (Cor-As) is sandwiched between TCNQs to construct a DDA-type alternating π column flanked by another coronene (Cor-B). Whereas solid-state (2)H NMR spectra of the 1:1 TCNQ complex formed with deuterated coronene confirmed the single in-plane 6-fold flipping motion of the coronenes, two unsynchronized motions were confirmed for the 3:1 TCNQ complex, which is consistent with a crystallographic study. Neutral [Ni(mnt)2] (mnt: maleonitriledithiolate) as an electron acceptor afforded a 5:2 complex with a DDA-type alternating π column flanked by another coronene, similar to the 3:1 TCNQ complex. The fact that the Cor-As in the [Ni(mnt)2] complex arrange in a non-parallel fashion must cause the fast in-plane rotation of Cor-A relative to that of Cor-B. This is in sharp contrast to the 3:1 TCNQ complex, in which the dimer of parallel Cor-As shows inter-column interactions with neighboring Cor-As. The solid-state (1)H NMR signal of the [Ni(mnt)2] complex suddenly broadens at temperatures below approximately 60 K, indicating that the in-plane rotation of the coronenes undergoes down to approximately 60 K; the rotational rate reaches the gigahertz regime at room temperature. Rotational barriers of these CT complexes, as estimated from variable-temperature spin-lattice relaxation time (T1) experiments, are significantly lower than that of pristine coronene. The investigated structure-property relationships indicate that the complexation not only facilitates the molecular rotation of coronenes but also provides a new solid-state rotor system that involves unsynchronized plural rotators.

Spatial Redistribution of Oxygen Ions in Oxide Resistance Switching Device after Forming Process

The change in the spatial distribution of oxygen ions after an initial voltage application called the forming process was investigated for oxide resistance switching devices by secondary ion mass spectrometry mapping. To track the motion of oxygen ions, tracer 18O ions were implanted in a planar Pt/CuO/Pt device. We found clear evidence for the oxygen reduction in the conductive bridge structure formed between two electrodes. In addition, the oxygen ions in the bridge structure drift to the anode, implying the oxygen diffusion (migration) induced by high electric field and/or current density. We discuss those results in terms of a filament model.

Two Superconducting Phases in the Isovalent Solid Solutions BaTi2Pn2O (Pn = As, Sb, and Bi)

We have recently reported two superconductors with the \(d^{1}\) square lattice, BaTi2Sb2O (\(T_{\text{c}}=1.2\) K) and BaTi2Bi2O (\(T_{\text{c}}=4.6\) K). In order to find a clue behind the superc...

Emergence of nontrivial magnetic excitations in a spin-liquid state of kagomé volborthite

Significance The Hall effect is due to the nature of the charge carriers in a conductor, which experience the Lorentz force in the presence of magnetic field. In antiferromagnetic insulators with geometrical frustrations, charge carries are absent, but the spin degrees of freedom form disordered liquid-like states with unusual excitations that can carry heat. Here, we report the observation of the thermal version of the Hall effect in a spin-liquid state of a quantum magnet with frustrated two-dimensional kagomé structure. Our finding implies the emergence of nontrivial excitations in a spin-liquid state that experience the fictitious Lorentz force, possibly related to the geometrical phase of the quantum-mechanical wave function in many-body systems. When quantum fluctuations destroy underlying long-range ordered states, novel quantum states emerge. Spin-liquid (SL) states of frustrated quantum antiferromagnets, in which highly correlated spins fluctuate down to very low temperatures, are prominent examples of such quantum states. SL states often exhibit exotic physical properties, but the precise nature of the elementary excitations behind such phenomena remains entirely elusive. Here, we use thermal Hall measurements that can capture the unexplored property of the elementary excitations in SL states, and report the observation of anomalous excitations that may unveil the unique features of the SL state. Our principal finding is a negative thermal Hall conductivity κxy which the charge-neutral spin excitations in a gapless SL state of the 2D kagomé insulator volborthite Cu3V2O7(OH)2⋅2H2O exhibit, in much the same way in which charged electrons show the conventional electric Hall effect. We find that κxy is absent in the high-temperature paramagnetic state and develops upon entering the SL state in accordance with the growth of the short-range spin correlations, demonstrating that κxy is a key signature of the elementary excitation formed in the SL state. These results suggest the emergence of nontrivial elementary excitations in the gapless SL state which feel the presence of fictitious magnetic flux, whose effective Lorentz force is found to be less than 1/100 of the force experienced by free electrons.

S-Wave Superconductivity in Superconducting BaTi[2]Sb[2]O Revealed by [121/123]Sb-NMR/Nuclear Quadrupole Resonance Measurements

We report the

Tc Enhancement by Aliovalent Anionic Substitution in Superconducting BaTi2(Sb1-xSnx)2O

^{121/123}

A-site-ordered perovskite MnCu3V4O12 with a 12-coordinated manganese(II).

Sb-NMR/nuclear quadrupole resonance (NQR) measurements on the newly-discovered superconductor BaTi