arXiv subjects
Publications and source records attributed to Hiroyuki Takeya.
We investigated pressure effects on transition temperature (Tc) of the new iron-based superconductor K0.8Fe2Se2 using a BeCu/NiCrAl hybrid-type clamped piston-cylinder cell. The Tc(onset) was 33K at 0.85 GPa. With increasing pressure, Tc(onset) gradually increased and reached 36.6 K at 2.03 GPa.
We synthesized the new Fe-based superconductor K0.8Fe2Se2 single crystals. The obtained single crystal exhibited a sharp superconducting transition, and the onset and zero-resistivity temperature was estimated to be 33 and 31.8 K, respectively. A high upper critical field of 192 T was obtained. Anisotropy of superconductivity of K0.8Fe2Se2 was ~3.6. Both the high upper critical field and comparably low anisotropy are advantageous for the application under high magnetic field.
We have fabricated the Fe(Se, Te) superconducting wire by a special process based on a powder-in-tube method. The pure Fe tube plays the role of not only the sheath but also the raw material for synthesizing the superconducting phases. We succeeded in observing zero resistivity current on the current-voltage measurements for the Fe(Se, Te) wire. Introduction of the pinning centers and fabricating a multi-core wire will enhance the critical current density for the next step.
We have performed soft x-ray and ultrahigh-resolution laser-excited photoemission measurements on tetragonal FeSe, which was recently identified as a superconductor. Energy dependent study of valence band is compared to band structure calculations and yields a reasonable assignment of partial densities of states. However, the sharp peak near the Fermi level slightly deviates from the calculated energy position, giving rise to the necessity of self-energy correction. We have also performed ultrahigh-resolution laser photoemission experiment on FeSe and observed the suppression of intensity around the Fermi level upon cooling.
We have performed x-ray photoemission spectroscopy on the system of noncentrosymmetric superconductor, Li$_2$(Pd$_x$Pt$_{1-x}$3)B. For Li$_2$Pt$_3$B, we found 2 major peaks with 2 other weak components, and the band calculations were in agreement with the observation. The assignment of valence band features using the calculated partial density of states determined that Pt 5d and B 2p contribute to the density of states at the Fermi level. The effect of antisymmetric spin-orbit coupling on the band structure might have been probed, and the analysis on the effect of Pt incorporation into the system indicates the smooth evolution of electronic structures. We presented the measurements of core levels (Pd 3d, Pt 4f, and B 1s) and discussed the chemical bonding states and electronic structures from them.
The coexistence of weak ferromagnetism and superconductivity in ErNi$_{2}$B$% _{2}$C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth $Δλ(T)$ of high-quality single crystals of ErNi$_{2}$B$_{2}$C. In addition to expected features at the Néel temperature $T_{N}$ = 6.0 K and weak ferromagnetic onset at $T_{WFM}=2.3 $K, $Δλ(T)$ rises to a maximum at $T_{m}=0.45$ K before dropping sharply down to $\sim $0.1 K. We assign the 0.45 K-maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $Δλ(T)$ as a consequence of increasing vortex density, and its subsequent decrease below $T_{m}$ as defect pinning suppresses vortex hopping.