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New superconductors discovered in the Akimitsu laboratory are reviewed here. These materials can be categorized into two groups:1) Cu-oxide superconductors.1-1 Cu-oxide system having CuO2 planes.1-2 Ladder lattice superconductor.2) Exploration of new metal-based superconductors.2-1 MgB2 and its application.2-2 Y2C3.2-3 Carrier-doped wide-gap semiconductors.2-4 New superconductor with a cage-type structure: R5T6Sn18 (R = Sc, Y, Lu; T = Rh, Ir).Finally, all of the new superconductors discovered in our laboratory are summarized. The outlook for the high-Tc superconductors and our present work are also described.

The temperature dependence of the resistivity ( ρ ) of Ag-doped Bi 2 Se 3 (Ag x Bi 2−x Se 3 ) shows insulating behavior above 35 K, but below 35 K, ρ suddenly decreases with decreasing temperature, in contrast to the metallic behavior for non-doped Bi 2 Se 3 at 1.5–300 K. This significant change in transport properties from metallic behavior clearly shows that the Ag doping of Bi 2 Se 3 can effectively tune the Fermi level downward. The Hall effect measurement shows that carrier is still electron in Ag x Bi 2−x Se 3 and the electron density changes with temperature to reasonably explain the transport properties. Furthermore, the positive gating of Ag x Bi 2−x Se 3 provides metallic behavior that is similar to that of non-doped Bi 2 Se 3 , indicating a successful upward tuning of the Fermi level.

In the light of the tremendous progress that has been made in raising the transition temperature of the copper oxide superconductors (for a review, see ref. 1 ), it is natural to wonder how high the transition temperature, T c , can be pushed in other classes of materials. At present, the highest reported values of T c for non-copper-oxide bulk superconductivity are 33 K in electron-doped Cs x Rb y C 60 (ref. 2 ), and 30 K in Ba 1- x K x BiO 3 (ref. 3 ). (Hole-doped C 60 was recently found 4 to be superconducting with a T c as high as 52 K, although the nature of the experiment meant that the supercurrents were confined to the surface of the C 60 crystal, rather than probing the bulk.) Here we report the discovery of bulk superconductivity in magnesium diboride, MgB 2 . Magnetization and resistivity measurements establish a transition temperature of 39 K, which we believe to be the highest yet determined for a non-copper-oxide bulk superconductor.

Doping a typical topological insulator, Bi 2 Se 3 , with Ag impurity causes a semiconductor–metal (S-M) transition at 35 K. To deepen the understanding of this phenomenon, structural and transport properties of Ag-doped Bi 2 Se 3 were studied. Single-crystal X-ray diffraction (SC-XRD) showed no structural transitions but slight shrinkage of the lattice, indicating no structural origin of the transition. To better understand electronic properties of Ag-doped Bi 2 Se 3 , extended analyses of Hall effect and electric-field effect were carried out. Hall effect measurements revealed that the reduction of resistance was accompanied by increases in not only carrier density but carrier mobility. The field-effect mobility is different for positive and negative gate voltages, indicating that the E F is located at around the bottom of the bulk conduction band (BCB) and that the carrier mobility in the bulk is larger than that at the bottom surface at all temperatures. The pinning of the E F at the BCB is found to be a key issue to induce the S-M transition, because the transition can be caused by depinning of the E F or the crossover between the bulk and the top surface transport.

New superconductors discovered in the Akimitsu laboratory are reviewed here. These materials can be categorized into two groups:1) Cu-oxide superconductors.1-1 Cu-oxide system having CuO planes.1-2 Ladder lattice superconductor.2) Exploration of new metal-based superconductors.2-1 MgB and its application.2-2 Y C .2-3 Carrier-doped wide-gap semiconductors.2-4 New superconductor with a cage-type structure: R T Sn (R = Sc, Y, Lu; T = Rh, Ir).Finally, all of the new superconductors discovered in our laboratory are summarized. The outlook for the high-T superconductors and our present work are also described.

We report a crystallization technique to make single crystals with homochiral crystallographic domain in CsCuCl3. By adapting crystallization technique with stirring, we succeeded in obtaining the mm-ordered enantiopure single crystals. The samples obtained without the stirring crystallization technique had zigzag shape, suggesting many crystalline nuclei combined during the crystallization process. The samples done with the stirring crystallization technique formed quart-crystal-like shape, suggesting only one crystalline nucleus grew. Absolute structure analysis using X-ray diffraction showed that the samples grown only by our technique had a homochiral crystalline domain.

We examined the low temperature magnetic properties of a single crystal of TmTi2Al20, a hole analogue of PrTi2Al20, by means of specific heat C(T) and magnetization M(T, H) measurements. At low field, an anomaly that can be ascribed to an antiferromagnetic ordering is observed in M(T) at TN 0.7 K. C(T) shows a very sharp λ-type anomaly at 0.7 K, confirming the phase transition. From the calculated magnetic entropy S(T), we found that the ground state of TmTi2Al20 is likely to be a magnetic Γ5 triplet in the J 6 multiplet.

We report the crystal structure parameters of clathrate-type copper-oxide Cu6O8YCl, which belongs to the Cu6O8MX (M cation, X anion) family, investigated by the powder neutron diffraction technique and the Rietveld method. The crystal structure of Cu6O8YCl was identified to be a cubic structure with the space group Fm m. The Cl atom is located at the 4a site, which is the center position of the Cu6O8 cage, and the Y atom occupies the 4b site in the cuboid space. The Y and Cl atoms have anomalously large atomic displacement parameters (Uiso) at room temperature, which suggests that Y and Cl atoms respectively present in a cuboid and in a Cu6O8 cage exhibit intensive motion such as rattling, as recently observed for clathrate compounds.

The magnetic states in the novel K2NiF4-type quasi-2-D spin-orbit-induced Mott insulator Ba2IrO4 was studied using magnetic susceptibility, muon spin rotation (µSR), and high-pressure resistivity measurements. In Ba2IrO4, a Néel ordered state was found to be stabilized at temperatures below TN (Néel temperature) ∼ 240 K. The critical exponent β ∼ 0.18 suggests a quasi-2-D Heisenberg antiferromagnetic state on the IrO2 planar square lattice. The magnetic moment is significantly reduced to ∼0.34 µB/Ir-atom, probably due to a low-dimensional quantum spin fluctuation with a large magnetic intra-plane correlation |J|. A non-Fermi liquid-like behavior with carrier scattering due to the spin fluctuation was observed in the metallic state under high pressures of more than 13.8 GPa.