Hiroshi Eisaki

Hard X-ray photoemission studies are carried out to evaluate the electronic structure of iron-based superconductors BaFe₂(As_1−xP_x)₂ in a wide range of the P content 0 ≤ x ≤ 0.61. As x increases, the Fe 2p_3/2 and Ba 3d_5/2 core levels linearly shift to the higher and lower binding energy sides, respectively, while the anion core levels show no appreciable shifts. These results can be explained in terms of the Madelung potential and the change of the hybridization between the Fe 3d and the anion orbitals. The increase of the hybridization with x leads to an increase of the band width, which is consistent with the spectrum shift near the Fermi level.

The Ba-Ir-Ge ternary compounds BaIr₂Ge₇ and Ba₃Ir₄Ge₁₆ exhibit superconductivity (SC) at 2.5 and 5.2 K, respectively. Detailed single-crystal structural analysis revealed that these compounds share unique quasi-two-dimensional networks composed of crown-shaped Ge rings that accommodate Ba atoms at the center, referred to as “edge-shared crown-shaped BaGe₁₆ polyhedra”. The layered Ba-Ge network yielded a modest anisotropy of 1.3–1.4 in the upper critical field, which is in good agreement with the band structure calculations. The Ba-Ge structural unit is similar to cage structures seen in various clathrates in which the anharmonic vibration of the central atoms, the so-called “rattling” behavior, brings about strong-coupling SC. However, each Ba-Ge unit is relatively small compared to these materials, which likely excludes the possibility of unconventional SC.

We have investigated orbital character and electron correlation effects on Fermi surfaces in the hole-overdoped iron pnictide superconductor KFe₂As₂, which shows a low T_c of ~4 K, by angle-resolved photoemission spectroscopy. From the polarization-dependence of the ARPES spectra, we have determined the orbital character of each Fermi surface. Electron mass renormalization of each band is quantitatively consistent with de Haas-van Alphen results. The outer β and middle ζ Fermi surfaces show large renormalization factor of m*/m_b ~6–7, while the inner α Fermi surface has a smaller factor m*/m_b ~2. Middle hole Fermi surface ζ has strong three-dimensionality compared to other Fermi surfaces, indicating the d_3z2–r2 orbital character, which may be related to the “octet-line nodes” recently observed by laser ARPES. The observed orbital-dependent mass renormalization would give constraints on the pairing mechanism with line nodes of this system.

Transverse-field muon spin rotation measurements were performed on the iron-based superconductor, KFe ₂ As ₂ , in order to investigate the superconducting gap mechanism. We found that the temperature dependence of the magnetic penetration depth λ shows multiple gap-like behavior and that λ depends on the field. These results suggest the existence of quasiparticle excitations outside the vortices.

We report ⁷⁵ As-nuclear quadrupole resonance (NQR) and ⁵⁷ Fe-nuclear magnetic resonance (NMR) studies on heavily electron-doped LaFeAsO-based compounds with T _c =5 and 8 K. The high quality of the samples was ensured by both narrow ⁵⁷ Fe-NMR and ⁷⁵ As-NQR spectra despite in the heavily overdoped state. Nuclear spin relaxation rate (1/ T ₁ ) measurements by ⁷⁵ As-NQR at zero field has revealed that a coherence peak just below T _c partially recovers in heavily electron-overdoped regimes where the nesting condition of hole and electron Fermi surfaces (FSs) becomes significantly worse. This result suggests that the interband scattering between the hole and electron FSs is suppressed by the heavily electron-overdoping because the hole FSs become smaller.

Phonons in single crystals of PrFeAsO _{1- y} are investigated using high-resolution inelastic x-ray scattering (IXS) and ab-initio pseudo-potential calculations. Extensive measurements show some changes in phonon spectra with temperature and/or doping. However, comparing our measurements with several ab-initio calculations we find that the experimentally observed changes are much smaller than differences between the experimental data and the calculations. Here we add to our previous work [Fukuda et al. : Phys. Rev. B 84 (2011) 064504] by showing more of the IXS spectra and details of some of the calculations. Agreement is improved if magnetism is included in the calculations, but still large disagreement remains. Several modifications of the calculations are considered and the best agreement is obtained from modifying the non-magnetic model by softening only the in-plane nearest-neighbor Fe–As force constant.

The Comment insists on the following: in our model it is assumed that the effective interactions have specific energy ranges within the single band with a cutoff at \omega_1 for the phononic part and a range from \omega_1 to \omega_2 in the AF channel. Our reply is that we assume that V_i(k,k')\neq 0 if |\xi_k|<\omega_i and |\xi_{k'}|<\omega_i, and otherwise V_i(k,k')= 0 (i=1,2), as stated in our paper. This is the model of BCS type with two attractive interactions, and this assumption is the characteristic of the BCS approximation. The claim "the integration limits have been modified such that the AF channel mediated pairing sets in where the ph-channel pairing terminates and is limited at an energy given by \omega_j=\omega_{AF}" in the Comment is wrong. We describe the model and the method to solve the gap equation in more detail

We report an ⁷⁵ As-nuclear quadrupole resonance (NQR) study on heavily electron-doped LaFeAsO _{1- x} F _x (La1111) with T _c =5 K. The measurement of the nuclear spin relaxation rate (1/ T ₁ ) has revealed that a Hebel–Slichter (HS) peak partially recovers in heavily electron-overdoped regimes where the nesting condition of hole and electron Fermi surfaces (FSs) becomes significantly worse. This is in contrast to previous results reported for optimally doped La1111 with T _c =28 K that there is a lack of the HS peak. These results indicate that the interband scattering between the hole and electron FSs is strongly suppressed by an almost vanishing hole FS because of the heavy electron-overdoping. Our findings strongly suggest that the sign reversal of the gap functions on different FSs, that is, the s _± -wave state, is realized in La1111 compounds. We remark that interband scattering on well-nested FSs is essential for stabilizing the s _± -wave state and enhancing T _c up to 28 K in LaFeAsO-based superconductors.