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We report the results of nuclear spin–lattice relaxation time, T 1 of 11 B in MgB 2 , Al-doped MgB 2 and NbB 2 . A T 1 T =constant behavior was observed in the normal state, indicating the absence of strong magnetic correlation. In the superconducting (SC) state, 1/ T 1 shows a tiny coherence peak just below T c and decreases exponentially, demonstrating an s-wave superconductivity. The magnitude of SC gap is estimated as 2 Δ / k B T c ∼5, which is quite larger than the weak-coupling value in the BCS theory, 2 Δ / k B T c =3.5. This rules out a possibility for the multiple SC gaps that was suggested in many literatures. The anisotropy in the upper critical field was also corroborated by the T 1 measurement on a bulk polycrystalline sample. In NbB 2 with a low value of T c =5 K, the T 1 measurement revealed the distinct coherence peak just below T c , followed by an exponential decrease with a magnitude of SC gap 2 Δ / k B T c =3.1. These data are consistent with the weak-coupling BCS theory. As Al 3+ is substituted for Mg 2+ , 1/ T 1 T , which is proportional to the square of the density of states at the Fermi level N ( E F ), decreases. The variation in T c against the relative changes in N ( E F ) deduced from 1/ T 1 T is well fitted in terms of the McMillan equation by assuming a characteristic phonon frequency ω ∼ 700 K and an electron–phonon coupling constant λ ∼ 0.87. Thus obtained values are in good agreement with the values suggested by the theoretical works. The high- T c superconductivity in MgB 2 is shown to occur through the strong coupling with high frequency phonons.

Sr2RuO4 with Tc = 0.7 K with the same layered perovskite structure as La2CuO4 is the exchange-enhanced Pauli paramagnet with the two dimensional (2D) electronic structure. The superconductivity of has been found to be unconventional from the nuclear relaxation behavior exhibiting a sharp decrease with no coherence peak just below Tc and the T1T=constant behavior well below Tc. In comparison with the high-Tc Bi2Sr2CaCu2O8+y (Bi2212) and the 3D analogue of exchange-enhanced Pauli paramagnet CeCo2, the similarity and dissimilarity are presented on the normal and superconducting states.

The existence of local moments in both Zn- and Ni-doped YBa 2 Cu 4 O 8 (Y1248) is confirmed by a Curie-law T -dependence of NMR linewidth and the Cu relaxation at low T . The reduction of Cu-NQR signal intensity in Ni-doped Y1248 suggests that, in contrast to the case of Ni-doped YBa 2 Cu 3 O 7 (Y1237), the Cu spin correlation around Ni is destroyed. This effect may be responsible for the larger reduction of T c by Ni-doping than in Y1237. For Cu sites which are away from an impurity by two lattice constants, 1/ T 1 T continues to increase below T ∗ , at which it has a maximum in the non-doped material. The effect in reducing T ∗ , which is sometimes called the spin gap temperature, is much more pronounced in Zn-doped samples, suggesting that the reduction of T ∗ is mainly due to the induction of magnetic moments in the d x 2 − y 2 orbital, which may modify Cu spin fluctuation.

We report a coexistence of superconductivity and antiferromagnetism in five-layered compound HgBa 2 Ca 4 Cu 5 O y (Hg-1245) with T c =108 K, which is composed of two types of CuO 2 planes in a unit cell, three inner planes (IP) and two outer planes (OP). The Cu-NMR study has revealed that the OP undergoes a superconducting (SC) transition at T c =108 K, whereas three underdoped IPs do an antiferromagnetic (AF) transition below T N ∼60 K with the Cu moments of ∼ 0.3–0.4 μ B . The metallic behavior observed at the IP well below T N points to a possibility that SC and AF order parameters coexist in a single CuO 2 plane.

63 Cu NMR measurements have been carried out in HgBa 2 Ca 2 Cu 3 O 8+δ (Hg1223) with the highest transition temperature, T c = 133 K, to date. The narrow NMR spectrum and the small residual density of states (DOS), N res , at the Fermi level in the superconducting state indicate that the Hg1223 possesses good quality.

