Hidekazu Mukuda

Spin-triplet superconductivity in Sr2RuO4 identified by 17O Knight shift

Superconductivity — one of the best understood many-body problems in physics — has again become a challenge following the discovery of unconventional superconducting materials: these include heavy-fermion, organic and the high-transition-temperature copper oxide superconductors. In conventional superconductors, the electrons form superconducting Cooper pairs in a spin-singlet state, which has zero total spin (S = 0). In principle, Cooper pairs can also form in a spin-triplet state (S = 1), analogous to the spin-triplet ‘p-wave’ state of paired neutral fermions in superfluid 3He (ref. 4). At present, the heavy-fermion compound UPt3 is the only known spin-triplet superconductor,, although the layered oxide superconductor Sr2RuO4 (ref. 7) is believed, on theoretical grounds, to be a promising candidate. The most direct means of identifying the spin state of Cooper pairs is from measurements of their spin susceptibility, which can be determined by the Knight shift (as probed by nuclear magnetic resonance (NMR)). Here we report Knight-shift measurements of Sr2RuO2 using 17O NMR. Our results show no change in spin susceptibility on passing through the superconducting transition temperature, which provides the definitive identification of Sr2RuO4 as a spin-triplet superconductor.

Uniform mixing of high-Tc superconductivity and antiferromagnetism on a single CuO2 plane of a Hg-based five-layered cuprate.

We report a site selective Cu-NMR study on underdoped Hg-based five-layered high-Tc cuprate HgBa2Ca4CU5O(12+delta) with a Tc = 72 K. Antiferromagnetism (AFM) has been found to take place at T(N) = 290 K, exhibiting a large antiferromagnetic moment of 0.67-0.69 microB at three inner planes (IP). This value is comparable to the values reported for nondoped cuprates, suggesting that the IP may be in a nearly nondoped regime. Most surprisingly, the AFM order is also detected with M(AFM)(OP) = 0.1 microB even at two outer planes (OP) that are responsible for the onset of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on a microscopic level with the AFM at OPs. This is the first microscopic evidence for the uniform mixed phase of AFM and SC on a single CuO2 plane in a simple environment without any vortex lattice and/or stripe order.

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.

High-Tc Superconductivity and Antiferromagnetism in Multilayered Copper Oxides –A New Paradigm of Superconducting Mechanism–

High-temperature superconductivity (HTSC) in copper oxides emerges on a layered CuO 2 plane when an antiferromagnetic Mott insulator is doped with mobile hole carriers. We review extensive studies of multilayered copper oxides by site-selective nuclear magnetic resonance (NMR), which have uncovered the intrinsic phase diagram of antiferromagnetism (AFM) and HTSC for a disorder-free CuO 2 plane with hole carriers. We present our experimental findings such as the existence of the AFM metallic state in doped Mott insulators, the uniformly mixed phase of AFM and HTSC, and the emergence of d -wave SC with a maximum T c just outside a critical carrier density, at which the AFM moment on a CuO 2 plane disappears. These results can be accounted for by the Mott physics based on the t – J model. The superexchange interaction J in among spins plays a vital role as a glue for Cooper pairs or mobile spin-singlet pairs, in contrast to the phonon-mediated attractive interaction among electrons established in the Bardeen...

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.

Genuine Phase Diagram of Homogeneously Doped CuO2 Plane in High-Tc Cuprate Superconductors

We report a genuine phase diagram for a disorder-free CuO 2 plane based on the precise evaluation of the local hole density ( N h ) by site-selective Cu-NMR studies on five-layered high- T c cuprates. It has been unraveled that (1) the antiferromagnetic metallic state (AFMM) is robust up to N h ≈0.17, (2) the uniformly mixed phase of superconductivity (SC) and AFMM is realized at N h ≤0.17, (3) the tetracritical point for the AFMM/(AFMM+SC)/SC/PM (paramagnetism) phases may be present at N h ≈0.15 and T ≈75 K, (4) T c is maximum close to a quantum critical point (QCP) at which the AFM order collapses, suggesting the intimate relationship between the high- T c SC and the AFM order. The results presented here strongly suggest that the AFM interaction plays the vital role as the glue for the Cooper pairs, which will lead us to a genuine understanding of why the T c of cuprate superconductors is so high.

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...

Microscopic evidence for evolution of superconductivity by effective carrier doping in boron-doped diamond: B11 -NMR study

We have investigated the superconductivity discovered in boron-doped diamonds by means of

Multiband superconductivity in filled-skutterudite compounds (Pr1- xLax)Os4Sb12 : An Sb nuclear-quadrupole-resonance study

^{11}\mathrm{B}\text{\ensuremath{-}}\mathrm{NMR}