The doping dependence of specific heat and magnetic susceptibility has been investigated for Li(x)ZrNCl superconductors derived from a band insulator. As the carrier concentration is decreased, the anisotropy of superconducting gap changes from highly anisotropic to almost isotropic. It was also found that, upon reducing carrier density, the superconducting coupling strength and the magnetic susceptibility are concomitantly enhanced in parallel with T(c), while the density of states at the Fermi level is kept almost constant. Theoretical calculations taking into account the on-site Coulomb interaction reproduced the experimental results, suggesting a possible pairing mediated by magnetic fluctuations, even in the doped band insulators.

The magnetic, transport and thermal properties of alkaline-earth-filled skutterudites AFe 4 Sb 12 (A = Ca, Sr, Ba) are reported. All three compounds show maximum magnetic susceptibility and thermopower at 50 K and a large electronic specific heat coefficient of 100 mJ/mol K 2 . The electrical resistivity exhibits a shoulder at 70 K and follows quadratic temperature dependence below 9 K. These properties are the characteristics of a nearly ferromagnetic metal. The suppression of ferromagnetic spin fluctuations of the Fe 3d band by applying magnetic fields was manifested as a negative magnetoresistance between 50 and 80 K.The concept of itinerant magnetism has been developed over the past three decades. 1) In weakly or nearly ferromagnetic systems, the temperature dependence of the amplitude of local spin density results in the Curie-Weiss behavior of magnetic susceptibility 1 at high temperatures. In an itinerant ferromagnet, the ordered moment can be much smaller than the effective moment. Low-energy spin fluctuations (SFs) significantly influence the transport and thermal properties at low temperatures. For example, SF scattering gives rise to quadratic T dependence of the electrical resistivity, and the temperature variation of SFs leads to the significant enhancement of the electronic specific heat. Furthermore, SFs in ZrZn 2 and Sr 2 RuO 4 are believed to act as the glue for electron pairing in unconventional superconducting states. 2,3) NaFe 4 Sb 12 is a newly discovered weak itinerant ferromagnet with a rather high T c of 85 K and a small ordered moment of 0.25 " B /Fe atom. 4) This compound belongs to the family of filled skutterudites with the chemical formula MT 4 X 12 , where M is an alkali, alkaline-earth or a rare-earth metal, T is Fe, Ru or Os, and X is P, As, or Sb. The T atom is located at the center of an X 6 octahedron, and the cornersharing TX 6 octahedra form a tilted array. Within this family, rare-earth filled compounds have been extensively studied because of their novel physical properties, such as the unconventional superconductivity in PrOs 4 Sb 12 . 5) However, the role of d electrons of T atoms in the physical properties has not been well understood so far. The discovery of itinerant ferromagnetism in NaFe 4 Sb 12 with Na þ ions has revealed the strong dependence of the d electron magnetism on the valence of the filler ion. In fact, LaFe 4 Sb 12 with La 3þ ions remains in an enhanced paramagnetic state. 6) These contrasting magnetic behaviors in turn suggest that the ground state of AFe 4 Sb 12 with divalent A ions should be close to a magnetic critical point at which an itinerant ferromagnetic state emerges. In this paper, we report the magnetic, transport, and thermal properties of AFe 4 Sb 12 (A = Ca, Sr, and Ba). The results show that all three are in a nearly ferromagnetic state with strong SFs of the Fe 3d electrons.The first investigation of the magnetic properties of AFe 4 Sb 12 (A = Ca, Sr, Ba) was performed by Danebrock et al. 7) The Curie-Weiss behavior with an effective magn...

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In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, for example, the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron–electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Here we observe, near the magnetic instability, a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance may arise from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations.

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