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The family of the superconducting quasiskutterudites (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) features a structural quantum critical point at x(c)=0.9, around which a dome-shaped variation of the superconducting transition temperature T(c) is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching x(c), which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of 2Δ/k(B)T(c) and ΔC/γT(c) beyond the Bardeen-Cooper-Schrieffer values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca(x)Sr(1-x))(3)Rh(4)Sn(13) thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

Co NQR studies were performed in recently discovered superconductor NaxCoO2·yH2O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, SC bilayer-hydrate sample with Tc ∼ 4.7 K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1/T1 in the SC sample, it was found that the coherence peak is absent just below Tc and that 1/T1 is proportional to temperature far below Tc. These results, which are in qualitatively agreement with the previous result by Fujimoto et al., suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T1T of the SC sample shows gradual increase below 100K down to Tc, whereas 1/T1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T1T and χ bulk in the SC sample, the increase of 1/T1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.

Temperature dependences of the electrical resistivity, magnetic susceptibility and specific heat have been measured on the single crystal Ca 3 Ir 4 Sn 13 . This compound shows a bulk superconducting transition at %7:0 K. We observe peak-like anomalies, which are attributed to the ferromagnetic spin-fluctuations, in susceptibility and resistivity. The resistivity shows a non Fermi-liquid behavior as & ¼ & 0 þ A Ã T n (n ¼ 1:2 for H ¼ 0) in normal state at low temperatures, and develops into a Fermi-liquid behavior by the suppression of the ferromagnetic spin fluctuation on increasing magnetic fields (n ¼ 2 for H ¼ 14 T). The superconductivity disappears for magnetic field H ! 7 T as well as the diminution of the ferromagnetic spin fluctuations. We find that the superconductivity coexists with the ferromagnetic spin fluctuations and seems to cooperate in this material. The large values of ÁC= n T c ¼ 2:78, KadowakiWoods ratio A= 2 n ¼ 1:14 Â 10 À5 mÁcm (KÁmol/mJ) 2 and Wilson ratio R w ¼ % 2 k 2 B 1 p =3" 2 B % 1:7, indicate the electrons are highly correlated in this system.Magnetic spin-fluctuations could be enhanced in materials close to the phase boundary of magnetic order and have negative effects on the occurrence of phonon-mediated superconductivity of BCS-type. In UGe 2 , the superconductivity is discovered in a limited pressure range on the border of ferromagnetism, and the superconductivity seems to originate from the same electrons that produce band magnetism. 1) Therefore the superconductivity occurs in the vicinity of the quantum critical point (QCP) case is naturally understood in terms of magnetic-interaction mediated one. For example, Mathur et al. put forward a ''magnetic glue'' model to explain the superconductivity in heavy-fermion system of CePd 2 Si 2 and CeIn 3 , in which superconductivity appears in the vicinity of antiferromagnetic QCP under the critical pressure. 2) The coexistence of superconductivity and spin fluctuations is first discovered in UPt 3 . 3) It is very rare that the material exhibits both spin fluctuations and superconductivity without actinide element. Very recently, the superconductivity discovered in the Ni-rich compound MgCNi 3 has been the focus of particular attention. 4) Band structural calculations reveal that the electronic state near Fermi surface is mainly of 3d orbitals of Ni. 5,6) The presence of ferromagnetic spin fluctuations is confirmed later by the NMR measurements. 7) Therefore the superconductivity in MgCNi 3 is expected to be near the ferromagnetism. 8) On the other hand, a coexistence of superconductivity and the spin fluctuations in Mo 3 Sb 7 is discovered by displaying a broad peak-like behavior in susceptibility and electrical resistivity. 9) However, Tran et al., suggest that a spin gap scenario is responsible for the anomaly in susceptibility. 10) The superconductivity in this system is still an open question.Ca 3 Ir 4 Sn 13 , which crystallizes in Pm " 3 3n space group (No. 223), were synthesized by Espinosa and his coworkers almost 30 years ago. ...

The magnetic properties of RTe 3 (RϭCe, Pr, Nd, Gd, Dy͒, which has a structure consisting of the alternate stacking of one RTe layer and two Te layers, were investigated using magnetization and electrical resistivity measurements. The magnetic transition temperatures were found at 3.0 K for RϭCe, 3.2 K for Nd, 11.5 K for Gd, and 4.5 K for Dy, respectively, and the Pr compound showed van Vleck paramagnetism. The ordered magnetic moment of RTe 3 lies within the layer. In contrast, the ordered magnetic moments of RTe 2 , having alternate stacking of one RTe and one Te layer, point to the layer-stacking direction, although the point symmetry at the R site and the coordination of the Te atoms to the R atoms are almost the same. The difference of the magnetic anisotropy in RTe 2 and RTe 3 was discussed in terms of the effective charge of the Te atoms in the Te layer with an electron charge transferred from the RTe layer. We propose magnetic compounds designed by charge introduction based on this result.