Kiichi Miyazawa

75As-NQR/NMR Studies on Oxygen-Deficient Iron-Based Oxypnictide Superconductors LaFeAsO1-y ( y = 0, 0.25, 0.4) and NdFeAsO0.6

We report 75 As-NQR/NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductors LaFeAsO 0.6 ( T c = 28 K) along with the results on LaFeAsO, LaFeAsO 0.75 ( T c = 20 K), and NdFeAsO 0.6 ( T c = 53 K). Nuclear spin–lattice relaxation rate 1/ T 1 of 75 As-NQR at zero field on LaFeAsO 0.6 has revealed a T 3 dependence below T c upon cooling without the coherence peak just below T c , evidencing the unconventional superconducting state with the line–node gap. We have found an intimate relationship between the nuclear quadrupole frequency ν Q of 75 As and T c for four samples used in this study. It implies microscopically that the local configuration of Fe and As atoms is significantly related to the T c of the Fe–oxypnictide superconductors, namely, the T c can be enhanced up to 50 K when the local configuration of Fe and As atoms is optimal, in which the band structure may be also optimized through the variation of hybridization between As 4 p orbitals and Fe 3 d orbitals.

Inverse iron isotope effect on the transition temperature of the (Ba,K)Fe2As2 superconductor.

We report that the (Ba,K)Fe(2)As(2) superconductor (transition temperature, T(c) approximately 38 K) has an inverse iron isotope coefficient alpha(Fe) = -0.18(3) (where T(c) approximately M(-alphaFe) and M is the iron isotope mass); i.e., the sample containing the large iron isotope mass depicts a higher T(c). Systematic inverse shifts in T(c) were clearly observed between the samples using three types of Fe isotopes ((54)Fe, natural Fe, and (57)Fe). This indicates the first evidence of the inverse isotope effect in high-T(c) superconductors. This anomalous mass dependence on T(c) implies an exotic coupling mechanism in Fe-based superconductors.

Strong-Coupling Spin-Singlet Superconductivity with Multiple Full Gaps in Hole-Doped Ba0.6K0.4Fe2As2 Probed by 57Fe-NMR

We present 57 Fe-NMR measurements of the novel normal and superconducting-state characteristics of the iron-arsenide superconductor Ba 0.6 K 0.4 Fe 2 As 2 ( T c = 38 K). In the normal state, the measured Knight shift and nuclear spin-lattice relaxation rate (1/ T 1 ) demonstrate the development of wave-number ( q )-dependent spin fluctuations, except at q = 0, which may originate from the nesting across the disconnected Fermi surfaces. In the superconducting state, the spin component in the 57 Fe-Knight shift decreases to almost zero at low temperatures, evidencing a spin-singlet superconducting state. The 57 Fe-1/ T 1 results are totally consistent with a s ± -wave model with multiple full gaps in the strong coupling regime. We demonstrate that the respective 1/ T 1 data for Ba 0.6 K 0.4 Fe 2 As 2 and LaFeAsO 0.7 , which seemingly follow a T 5 - and a T 3 -like behaviors below T c , are consistently explained in terms of this model only by changing the size of the superconducting gap.

75As NMR Study of Hole-Doped Superconductor Ba1-xKxFe2As2 (Tc≃38 K)

We report the 75 As nuclear magnetic resonance (NMR) measurement of the hole-doped superconductor Ba 1- x K x Fe 2 As 2 with different lattice parameters and different superconducting volume fractions ( T c ≃38 K). 75 As-NMR spectra revealed that the magnetically ordered and superconducting phases are microscopically separated. The spin-lattice relaxation rate 1/ T 1 in the normal state reflects the existence of a large two-dimensional antiferromagnetic spin fluctuation. The 1/ T 1 in the superconducting state down to the lowest measurement temperature T varies close to T 3 . In addition, it exhibits no coherence peak just below T c . This shows a T dependence similar to those of other iron pnictides.

Suppression of Magnetic Order by Pressure in BaFe2As2

We performed the dc resistivity and the ZF 75As-NMR measurement of BaFe2As2 under high pressure. The T-P phase diagram of BaFe2As2 determined from resistivity anomalies and the ZF 75As-NMR clearly revealed that the SDW anomaly is quite robust against P.

