Yasuoka Hiroshi

Anisotropic Superconducting Gap in Transuranium Superconductor PuRhGa5: Ga NQR Study on a Single Crystal

69,71 Ga NMR/NQR studies have been performed on a single crystal of the transuranium superconductor PuRhGa 5 with T c ≃9 K. We have observed a 69 Ga NQR line at ∼29.15 MHz, and assigned it to the 4 i Ga site using the NMR results. The 69 Ga NQR spin–lattice relaxation rate 1/ T 1 shows no coherence peak just below T c , but obeys T 3 behavior below T c . This result strongly suggests that PuRhGa 5 is an unconventional superconductor having an anisotropic superconducting gap. The gap amplitude 2Δ 0 ( T →0)≃5 k B T c and the residual density of states N res ( T =0)/ N 0 ( T = T c )≃0.25 have been determined assuming a simple polar function of the form Δ(θ,φ)=Δ 0 cos θ, where θ and φ are angular parameters on the Fermi surface.

Itinerant Electron Ferromagnetism in Y4Co3–59Co NMR

The 59 Co NMR experiment has been performed in the normal state of Y 4 Co 3 , a new type of magnetic superconductor. The spin echo spectrum at a constant frequency (53 MHz) consists of three kinds of resonance signals corresponding to the three crystallographically inequivalent Co sites. Only one of these sites shows the large negative Knight shift and identified as Co-2b sites which form a linear chain along the c -axis. The zero field ferromagnetic nuclear resonance has been also observed around 18 MHz with the half width of 9 MHz at 2.1 K, which gives the ferromagnetic moment of 0.23 µ B , indicating the weak itinerant ferromagnetism in this compound.

27Al NMR Evidence for the Strong-Coupling d-Wave Superconductivity in NpPd5Al2

We present 27 Al NMR studies for a single crystal of the newly developed superconductor NpPd 5 Al 2 ( T c = 4.9 K). In the superconducting state, the 27 Al NMR spectrum shifts to lower frequencies owing to a decrease of the spin part of the Knight shift, and exhibits broadening characteristic of the local field distribution associated with a vortex lattice. We have also found that the 27 Al nuclear spin–lattice relaxation rate (1/ T 1 ) decreases just below T c with no coherence peak and shows T 3 behavior at low temperatures. The observed temperature dependences of the Knight shift and 1/ T 1 in general accord with those assuming a line-node superconducting gap with the gap value of 2Δ/ k B T c ≃6.4. Therefore, the present NMR data provides evidence for strong-coupling d -wave superconductivity in NpPd 5 Al 2 .

Nuclear Magnetic Relaxation in a Dense Kondo Compound CeCu6

63 Cu nuclear spin-lattice relaxation rate (1/ T 1 ) has been measured in a dense Kondo compound CeCu 6 . The nuclear relaxation was not expressed by a single exponential time dependence probably because of the many inequivalent Cu sites and/or the anisotropy in 1/ T 1 . The relaxation rate of the Ce magnetic moments (1/τ) deduced from the largest value of 1/ T 1 showed the similar temperature dependence as those in CeAl 3 and CeCu 2 Si 2 obtained from the neutron scattering experiments, but much larger value than these compounds, although considerable ambiguity is present in the estimation of the absolute value of 1/τ.

Ti and Be NMR Studies of a Nearly Ferromagnet TiBe2

The microscopic magnetic properties of an itinerant electron paramagnet TiBe 2 have been investigated by Ti and Be NMR at temperatures from 1.4 to 280 K. The Knight shift K of the Ti resonance was found to be linear in the susceptibility χ, while the temperature and the external field dependences of the Be resonance spectrum could be explained consistently from the combined effect of anisotropic hyperfine and electric quadrupole interactions. From the analysis of the Knight shift and the nuclear spin-lattice relaxation time T 1 of Be at low temperatures, it is shown that only the long-wave components of spin fluctuations are strongly exchange-enhanced and play a predominant role. It is also shown that although the spin contribution to 1/ T 1 varies above about 40 K as χ spin T in accord with a theoretical calculation for weakly or nearly ferromagnetic metals, deviation from the above relation is quite marked at lower temperatures.

