Tomohisa Takamatsu

Low-Temperature Synthesis of the Infinite-Layer Compound LaNiO2 by Soft-Chemical Techniques

Bulk samples of LaNiO2 with the so-called infinite-layer structure have successfully been synthesized. First, polycrystalline samples of LaNiO3 with the perovskite structure were prepared using molten KOH at a temperature of 400–450 °C. Then, they were finely ground with a double stoichiometric excess of reductant CaH2 and sealed in an evacuated Pyrex tube. The sealed ampoule was then heated at 300 °C for 24 h. Powder X-ray diffraction analysis has revealed that the product obtained after removing the residual CaH2 and the byproduct CaO is of almost single phase with tetragonal symmetry, indicating the formation of the infinite-layer compound LaNiO2.

Pairing Symmetry Studied from Impurity Effects in the Undoped Superconductor T′-La1.8Eu0.2CuO4

We have investigated the effects of magnetic Ni and nonmagnetic Zn impurities on the superconductivity in undoped T′-La1.8Eu0.2CuO4 (T′-LECO) with the Nd2CuO4-type structure, using polycrystalline bulk samples, to clarify the pairing symmetry. It has been found that the suppression rates of the superconducting transition temperature Tc by both Ni and Zn impurities are nearly the same and are very similar to those in the optimally doped and overdoped regimes of hole-doped T-La2−xSrxCuO4 with the K2NiF4-type structure. These results strongly suggest that the superconductivity in undoped T′-LECO has d-wave symmetry and is mediated by the spin fluctuation.

Effects of Ge substitution on thermoelectric properties of CrSi2

Polycrystalline Cr(Si1− x Ge x )2 samples were prepared using arc melting and spark plasma sintering methods. Single-phase Cr(Si1− x Ge x )2 samples were obtained for the compositional range of 0 ≤ x ≤ 0.015 and the lattice parameters monotonically increased with x. In the range of single phases, the electrical conductivity and Seebeck coefficient increased and decreased with increasing x, respectively. The partial substitution of Ge effectively reduced the thermal conductivity to ~80%, which resulted in the increase in the ZT of Cr(Si1− x Ge x )2 samples from 0.16 (x = 0) to 0.25 (x = 0.015) at 600 K. From the results of first-principles calculation for transport properties, it can be concluded that the origin of the increase in electrical conductivity and the decrease in Seebeck coefficient of Ge-substituted samples is the decrease in carrier effective mass.

Impurity Effects on the Electronic State in the Undoped (Ce-free) Superconductor T′-La1.8Eu0.2CuO4 Studied by Muon Spin Relaxation

To investigate the electronic state in the undoped (Ce-free) superconductor T′-La1.8Eu0.2CuO4 (T′-LECO), we have performed muon spin relaxation measurements using impurity-substituted superconducti...

Search for New Superconductors by Soft-Chemical Techniques

Our Recent Works on the Search for New Superconductors Using Soft-Chemical Techniques Are Reviewed. We Have Found New Mg-Intercalated Superconductors of MgxZrNCl and MgxHFNCl with Tc ~ 13 K and 25 K, Respectively. Moreover, we Have Succeeded in Preparing Bulk Samples of Hole-Doped (La,Ca)2CuO4 with the Nd2CuO4-Type, Namely, the so-Called T’-Type Structure by Reduction of (La,Ca)2CuO4 with the K2NiF4-Type, Namely, the so-Called T-Type Structure Using CaH2 at a Temperature as Low as 300°C and Subsequent Oxidation. This Result Is in Contrast with the Synthesis of (La,Ca)2CuO4 with the K2NiF4-Type Structure by the Conventional Solid-State Reaction Method at a Temperature as High as ~1000°C.