Spin Fluctuations and Unconventional Superconductivity in the Fe-Based Oxypnictide Superconductor LaFeAsO0.7 Probed by 57Fe-NMR

We report 57 Fe-NMR studies on the oxygen-deficient iron (Fe)-based oxypnictide superconductor LaFeAsO 0.7 ( T c = 28 K) enriched by 57 Fe isotope. In the superconducting state, the spin component of 57 Fe-Knight shift decreases to almost zero at low temperatures, and the nuclear spin–lattice relaxation rate 57 (1/ T 1 ) exhibits a T 3 -like dependence without the coherence peak just below T c , which provide firm evidence of the unconventional superconducting state formed by spin-singlet Cooper pairing. All these events below T c are consistently argued in terms of the extended s ± -wave pairing with a sign reversal of the order parameter among Fermi surfaces. In the normal state, we found the remarkable decrease in 1/ T 1 T upon cooling for both the Fe and As sites, which originates from the decrease in low-energy spectral weight of spin fluctuations over whole q space upon cooling below room temperature. Such behavior has never been observed for other strongly correlated superconductors where an antife...

Superconductivity at 26 K in (Ca1-xNax)Fe2As2

The ternary iron arsenide CaFe2As2 becomes superconducting by hole doping which was achieved by the partial substitution of Ca by Na. We have found bulk superconductivity at 26.0 K in a sample of (Ca1-xNax)Fe2As2 with a nominal composition of x = 0.4. The a-parameter shrank and the c-parameter expanded by the Na substitution. The parent compound CaFe2As2 forms the tetragonal ThCr2Si2-type structure, which is a poor metal and decrease of resistivity started below 160 K, presumably associated with the spin-density wave order. By substituting Ca with Na, holes are introduced to the FeAs layers, which weakens the resistive anomaly and induces the superconductivity.

Absence of an appreciable iron isotope effect on the transition temperature of the optimally doped SmFeAsO(1-y) Superconductor.

We report the iron (Fe) isotope effect on the transition temperature (T(c)) in oxygen-deficient SmFeAsO(1-y), a 50-K-class, Fe-based superconductor. For the optimally doped samples with T(c) = 54  K, a change of the average atomic mass of Fe (M(Fe)) causes a negligibly small shift in T(c), with the Fe isotope coefficient (α(Fe)) as small as -0.024 ± 0.015 (where α(Fe)=-d lnT(c)/dlnM(Fe)). This result contrasts with the finite, inverse isotope shift observed in optimally doped (Ba,K)Fe2As2, indicating that the contribution of the electron-phonon interaction markedly differs between these two Fe-based high-T(c) superconductors.

Two-Dimensional Spin Density Wave State in LaFeAsO

The parent compound of the Fe oxypnictide superconductor, LaFeAsO, has been studied by pulsed neutron powder inelastic scattering measurement. The inelastic scattering intensity along the Q -axis at T =140 K exhibits a prominent asymmetric peak ascribed to the (π,π, l ) magnetic rod, suggesting the two-dimensionality of the spin density wave above the magnetic transition temperature. It persists even in the tetragonal phase up to room temperature, which is well above the magnetic transition temperature, corresponding to two antiferromagnetic sublattices or domains with a strong effective antiferromagnetic interaction J 2 between next-nearest-neighbour Fe magnetic moments in a single Fe square lattice.

Possible hydrogen doping and enhancement of Tc (=35 K) in a LaFeAsO-based superconductor

We report that the incorporation of hydroxide ions (OH)− significantly enhances the superconducting transition temperature (Tc) in the LnFeAsO-based superconductors (Ln1111: Ln=La, Ce, and Pr). For La1111, Tc of the (OH)− incorporated sample synthesized using high-pressure technique becomes 35 K, which is higher by 7 K than the typical optimally doped La1111 superconductors. Similar enhancement in Tc is also observed for Ce1111 and Pr1111. H1-NMR measurement have confirmed the existence of hydrogen atoms in the samples. Accompanying the (OH)− incorporation, the lattice parameters are largely contracted, down to the values which have never been attained by any other dopings/substitutions.