Takahito Saito

Evidence for Ferromagnetic Fluctuations in Filled Skutterudite LaFe4Sb12:Sb-NQR and La-NMR

We report the results of 121,123 Sb-nuclear quadrupole resonance (NQR) and 139 La-NMR measurements on a filled skutterudite antimonide LaFe 4 Sb 12 in order to investigate the magnetic properties at low temperatures from a microscopic point of view. The nuclear spin–lattice relaxation time T 1 of Sb nuclei deviates from the relation T 1 T = constant above 4.2 K, where 1/ T 1 T has a Curie–Weiss temperature dependence 1/ T 1 T = C /( T +θ) with θ∼30 K. The temperature dependence of the Knight shift of 139 La nuclei, which is related to the susceptibility at q = 0, is scaled to that of 1/ T 1 T above 40 K. This relation strongly suggests that ferromagnetic fluctuations are predominant in LaFe 4 Sb 12 . We also point out that LaFe 4 Sb 12 is situated close to the ferromagnetic instability due to the small Weiss temperature in the Curie–Weiss behavior of 1/ T 1 T and the Knight shift.

Electrical, Magnetic and NMR Studies of Ge-Based Filled Skutterudites RPt4Ge12 (R = La, Ce, Pr, Nd)

We have synthesized the polycrystalline samples of Ge-based filled skutterudites R Pt 4 Ge 12 ( R =La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt 4 Ge 12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt 4 Ge 12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt 4 Ge 12 and NdPt 4 Ge 12 show the well-localized nature of 4 f -electrons. The NMR measurements in CePt 4 Ge 12 revealed that the nuclear spin–lattice relaxation rate 1/ T 1 shows an activation-type temperature dependence 1/ T 1 ∝exp (-Δ/ k B T ) with an energy gap Δ/ k B =150 K above 100 K, whereas it is proportional to the temperature belo...

Electronic States and Superconductivity in Alkali-Intercalated Fullerides: 13C-NMR Study in Na2RbC60, Na2CsC60, K3C60, K2RbC60, K2CsC60, KRbCsC60, Rb2CsC60 and RbCs2C60

A systematic 13 C NMR experiment is reported in alkali-fullerides A 3 C 60 (where A 3 is alkali-metals) between superconducting transition temperature T c and ∼100 K. Careful 13 C NMR spin-lattice relaxation time study clarified the properties of the conduction electron wave functions at Fermi surface. An upper limit of Stoner exchange parameter was estimated to be ∼3 eV/(spin-carbon), which is far from spin-wave instability criterion. Based on these studies, a variation of the density of states at Fermi level, N ( E f ), with T c was deduced. The results were analyzed in terms of McMillan formula, leading to weak or medium electron-phonon coupling constants and relevant phonon energy larger than 600 K. It was strongly suggested that the A 3 C 60 superconductor is described within a framework of the conventional phonon-mediated BCS superconductivity.

Electronic State and Arrangement of Hydrogen Atoms in Ternary Graphite Intercalation Compounds C8KH0.61 –ESR, 1H NMR and 13C NMR Studies

We have studied ESR, 1 H NMR and 13 C NMR in ternary graphite intercalation compounds C 8 KH 0.61 prepared from the highly oriented pyrolitic graphite (HOPG). Although it has been controversial whether the hydrogen atom has a localized moment, our experiment proves non-existence of the localized moment. It is shown that the absorbed hydrogen molecules are dissolved into an atomic state and that the hydrogen atoms have a metallic character. The electronic state of hydrogen atoms and their geometrical arrangement are discussed.

1H NMR in hydrogen chemisorbed K-GIC C8KH23: Metallic electronic state at hydrogen site

Abstract The 1H NMR was observed in hydrogen chemisorbed K-GIC C8KH 2 3 and the spin lattice relaxation rate T−11 of 1H was measured at the temperature range from 1.3 to 300 K. The temperature dependence of T−11 obeyed the T1T = const. law below 100 K and well characterized the metallic Fermi surface. Together with the observed small angular dependence of T−11, the existence of delocalized s-wave electrons on hydrogen was confirmed.

NMR evidence of BCS superconductivity in Y(Ni1−xPtx)2B2C

Abstract 13 C and 11 B NMR studies have been made on the quaternary superconductor Y(Ni1−xPtx)2B2C with x=0 and 0.4. Temperature dependence of Knight shift and spin-lattice relaxation rate (SLRR) in the normal state was explained consistently among all the observed nuclei by Korringa mechanism without taking account of antiferromagnetic spin fluctuation. Bellow the superconducting transition temperature we observed a Hebel–Slichter peak in case of 13 C SLRR. These results mean that the borocarbide is a normal metal and a phonon-mediated superconductor

13C NMR study of borocarbide superconductor YNi2B2C

Abstract 13 C NMR study has been made of the quaternary superconductor YNi 2 B 2 C. Temperature dependence of T 1 and K in the normal state showed normal metal behavior. Temperature dependence of T 1 in the superconducting state strongly suggests that the quaternary borocarbide is a conventional BCS superconductor.

NMR Studies on Orientational Ordering Phase Transition in Na2CsC60

Sodium-containing alkali metal fulleride, Na 2 CsC 60 , was studied by 13 C and 23 Na NMR below and above the face-centered cubic ( f c c ) to simple cubic ( s c ) phase transition temperature, T m . It is shown that the C 60 molecules rotate faster than the rotational correlation time of 1.6×10 -5 sec above ∼140 K and 2.4×10 -9 sec around 300 K. Using 23 Na NMR spectra broadened by the second-order quadrupole interaction, the electric field gradient at Na sites, e 2 q Q / h , was estimated to be 940±20 kHz and less than 0.1 kHz below and above T m , respectively. The detailed analysis suggests that Na atoms in s c phase have a covalent character with C 60 molecules. It is also suggested that Na 2 CsC 60 is the first metallic plastic crystal.

Oxygen concentration dependence of superconductivity in the Pr substituted Y1−xPrxBa2Cu3O7−y

Abstract The oxygen concentration dependence of T c is examined in Y 1−x Pr x Ba 2 Cu 3 O 7− y . The T c decreases with decreasing the oxygen concentration 7− y in not only Ortho I (6.75≦7− y ≦7.0) but also the region between Ortho II and Tetra (7− y ≦6.6) for the Pr contents of 0≦ x T c increases conversely with decrease of 7− y for 0.3≦ x y ≦6.75). The Pr content dependence of T c for Ortho II is quite different from that for Ortho I, and is preferably similar to that in the stoichiometric oxide superconductor Y 1−x Pr x Ba 2 Cu 4 O 8 .

Oxygen concentration dependence of superconductivity in Pr-substituted Ln1−xPrxBa2Cu3O7−y (Ln = Y, Nd, Gd)

The Pr content x dependence of the superconducting transition temperature Tc is examined for various oxygen concentrations by quenching the samples of the Pr-substituted Ln1−xPrxBa2Cu3O7−y, where Ln is Y, Nd and Gd. In the case of Ln = Y, the initial rate of decrease of Tc by Pr substitution (−dTcdx) decreases with decreasing oxygen concentration for 6.6 < 7 − y < 7.0, then becomes constant for 7 − y < 6.6. In contrast, the effect of Tc-suppression by Pr in the case of Ln = Nd does not depend upon the oxygen concentration, and is much stronger than that in the case of Ln = Y. In the case of Ln = Gd, intermediate behavior between those of Y and Nd is observed, corresponding to the ionic size of Ln.