H. Kotegawa

Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: an Sb nuclear quadrupole resonance study.

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with T(c)=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate, 1/T(1), and NQR frequency unravel a low-lying crystal-electric-field splitting below T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.

Evidence for strong-coupling s-wave superconductivity in MgB2: (11)B NMR Study.

We have investigated a gap structure in a newly discovered superconductor, MgB2, through measurement of the (11)B nuclear spin-lattice relaxation rate, (11)(1/T(1)). (11)(1/T(1)) is proportional to the temperature (T) in the normal state, and decreases exponentially in the superconducting (SC) state, revealing a tiny coherence peak just below T(c). The T dependence of 1/T(1) in the SC state can be accounted for by an s-wave SC model with a large gap size of 2Delta/k(B)T(c) approximately 5 which suggests it is in a strong-coupling regime.

NMR study of carrier distribution and superconductivity in multilayered high-Tc cuprates

Abstract We report 63 Cu-Knight shift measurement on multilayered high- T c cuprate oxides that include inequivalent outer (OP) and inner (IP) CuO 2 planes in a unit cell with number of planes n =3–5. Using an experimental relation between the spin part of Knight shift ( K s ) and the carrier concentration ( N h ) reported in n =1 and 2 cuprates, the local carrier concentrations N h (OP) in the OP and N h (IP) in the IP have been deduced. We have found that N h (OP) is larger than N h (IP) in all the systems. The difference in the doping level increases as total-carrier content δ and n increase. Imbalance between N h (OP) and N h (IP) is suggested to be caused by a mechanism that the electrostatic potential associated with the apical oxygen has more attraction for holes in the OP than in the IP. It is also suggested that T c of Hg1223 ( n =3) is the highest ( T c =133xa0K) to date, due to N h (IP) optimized to N h,optimum ∼0.2. From the fact that N h (OP)> N h,optimum , we propose that if N h (OP) could also be optimized in addition to optimized N h (IP), T c might be raised higher than 133xa0K.

Sb-NQR probe for superconducting properties in the Pr-based filled-skutterudite compound PrRu4Sb12

We report the electronic and superconducting properties in the Pr-based filled-skutterudite superconductor PrRu 4 Sb 1 2 with T c =1.3 K via the measurements of nuclear-quadrupole-resonance frequency ν Q and nuclear-spin-lattice-relaxation time T 1 of Sb nuclei. The temperature dependence of v Q has revealed the energy scheme of Pr 3 + crystal electric field that is consistent with an energy separation Δ C E F ∼70 K between the ground state and the first-excited state. In the normal state, the Korringa relation of (1/T 1 T) P r =const is valid, with [( 1/T 1 T) p r /(1/T 1 T) L a ] 1 / 2 ∼1.44, where (1/T 1 T) L a is for LaRu 4 Sb 1 2 . These results are understood in terms of a conventional Fermi-liquid picture in which the Pr-4f 2 state derives neither magnetic nor quadrupolar degrees of freedom at low temperatures. In the superconducting state, 1/T 1 shows a distinct coherence peak just below T c , followed by an exponential decrease with a value of 2Δ/k B T c =3.1. These results demonstrate that PrRu 4 Sb 1 2 is a typical weak-coupling s-wave superconductor, in strong contrast with the heavy-fermion superconductor PrOs 4 Sb 1 2 that is in an unconventional strong coupling regime. The present study on PrRu 4 Sb 1 2 highlights that the Pr-4f 2 -derived nonmagnetic doublet plays a key role in the unconventional electronic and superconducting properties in PrOs 4 Sb 1 2 .

Carrier distribution and superconductivity in multilayer high-Tc cuprates proved by 63Cu NMR

The carrier distribution among the inequivalent CuO2planes in multilayer cuprates is investigated by63Cu Knight shift measurement with varying the carrier concentration, δ and the number of CuO2plane in a unit cell, n. We found that 70 (60)% of total carriers are doped selectively into two outer CuO2planes in n = 3 (4). This indicates resultantly that the inhomogeneity of carrier distribution increases with increasing δ and n in multilayer cuprates.

Evidence for high-frequency phonon mediated S-wave superconductivity: 11B NMR study of Al-doped MgB2

We report

Evidence for strong-coupling s-wave superconductivity in MgB2: 11B-NMR study of MgB2 and the related materials

^{11}

Coexistence of ferromagnetism and superconductivity in magnetic superconductor RuSr2YCu2O8 revealed by 63Cu-NMR

B-NMR study on Al-doped MgB

Microscopic coexistence of antiferromagnetism and superconductivity in HgBa2Ca4Cu5Oy:Cu-NMR study

_2

Unconventional superconducting characteristics in the multilayered high-Tc cuprate (Cu0.6C0.4)Ba2Ca3Cu4O12+y probed by 63Cu NMR

that addresses a possible mechanism for a high superconducting (SC) transition temperature (