M. Yashima

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.

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.

Enhancement of Superconducting Transition Temperature due to the Strong Antiferromagnetic Spin Fluctuations in the Noncentrosymmetric Heavy-Fermion Superconductor CeIrSi3 : A 29Si NMR Study under Pressure

We report a (29)Si NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi(3) without inversion symmetry. In the SC state at P = 2.7-2.8 GPa, the temperature (T) dependence of the nuclear-spin lattice relaxation rate 1/T(1) below T(c) exhibits a T(3) behavior without any coherence peak just below T(c), revealing the presence of line nodes in the SC gap. In the normal state, 1/T(1) follows a square root T-like behavior, suggesting that the SC emerges under the non-Fermi-liquid state dominated by AFM spin fluctuations enhanced around a quantum critical point. The reason why the maximum T(c) in CeIrSi(3) is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.

Anisotropic Spin Fluctuations in Heavy-Fermion Superconductor CeCoIn5: In-NQR and Co-NMR Studies

We report In-NQR and Co-NMR experiments of CeCoIn 5 that undergoes a superconducting transition with a record high T c = 2.3 K to date among heavy-fermion superconductors. At zero magnetic field, a...

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...

Magnetic Criticality and Unconventional Superconductivity in CeCoIn5: Study of 115In-Nuclear Quadrupole Resonance under Pressure

We report the systematic evolution of the superconducting (SC) characteristics of the heavy-fermion (HF) superconductor CeCoIn 5 via nuclear-quadrupole-resonance (NQR) measurement under pressure ( P ). The application of P significantly suppresses the nuclear spin–lattice relaxation rate 1/ T 1 that is dominated by antiferromagnetic (AFM) spin fluctuations (SFs) specific to a quantum critical point (QCP). It is demonstrated that the marked suppression of AFM SFs leads to a reduction in the SC energy gap or in the coupling strength of the Cooper pair. T c , nevertheless, increases with increasing P due to the increase in HF bandwidth. This is expected to make the lifetime of quasi-particles sufficiently long.

Enhancing the Superconducting Transition Temperature of CeRh~1~-~xIr~xIn~5 due to the Strong-Coupling Effects of Antiferromagnetic Spin Fluctuations: An ^1^1^5In Nuclear Quadrupole Resonance Study

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh(1-x)Ir(x)In(5) via an (115)In nuclear-quadrupole-resonance experiment. The nuclear spin-lattice relaxation rate 1/T(1) has revealed the marked development of AFM spin fluctuations as approaching an AFM ordered state. Concomitantly, the superconducting transition temperature T(c) and the energy gap Delta0 increase drastically from T(c)= 0.4K and 2Delta0/k(B)T(c)=5 in CeIrIn(5) up to T(c) =1.2K and 2Delta0/k(B)T(c) =8.3 in CeRh(0.3)Ir(0.7)In5 , respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the strong-coupling unconventional SC in HF compounds.

Doping Dependence of Normal-State Properties in Iron-Based Oxypnictide Superconductor LaFeAsO1-y Probed by 57Fe-NMR and 75As-NMR/NQR

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 re...

Spin susceptibility of noncentrosymmetric heavy-fermion superconductor CeIrSi3 under pressure: 29Si Knight-shift study on single crystal.

We report 29Si NMR study on a single crystal of the heavy-fermion superconductor CeIrSi3 without an inversion symmetry along the c axis. The 29Si Knight-shift measurements under pressure have revealed that the spin susceptibility for the ab plane decreases slightly below T(c), whereas along the c axis it does not change at all. The result can be accounted for by the spin susceptibility in the superconducting state being dominated by the strong antisymmetric (Rashba-type) spin-orbit interaction that originates from the absence of an inversion center along the c axis and it being much larger than superconducting condensation energy. This is the first observation which exhibits an anisotropy of the spin susceptibility below T(c) in the noncentrosymmetric superconductor dominated by strong Rashba-type spin-orbit interaction.

Multiband Superconductivity in Heavy Fermion Compound CePt3Si without Inversion Symmetry: An NMR Study on a High-Quality Single Crystal

We report on novel superconducting characteristics of the heavy fermion (HF) superconductor CePt 3 Si without inversion symmetry through 195 Pt-NMR study on a single crystal with T c = 0.46 K that is lower than T c ∼0.75 K for polycrystals. We show that the intrinsic superconducting characteristics inherent to CePt 3 Si can be understood in terms of the unconventional strong-coupling state with a line–node gap below T c = 0.46 K. The mystery about the sample dependence of T c is explained by the fact that more or less polycrystals and single crystals inevitably contain some disordered domains, which exhibit a conventional BCS s -wave superconductivity (SC) below 0.8 K. In contrast, the Neel temperature T N ∼2.2 K is present regardless of the quality of samples, revealing that the Fermi surface responsible for SC differ from that for the antiferromagnetic order. These unusual characteristics of CePt 3 Si can be also described by a multiband model; in the homogeneous domains, the coherent HF bands are respo...