Approaching a quantum critical point (QCP) has been an effective route to stabilize superconductivity. While the role of magnetic QCPs has been extensively discussed, similar exploration of a structural QCP is scarce due to the lack of suitable systems with a continuous structural transition that can be conveniently tuned to 0 K. Using inelastic X-ray scattering, we examine the phonon spectrum of the nonmagnetic quasi-skutterudite (CaxSr1−x)3Rh4Sn13, which represents a precious system to explore the interplay between structural instabilities and superconductivity by tuning the Ca concentration x. We unambiguously detect the softening of phonon modes around the M point on cooling towards the structural transition. Intriguingly, at x = 0.85, the soft mode energy squared at the M point extrapolates to zero at (−5.7 ± 7.7) K, providing the first compelling microscopic evidence of a structural QCP in (CaxSr1−x)3Rh4Sn13. The enhanced phonon density-of-states at low energy provides the essential ingredient for realizing strong-coupling superconductivity near the structural QCP. arXiv:1810.02248v1 [cond-mat.supr-con]

We have systematically investigated magnetic properties of La and Co substituted SrFe12O19 using single crystals. By utilizing travelling solvent floating zone technique, we have grown single crystals over wide range of substitution content, and examined their saturation moments and magnetocrystalline anisotropy. With increasing content of La and Co, saturation moment at 300 K monotonically increases. The increment of magnetocrystalline anisotropy is almost proportional to the content of Co. In addition, further La substitution also enhances magnetocrystalline anisotropy, which demonstrates that Fe 2+ plays an important role to enhance magnetocrystalline anisotropy. We have analyzed magnetocrystalline anisotropy energy as a function of substitution content, and elucidate the effects of Co 2+ and Fe 2+ .

To specify preferential occupation sites of Co substituents and to clarify charge and spin states of Co ions in (La, Co)-cosubstituted hexagonal magnetoplumbite-type (M-type) Sr ferrite Sr1−xLaxFe12−yCoyO19 (x, y ≤ 0.4), 57 Fe and 59 Co nuclear magnetic resonance (NMR) spectra are measured under zero and external magnetic fields using powdered and single crystalline specimens. For comparison, NMR investigations of non-doped and La-or Co-doped M-type Sr ferrites are also performed. Ferrimagnetic M-type Sr ferrite contains the following five crystallographic Fe sites: the majority spin sites 12k, 2a, and 2b, and the minority spin sites 4f1 and 4f2. Based on 57 Fe and 59 Co NMR, a plausible model of (La, Co)-codoped Sr ferrite is deduced. To a considerable degree, the charge compensation between La 3+ and Co 2+ works in the equal (La, Co)-codoped case, where more than half of the Co ions are considered to be present in the minority spin 4f1 sites at the center of the oxygen tetrahedra, with the S=3/2 state carrying a small orbital moment owing to spin-orbit interaction. The remaining small number of high-spin Co 2+ (S=3/2, L=1) ions with unquenched orbital moments would be distributed to the other octahedral 12k, 2a, and 4f2 sites.

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