No abstract

We report 11 B and 195 Pt NMR measurements in non-centrosymmetric superconductor Li2Pt3B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature Tc. With decreasing temperature (T ) below Tc, the spinlattice relaxation rate 1/T1 decreases with no coherence peak and is in proportion to T 3 . These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast with those in the isostructural Li2Pd3B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of strong electron-electron correlations.PACS numbers: 74.25. Bt, 74.25.Jb, 74.70.Dd In most superconducting materials, there is an inversion center in the crystal which guarantees the parity conservation. In conventional superconductors, such as Al, where the Cooper pair is formed by the attractive force produced by lattice vibration, the orbital wave function (OWF) of the Cooper pairs is in the s-wave form. Since an electron must obey the Fermi statistics, the two spins of such Cooper pair must be in the singlet state. This is also true in most strongly correlated electron systems such as high transition-temperature (T c ) copperoxides [1], cobalt oxide Na x CoO 2 ·1.3H 2 O [2] and many heavy-fermion compounds [3], where the OWF is also symmetric although it has nodes (zeroes). In contrast, if the OWF is asymmetric about the origin with nodes, e.g., a p-wave function, the Cooper pair must be in the spin-triplet state. However, when a superconductor lacks a crystal inversion center, the above-described rule (parity conservation) is violated due to the asymmetric spin-orbit coupling (SOC), and the pairing symmetry becomes nontrivial [8,9,10,11].In this Letter, we present NMR (nuclear magnetic resonance) evidence that increasing the strength of the SOC drastically changes the electron pairing symmetry in non-centrosymmetric superconductors Li 2 Pt(Pd) 3 B. The perovskite-like cubic compounds Li 2 Pd 3 B and Li 2 Pt 3 B are superconducting at T c ∼ 7 K and ∼2.7 K, respectively [12,13]. The inversion symmetry breaking effect is much larger compared to known compounds such as CePt 3 Si (Ref. [14]); all the elements, including the heavy element Pt(Pd), are located in non-centrosymmetric positions, while in CePt 3 Si the main effect comes from non-centrosymmetric Si which is a much lighter element. Also, in CePt 3 Si or UIr (Ref. [15]), the correlated felectrons play a major role in determining the superconducting properties [16,17]; note that the 4f 0 analog of the former compound, LaPt 3 Si, is a conventional superconductor [18]. However, there are no electron correlations in Li 2 Pd 3 B [19,20], which turns out to be also true in Li 2 Pt 3 B (see below). Li 2 Pd 3 B is a spin singlet, s-wave superconductor as...

No abstract

The design, synthesis, and biological activity of benzimidazole-7-carboxylic acids bearing 5-oxo-1,2,4-oxadiazole, 5-oxo-1,2,4-thiadiazole, 5-thioxo-1,2,4-oxadiazole, and 2-oxo-1,2,3,5-oxathiadiazole rings are described. These compounds were efficiently prepared from the key intermediates, the amidoximes 4. The synthesized compounds were evaluated for in vitro and in vivo angiotensin II (AII) receptor antagonistic activities. Most were found to have high affinity for the AT1 receptor (IC50 value, 10(-6)-10(-7)M) and to inhibit the AII-induced pressor response (more than 50% inhibition at 1 mg/kg po). The 5-oxo-1,2,4-oxadiazole, 5-oxo-1,2,4-thiadiazole, and 5-thioxo-1,2,4-oxadiazole derivatives showed stronger inhibitory effects than the corresponding tetrazole derivatives, while their binding affinities were weaker. This might be ascribed to their improved bioavailability by increased lipophilicity. The 5-oxo-1,2,4-oxadiazole derivative 2 (TAK-536) and 5-oxo-1,2,4-thiadiazole derivative 8f showed efficient oral bioavailability without prodrug formation. This study showed that the 5-oxo-1,2,4-oxadiazole ring and its thio analog, the 5-oxo-1,2,4-thiadiazole ring, could be lipophilic bioisosteres for the tetrazole ring in nonpeptide AII receptor antagonists.

No abstract

Environment-friendly proton-exchange-membrane fuel cells (PEMFC) are considered a possible answer to environmental and energy problems. [1][2][3][4][5][6][7][8] To make fuel-cell automobiles a reality, the activity and life of the Pt/C cathode catalyst must be improved. Towards this goal, we have developed a novel time-gating quick XAFS (QXAFS) technique with 1-s time resolution and an energy-dispersive XAFS (DXAFS) system with 4-ms time resolution. Using these techniques, we have observed the electrochemical reaction mechanism and found evidence for dynamic surface events involving Pt dissolution at the Pt/C cathode, the reaction kinetics of the electrontransfer processes, redox structural changes (eight elementary steps), and a significant time lag among those events for the first time under operando fuel-cell conditions. Measurement of the current in a PEMFC in real time shows that a power-on process from open circuit to an operating state brings about rapid electrochemical reactions on its electrode surfaces, which are completed within a few seconds. Such power-on and -off processes (voltage change from open-circuit voltage (OCV = e.g. 1.0 V) to operating voltage (e.g. 0.4 V)) with huge energy transfer are indispensable for commercial applications of fuel-cell systems. However, surface atoms of the active metal particles tend to dissolve slightly into the electrolyte that is in contact with the cathode catalyst layer, and an undesired Pt particle (or layer) deposits in the electrolyte. [9,10] This effect is a problem because automobiles, in particular, require continual repetition of the on/off processes with rapid changes in cell voltages to alter the cars speed.To overcome these serious problems, reaction mechanisms on the electrode surfaces must be investigated in situ during voltage-stepping processes in real time. However, to the best of our knowledge, there are no reports that have fully explored and determined the reaction kinetics of both the structural changes of the metal catalysts and the electrochemical reactions on the electrode surfaces in PEMFCs. We have investigated the mechanism of the electrochemical processes involved in rapid voltage-controlled processes on a Pt/C catalyst. From the viewpoints of electrification and structural changes in the Pt catalyst, in situ time-resolved quick X-ray absorption fine structure (QXAFS) spectroscopy was used to monitor directly the chemical bonding and electronic states in the Pt nanoparticles that act as the electrodes. [11][12][13][14][15][16][17] Acquisition of a QXAFS spectrum at a Pt L III edge requires at least 15 s because of the slow mechanical rotation of the monochromator; however, the time resolution (15 s) is too slow to observe the reaction mechanism of the target processes. Herein, we propose a novel time-gating quick XAFS (TG-QXAFS), with 1-s time resolution, for the first time. The in situ TG-QXAFS measurements of the potential stepping processes in real time revealed extraordinary time lags between electrification and the redox chemical p...

No abstract

Composite electrodes containing active materials, carbon and binder are widely used in lithium-ion batteries. Since the electrode reaction occurs preferentially in regions with lower resistance, reaction distribution can be happened within composite electrodes. We investigate the relationship between the reaction distribution with depth direction and electronic/ionic conductivity in composite electrodes with changing electrode porosities. Two dimensional X-ray absorption spectroscopy shows that the reaction distribution is happened in lower porosity electrodes. Our developed 6-probe method can measure electronic/ionic conductivity in composite electrodes. The ionic conductivity is decreased for lower porosity electrodes, which governs the reaction distribution of composite electrodes and their performances.