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We report 73 Ge-NQR studies on filled-skutterudite superconductors LaPt 4 Ge 12 and PrPt 4 Ge 12 under zero external field. In PrPt 4 Ge 12 , the measurement of the nuclear spin-lattice relaxation rate 1/ T 1 has revealed a distinct coherence peak just below T c followed by an exponential decrease well below T c , evidencing that PrPt 4 Ge 12 is an s -wave superconductor with the isotropic gap in a weak-coupling regime. In LaPt 4 Ge 12 , the 1/ T 1 exhibits the exponential decrease well below T c , suggesting the isotropic s -wave superconductivity. Even though 73 Ge-NQR-1/ T 1 in LaPt 4 Ge 12 has been measured under zero field, its coherence peak was absent, which contrasts with the result in Pr-compound. This may point to a possible anisotropy in the Fermi surface and/or some damping effect of quasiparticles in association with electron–phonon interactions.

We report a microscopic investigation of the superconducting clathrates Ag 6 O 8 Ag X ( X =NO 3 and HF 2 ), which are composed of Ag 6 O 8 polyhedral cages and a guest molecule X inside the cages. Site-selective NMR measurements have revealed that the trigonal planar NO 3 molecule in Ag 6 O 8 AgNO 3 is confined in the (001) plane in a low-temperature tetragonal phase, while the uniaxial HF 2 molecule in Ag 6 O 8 AgHF 2 is preferentially directed along the c -axis in a low-temperature phase. These results point to the fact that the distortion of the cages takes place so as to minimize the entropy of the molecular motions of the guest ions inside the cages as the cage volume contracts thermally upon cooling. Note that the difference in T c between these compounds may be primarily ascribed to a difference in their Debye frequencies, but neither to anharmonic vibrations of the guest molecules called rattling nor to other possible remaining degrees of freedom in the molecular dynamics.

We report on the results of the 73 Ge-NQR measurements for M Pt 4 Ge 12 ( M = Sr, Ba) to clarify the microscopic electronic states and the relationship between superconductivity and rattling. In the normal state, 1/ T 1 is proportional to temperature, showing a Korringa relation 1/ T 1 T = 0.020 and 0.016 (sK) -1 for SrPt 4 Ge 12 and BaPt 4 Ge 12 , respectively. Also, in the superconducting state, 1/ T 1 shows a distinct coherence peak just below T C , which suggests a conventional BCS superconductor.

We report on the superconducting characteristics of the filled skutterudites LaPt 4 Ge 12 ( T c =8.3 K) and PrPt 4 Ge 12 ( T c =7.9 K) by 73 Ge-nuclear quadrupole resonance (NQR) at a zero field. In PrPt 4 Ge 12 , the nuclear spin–lattice relaxation rate 73 (1/ T 1 ) has revealed a coherence peak just below T c , indicating that the superconductivity of PrPt 4 Ge 12 is accounted for in terms of the conventional BCS regime. The result was consistently reproduced by not only an isotropic s -wave gap model but also an anisotropic s -wave gap model with a point node. In the conventional superconductor LaPt 4 Ge 12 , the coherence peak in 73 (1/ T 1 ) is markedly suppressed owing to some damping effect of quasiparticles. We note that the T c s of LaPt 4 Ge 12 and PrPt 4 Ge 12 being higher than that of other R Pt 4 Ge 12 may be attributed to the larger density of states, not to some local anharmonic phonon modes derived from the possible rattling of R ions.

We report on the normal-state properties of a superconducting type-III clathrate compound Ba 24 Ge 100 investigated by 73 Ge-NMR at cage sites and by 135/137 Ba-NMR at guest sites at ambient pressure ( P =0) and 2.7 GPa. The measurements of the nuclear spin-lattice relaxation rate 1/ T 1 have revealed a significant increase in 1/ T 1 T of approximately 200 K at P =0, which may be relevant to the rattling motion of Ba atoms in Ge cages. This increase in 1/ T 1 T at high temperatures is markedly suppressed at P =2.7 GPa. As a result of the suppression of the successive structural transformations, the 1/ T 1 T =const. in a low-temperature regime is increased, proving the increase in the density of states at the Fermi level, N ( E F ), at P . It is this increase in N ( E F ) that increases the superconducting transition temperature T c from 0.24 K at P =0 to 3.8 K at P ∼2.7 GPa.

Through 73Ge-NMR measurements and ab initio calculations, we have studied electronic properties of a type-III clathrate compound Ba24Ge100. At ambient pressure, the nuclear spin-lattice relaxation rate 1/T1T has a peak around 200K followed by an activation type decrease with decreasing temperature and it becomes constant at very low temperatures. Near the peak temperature, the successive structural transformations have been observed at TS1 = 215K and TS2 = 180K. The constant value of 1/T1T at low temperatures is considerably smaller than the value at temperatures higher than TS1, which implies that the density of states at the Fermi level D(EF) significantly decreases from the high temperature value at ambient pressure. From ab initio calculations on Ba24Ge100 as well as Ba24Si100, we speculate the phase changes of Ba24Ge100 in the relevant region of the T-P plane. Our results of NMR measurements and D(EF) calculations for the P4132 and an R3 structures imply that, at high pressures ~ 2.8 GPa, there should be increase of D(EF) from the value at ambient pressure and low temperatures. We conclude that this result is predominantly related with the large enhancement of the superconducting transition temperature Tc from 0.24 K (at P = 0) to 3.8 K (at P = 2.7 GPa) observed in Ba24Ge100