Anomalous Temperature Dependence of 139La Nuclear Spin-Lattice Relaxation in La2CuO4-δ and (La0.9Ca0.1)2CuO4-δ

Nuclear spin-lattice relaxation time ( T 1 ) of 139 La in the antiferromagnet La 2 CuO 4-δ and the superconductor (La 0.9 Ca 0.1 ) 2 CuO 4-δ has been measured in a zero applied field and temperature ranges of 1.7 K–299 K and 1.8 K–130 K, respectively. A divergent behavior of the relaxation rate (1/ T 1 ) toward the Neel temperature ( T N ≃250 K) which is characteristic of itinerant antiferromagnets has been observed in La 2 CuO 4-δ above T N ≃70 K. An anomalous enhancement of 1/ T 1 ( T ) below about 70 K has been observed in both oxides. Because of this unclarified enhancement, the change of 1/ T 1 ( T ) which is associated with the superconducting transition was not observed in (La 0.9 Ca 0.1 ) 2 CuO 4-δ below T c ≃18 K.

59Co NMR in an Antiferromagnetic CoO Single Crystal

The investigation of 59 Co NMR in CoO single crystal has been made in the antiferromagnetic state under an external magnetic field. The observed value of the hyperfine field at 59 Co nucleus in zero external field is found to be +495.3±0.5 kOe at 4.2 K, which is in good agreement with the theoretically estimated value of +485 kOe. Analysing the angular dependence of the NMR spectra in the external field of 15.0 kOe, the angle between the hyperfine field and the (001) axis is determined to be 21.1±0.5° at 4.2 K which differs from corresponding angle of the spin moments. The angle increases slightly when the temperature is increased.

NMR of 51V in the Antiferromagnetic State of V5S8

Nuclear magnetic resonance of 51 V has been investigated in the antiferromagnetic state of polycrystalline V 5 S 8 at liquid helium temperatures. Two signals with hyperfine fields of 10.1±0.1 kOe and 14.7±0.1 kOe, and one without hyperfine field but with a shift of 2.5±0.5 were observed and assigned to the three different vanadium sites. The magnitudes of the moments estimated are 0.15µ B , 0.22µ B and 0µ B for the three sites in the antiferromagnetic state. This suggests V 5 S 8 is a three-dimensional weakly magnetic system.

NMR Shift Measurements of 69Ga in Unconventional Superconductor PuRhGa5

The NMR shift ( K ) at the Ga(2) site (the 4 i site) has been measured in PuRhGa 5 , a compound which shows anisotropic superconductivity below T c ≈9 K. In the normal state, K ( T ) is very nearly independent of T below ∼30 K. This result is consistent with the relaxation rate measurements. This Korringa-like behavior indicates the formation of Fermi-liquid state below ∼30 K. The transferred hyperfine coupling constants have been estimated from the relation between K and the bulk susceptibility, showing a strong anisotropy. Measurements of K ( T ) in the superconducting state have also been performed. A decrease of K in the superconducting state has been observed, which strongly suggests a spin-singlet pairing.

59Co NMR in an Antiferromagnetic KCoF3 Single Crystal

The properties of the 59 Co NMR in a single crystal of KCoF 3 were studied by a transient NMR technique at low temperatures. The observed spectrum is composed of seven distinct lines split by the nuclear-quadrupole interaction. The frequency of the central (±1/2↔±1/2) transition extrapolates to 372.7±0.05 MHz at 0 K, corresponding to the hyperfine field of 370.7±0.05 kOe. The line separations were not equal having the average value of 2.1±0.5 MHz. The hyperfine field and the quadrupole interaction agree with the calculated values based on the Co 2+ ground state wave function. A large shift as much as 34% was observed for the nuclear g-factor, which is expected mainly from the residual orbital moment of the Co 2+ ions. It was also found that external fields above 35 kOe along one of the c -axis make all the domains in the antiferromagnetic state to be perpendicular to the field.