Hideki Tou

Anion height dependence of Tc for the Fe-based superconductor

We have established a plot of the anion height dependence of superconducting transition temperature Tc for the typical Fe-based superconductors. The plot showed a symmetric curve with a peak around 1.38??. Both data at ambient pressure and under high pressure obeyed the unique curve. This plot will be one of the key strategies for both understanding the mechanism of Fe-based superconductivity and searching for the new Fe-based superconductors with higher? Tc.

Precise Pressure Dependence of the Superconducting Transition Temperature of FeSe: Resistivity and 77Se-NMR Study

We report the precise pressure dependence of FeSe from a resistivity measurement up to 4.15 GPa. Superconducting transition temperature ( T c ) increases sensitively under pressure, but shows a plateau between 0.5–1.5 GPa. The maximum T c , which is determined by zero resistance, is 21 K at approximately 3.5 GPa. The onset value reaches ∼37 K at 4.15 GPa. We also measure the nuclear spin–lattice relaxation rate 1/ T 1 under pressure using 77 Se-NMR measurement. 1/ T 1 shows that bulk superconductivity is realized in the zero-resistance state. The pressure dependence of 1/ T 1 T just above T c shows a plateau as well as the pressure dependence of T c , which gives clear evidence of the close relationship between 1/ T 1 T and T c . Spin fluctuations are suggested to contribute to the mechanism of superconductivity.

Abrupt Emergence of Pressure-Induced Superconductivity of 34 K in SrFe2As2: A Resistivity Study under Pressure

We report resistivity measurement under pressure in single crystals of SrFe 2 As 2 , which is one of the parent materials of Fe-based superconductors. The structural and antiferromagnetic (AFM) transition of T 0 = 198 K at ambient pressure is suppressed under pressure, and the ordered phase disappears above P c ∼3.6–3.7 GPa. Superconductivity with a sharp transition appears accompanied by the suppression of the AFM state. T c exhibits a maximum of 34.1 K, which is realized close to the phase boundary at P c . This T c is the highest among those of the stoichiometric Fe-based superconductors.

d-Wave Superconductivity in Antiferromagnetic Heavy-Fermion Compound UPd2Al3 –Evidence from27Al NMR/NQR Studies–

The nuclear-spin-lattice relaxation rate, 27 (1/ T 1 ), of the high-quality polycrystalline UPd 2 Al 3 with a record T c =2 K and the narrowest Al NQR linewidth to date of 12 kHz has been measured in zero magnetic field by the NQR technique. 27 (1/ T 1 ) below T c has been found to obey the T 3 - law at least down to 0.18 K, giving strong evidence that the energy gap vanishes along lines on the Fermi surface. From the analysis of 27 Al Knight shift below T c , it is shown that the anisotropic residual Knight shift originates from the f -electron susceptibility in the antiferromagnetic state, whereas the isotropic reduction of the shift below T c is due to a singlet pairing nature of the superconductivity. Both the results of 27 (1/ T 1 ) and 27 Al Knight shift in UPd 2 Al 3 can be reproduced consistently by a d-wave pairing model characterized by lines of vanishing gap on the Fermi surface.

Evidence for Unconventional Superconductivity in Arsenic-Free Iron-Based Superconductor FeSe: A 77Se-NMR Study

We report the results of 77 Se-nuclear magnetic resonance (NMR) in α-FeSe, which exhibits a similar crystal structure to the LaFeAsO 1- x F x superconductor and shows superconductivity at 8 K. The nuclear-spin lattice relaxation rate 1/ T 1 shows T 3 behavior below the superconducting transition temperature T c without a coherence peak. The T 1 T = const. behavior, indicative of the Fermi liquid state, can be seen in a wide temperature range above T c . The superconductivity in α-FeSe is also an unconventional one as well as LaFeAsO 1- x F x and related materials. The FeAs layer is not essential for the occurrence of the unconventional superconductivity.

Superconductivity in a new iron pnictide oxide (Fe2As2)(Sr4(Mg, Ti)2O6)

We have discovered a new iron pnictide oxide superconductor (Fe2As2)(Sr4(Mg, Ti)2O6). This material is isostructural with (Fe2As2)(Sr4M2O6) (M = Sc, Cr, V), which were found in previous studies. The structure of this compound is tetragonal with a space group of P4/nmm and consists of the anti-fluorite type FeAs layer and perovskite-type block layer. The lattice constants are a = 3.935?? and c = 15.952?? for (Fe2As2)(Sr4MgTiO6). Bulk superconductivity with a Tc(onset) of ~ 26?K was observed for a partially Co-substituted sample. Moreover, Co-free and Ti-rich samples exhibited higher Tc(onset)s above 35?K, which were further enhanced by applying high pressures up to ~ 43?K.

Anomaly of X-ray Diffraction Profile in Single-Walled Carbon Nanotubes

X-ray diffraction (XRD) studies on single-walled carbon nanotube (SWNT) samples prepared by the arc-discharge method were reported. The XRD profile was basically explained to be a result of triangular packing of SWNTs with a lattice constant of 17.1 A and an average nanotube radius of 7.1 A. We found an anomalous change in XRD profiles before and after heat-treatment of the SWNT samples in air at ~350°C. Combined with gravimetric measurements and resistivity measurements, a detailed simulation of the XRD profiles showed that air (oxygen, and/or nitrogen and/or water) can be condensed inside the SWNTs.

Superconductivity of 2.2 K under Pressure in Helimagnet CrAs

We report resistivity measurements of the helimagnet CrAs under pressures. The helimagnetic transition with TN ∼ 265 K at ambient pressure is completely suppressed above a critical pressure of Pc ∼ 0.7 GPa, and superconductivity is observed at ∼2.2 K for zero resistance, which exists in a wide pressure range extending beyond 3 GPa. Both the upper critical field Hc2 and the coefficient A in the resistivity increase toward Pc, suggesting that the superconductivity of CrAs is mediated by electronic correlations enhanced in the vicinity of the helimagnetic phase.

Effect of Uniaxial Stress for Pressure-Induced Superconductor SrFe2As2

We report that the pressure–temperature phase diagram of single-crystalline SrFe 2 As 2 is easily affected by the hydrostaticity of a pressure-transmitting medium. For all of the three mediums we used, superconductivity with zero resistance appears, accompanied by the suppression of an antiferromagnetic (orthorhombic) phase, but the critical pressure P c was found to depend on the type of medium. P c was estimated to be 4.4 GPa under almost hydrostatic condition, but it decreased to 3.4–3.7 GPa with the use of the medium already solidified at room temperature. The uniaxial stress along the c -axis is suggested to aid in the suppression of the antiferromagnetic (orthorhombic) phase. The pressure effect of BaFe 2 As 2 is also reported.

C70 Molecular Stumbling inside Single-Walled Carbon Nanotubes

We report the structural study of C 70 -one-dimensional (1D) crystal formed inside single-walled carbon nanotubes (SWNTs). X-ray diffraction measurements were performed between 100 K and 999 K on C...