Hijiri Kito

In ZrP_1.25Se_0.75, the superconducting transition temperature T _c is increased from 5.8 K to 7.6 K when Sc atoms are partially substituted for Zr atoms having almost the same atomic radius as Sc atoms. Transport measurements show that holes are doped with substituting Sc atoms. The density of states in (Zr, Sc)P_1.25Se_0.75 is derived from band structure calculations and its implication in the doping behaviour and the increase of T _c is discussed.

It is shown that the maximum superconducting transition temperature, T c for ZrP 1.25 Se 0.75 is increased from 5.8 K to 7.2 K by partially substituting a non-magnetic rare earth atom Lu for Zr and on the other hand is decreased by substituting other non-magnetic rare earth atoms Y and La. Obviously, T c is raised when lattice constants in the a-axis and the c-axis increase with replacing Zr. Its implications for further increasing T c are discussed.

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 on the observation of high- T _c superconductivity (SC) emerging with the background of an antiferromagnetic (AFM) order in the five-layered cuprate Ba ₂ Ca ₄ Cu ₅ O ₁₀ (F,O) ₂ through ¹⁹ F-NMR and zero-field Cu-NMR studies. The measurements of spectrum and nuclear spin–lattice relaxation rates ¹⁹ (1/ T ₁ ) of ¹⁹ F-NMR give convincing evidence for the AFM order taking place below T _N = 175 K and for the onset of SC below T _c = 52 K, hence both coexisting. The zero-field Cu-NMR study has revealed that AFM moments at Cu sites are 0.14 µ _B at outer CuO ₂ layers and 0.20 µ _B at inner ones. We remark that an intimate coupling exists between the AFM state and the SC order parameter below T _c = 52 K; the spin alignment in the AFM state is presumably changed in the SC-AFM mixed state.

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.

Using a cubic-anvil high-pressure apparatus, ternary iridium phosphides MIrP (M=Ti, Zr, Nb, Mo) and MgRuP have been prepared by reaction of stoichiometric amounts of each metal and phosphide powders at around 2 Gpa and above 1523 K for the first time. The structure of these compounds prepared at high-pressure has been characterized by X-ray powder diffraction. Diffraction lines of these compounds are assigned by the index of the Co2Si-type structure. The electrical resistivity and the d.c magnetic susceptibility of MIrP (M=Ti, Zr, Nb, Mo) have measured at low temperatures. Unfortunately, no superconducting transition for MIrP (M=Ti, Zr, Nb, Mo) and MgRuP are observed down to 2 K.

Using a cubic-anvil high-pressure apparatus, ternary iridium phosphides MIrP (M=Ti, Zr, Nb, Mo) and MgRuP have been prepared by reaction of stoichiometric amounts of each metal and phosphide powders at around 2 Gpa and above 1523 K for the first time. The structure of these compounds prepared at high-pressure has been characterized by X-ray powder diffraction. Diffraction lines of these compounds are assigned by the index of the Co₂Si-type structure. The electrical resistivity and the d.c magnetic susceptibility of MIrP (M=Ti, Zr, Nb, Mo) have measured at low temperatures. Unfortunately, no superconducting transition for MIrP (M=Ti, Zr, Nb, Mo) and MgRuP are observed down to 2 K.

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 systematic ⁵⁷ Fe-NMR and ⁷⁵ As-NMR/NQR studies on an underdoped sample ( T _c =20 K), an optimally doped sample ( T _c =28 K), and an overdoped sample ( T _c =22 K) of oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO _{1- y} . A microscopic phase separation between superconducting domains and magnetic domains is shown to take place in the underdoped sample, indicating a local inhomogeneity in association with the density distribution of oxygen deficiencies. As a result, 1/ T ₁ T in the normal state of the superconducting domain decreases significantly upon cooling at both the Fe and As sites regardless of the electron-doping level in LaFeAsO _{1- y} . On the basis of this result, we claim that 1/ T ₁ T is not always enhanced by antiferromagnetic fluctuations close to an antiferromagnetic phase in the underdoped superconducting sample. This contrasts with the behavior in hole-doped Ba _0.6 K _0.4 Fe ₂ As ₂ ( T _c =38 K), which exhibits a significant increase in 1/ T ₁ T upon cooling. We remark that the crucial difference between the normal-state properties of LaFeAsO _{1- y} and Ba _0.6 K _0.4 Fe ₂ As ₂ originates from the fact that the relevant Fermi surface topologies are differently modified depending on whether electrons or holes are doped into the FeAs layers.