We report on a possible onset of magnetism in F-substituted four-layered high- T c compounds Ba 2 Ca 3 Cu 4 O 8 (O 1− y F y ) 2 (2 y = 1.2, 1.4, 1.6 and 2.0) which are composed of two types of CuO 2 planes in a unit cell; two inner planes (IP’s) and two outer planes (OP’s). The Cu NMR study has revealed that the hole density at the OP’s is slightly larger than that at the IP’s, and a total carrier density decreases as F content increases. The F NMR study has probed the antiferromagnetic order below T N ≃ 100 K in 0234F(2.0) which undergoes a superconducting transition below T c = 55 K. This result gives evidence for the coexistence of antiferromagnetism and superconductivity even in four-layered high- T c cuprates as well as for five-layered Hg-based multilayered high- T c cuprates.

We have studied the chemical-pressure effect on the Neel temperature ( T N ) due to the substitution of Ir for Rh in the layered heavy fermion compound CeRhIn 5 by nuclear quadrupolar resonance (NQR) technique. We find that T N increases slightly upon replacing Ir for Rh up to the Ir content of 0.45. This feature resembles that in hydrostatically pressurized CeRhIn 5 below 1.0 GPa where T N shows a similar dependence against pressure.

We have studied the ground state of the overdoped single-layered cuprate superconductor Bi 2.1 Sr 1.9 CuO 6+ δ ( T c =8 K) by applying a strong magnetic field of 15.3 T to suppress superconductivity. In the normal state down to T =1.6 K induced by the field, 63 Cu nuclear spin–lattice relaxation rate (1/ T 1 ) is found to obey a T 1 T =const. relation, indicating that the ground state in the overdoped regime is a Fermi liquid state. Our result is at variance with a recent study by scanning tunneling microscopy (STM) that finds a pseudogap in a similar sample. The implication on the phase diagram is discussed.

We report 63Cu-NMR studies of the multilayered high-Tc cuprate (Cu0.6C0.4)Ba2Ca3Cu4O12+y(Cu1234) with Tc = 117 K. In the normal state, the Knight shift give evidence that the inner CuO2 planes (IP) are underdoped, whereas the outer ones (OP) are heavily overdoped. In the superconducting (SC) state, it is shown that the SC gap fully develops below Tc=117 K in the IP, whereas that in the OP develops moderately below Tc = 117 K, but markedly below Tc2 = 60 K. We propose that these dissimilarities are caused by the large difference of doping levels between the IP and the OP, and that the bulk SC transition at Tc = 117 K is triggered by the IP in Cu1234.

We have investigated an influence of Zn- and Ni-doping at CuO 2 plane in various high- T c compounds. In optimum-doped LSCO, La 1.825 Sr 0.175 CuO 4 , magnetic Ni as well as nonmagnetic Zn induce a large residual density of states (RDOS) in the superconducting state. The induction of RDOS by slight Zn-doping is considered to be a common feature in optimum-doped high- T c compounds.

From Cu-NQR measurements in YBa2(Cu1−x)3O7, all the 63(1/T1) are found to be on a universal curve, when 1/T1 is plotted against t = T/Tc(x) above Tc. This result indicates that Tc is intimately correlated with the Cu antiferromagnetic spin fluctuation (AFSF) that governs 63(1/T1 to Tc is a direct evidence for the pairing mechanism to be magnetic in origin. The reduction in Tc by the Ni substitution is considered to be due to the decrease in the characteristic energy, ΓQ of AFSF around Q = (π/a, π/a).

Ru NQR study in single crystals of Sr 2 RuO 4 ( T c ∼ 0.7 K) has been carried out to investigate a superconducting pairing state. In the superconducting state, 1/ T 1 exhibits a sharp decrease with no coherence peak just below T c and the T 1 T = constant behavior well below T c , suggesting that the anisotropic pairing state is realized as in high- T c and heavy-fermion superconductors.

The normal and superconducting properties in single crystals Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) and Bi 2 Sr 2 (Ca 0.95 Y 0.05 )Cu 2 O 8 (5%Y-doped Bi2212) have been investigated by systematic measurements of the 63 Cu Knight shift, 63 K(T) and the nuclear spin-lattice relaxation time, 63 T 1 .

63 Cu NMR in Tl 2 Ba 2 CuO 6+y with T c -72, 40, 0 K has been investigated, using oriented powder sample. The spin part of Knight shift increases with hole content, suggesting that the isotropic transferred hyperfine field from the nearest neighbor Cu sites is enhanced. The anisotropy of nuclear spin relaxation rate decreases with hole content. From the analysis of anisotropy, the Cu antiferromagnetic (AF) spin correlation diminishes gradually with decreasing T c .