Yasuyuki Nakajima

The critical current density, J _c , in high-quality single crystals of iron-based superconductors (IBS) has reached larger than 10 ⁵ A/cm ² at 4.2 K. This large value of J _c urged the development of potential superconducting wires or tapes based on IBS. We have fabricated superconducting wires and tapes of IBS using several techniques and made an extensive characterization by structural, transport, and magnetic measurements. The powder-in-tube method is used for (Ba, K)Fe ₂ As ₂ and Ba(Fe, Co) ₂ As ₂ wires and the diffusion method is used for FeSe tape. Intra- and inter-granular J _c 's have been evaluated by direct transport and indirect magnetization measurements as well as using magneto-optical (MO) imaging. The intergranular J _c 's at 4.2 K under self-field for (Ba, K)Fe ₂ As ₂ and Ba(Fe, Co) ₂ As ₂ wires and FeSe tapes are 1.3 × 10 ⁴ A/cm ² , 3.8 × 10 ³ A/cm ² , and 600 A/cm ² , respectively. On the other hand, MO images have demonstrated that the intragranular J _c is at least one order of magnitude larger than the intergranular J _c , which is consistent with the magnetic evaluation.

We report the thermal conductivity in Co-doped BaFe₂As₂ at under-, optimal- and over-doping levels. We find the enhancement of the thermal conductivity divided by temperature κ/T in the superconducting state at all doping levels, which is strikingly similar to those observed in the strongly correlated superconductors, such as high-T_c cuprates and heavy fermion systems. We also find that the enhancement of κ/T is strongly suppressed by applying magnetic fields. The analysis based on the vortex-scattering model unveils that the enhancement of κ/T originates from the steep increase of the quasi-particle mean-free path possibly due to the reduction of the inelastic scattering in the superconducting state. Strong enhancement of κ_e/T of over-doping level may suggest that quasiparticle population still remains even at low temperatures.

We have performed an angle-resolved photoemission spectroscopy (ARPES) study of the undoped and electron-doped iron pnictides BaFe _{2- x} Co _x As ₂ (Ba122) ( x = 0, 0.14) and studied the Fermi surfaces (FSs) and band dispersions near the Fermi level. The FS sheets we observed are consistent with the shrinkage of the hole-like pockets around the Brillouin Zone (BZ) center and the expansion of the electron pockets around the BZ corner in the electron-doped compound as compared to the undoped parent compound. Band dispersions and FSs around the BZ center strongly depend on the photon energy, indicating a three-dimensional (3D) electronic structure. This observation suggests that antiferromagnetism and superconductivity in the pnictides have to be described in terms of an orbital-dependent 3D electronic structure, where FS nesting is not necessarily strong.

We report the low-temperature specific heat of ternary transition-metal silicide superconductor Sc₅Ir₄Si₁₀. Our detailed specific-heat studies reveal that the temperature dependence of specific heat in Sc₅Ir₄Si₁₀ deviates from that for a BCS weak-coupling superconductor and can be explained by the phenomenological two-gap model. These results suggest the existence of two distinct superconducting gaps in Sc₅Ir₄Si₁₀.

We report the real-space observation of magnetic domains in the ac-plane at the metamagnetic transition in a triple-layer ruthenate Sr₄Ru₃O₁₀ by magneto-optical imaging and scanning Hall probe measurements. We find anisotropic magnetic domain structures with weak c-axis correlation at low fields. The magnetic inhomogeneities are removed above the metamagnetic transition field. These changes of magnetic structure support the phase-separation scenario for the ultra-sharp steps in magnetoresistance.