Spin fluctuations and superconductivity in Y5Rh6Sn18 doped with Pd and Co: Evidence of peak effect in the superconducting mixed state

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We performed a systematic study of the nonmagnetic skutterudite-related ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ superconductor, where the lattice disorder on the coherence length scale $\ensuremath{\xi}$ additionally generates a nonhomogeneous, high-temperature superconducting phase with disorder-enhanced critical temperature ${T}_{c}^{★}$. We have previously discussed various possibilities of local atomic disorder; one of the possibilities is doping. Our present studies focus on the series of ${\mathrm{Y}}_{5\ensuremath{-}\ensuremath{\delta}}({\mathrm{Rh}}_{5.5}{M}_{0.5}){\mathrm{Sn}}_{18}$ compounds ($\ensuremath{\delta}\ensuremath{\ll}1$), where the dopants $M=$ Co, Ir, Ru, and Pd, when they are smaller (Co) or larger (Pd) than Rh, generate the peak effect in fields smaller than the critical field ${H}_{c2}$. This phenomenon manifests itself as a weak peak in the real and imaginary parts of ac susceptibility and is more distinct in magnetoresistance. Using a simple theoretical model, we demonstrate that the effectiveness of this mechanism depends not only on the magnitude of the difference between the size of the dopant and the host atom, but also on whether the dopant is smaller or larger. The agreement between this prediction and our experimental data strongly supports the impurity-based scenario of the observed peak effect. Magnetoresistance isotherms of the remaining samples ($M=$ Ir and Ru) with the radius of $M$ being very similar to that of Rh show, however, a weak peak-effect-like behavior, which is mainly due to vacancies $\ensuremath{\delta}$ at Y sites, while the corresponding ac susceptibility isotherms exhibit a distinct peak at $H\ensuremath{\sim}{H}_{c2}$. This field-dependent ${\ensuremath{\chi}}_{\mathrm{ac}}$ anomaly appears to be similar in nature to the peak effect; however, it cannot be attributed to pinning and appears to be an equilibrium property of the system. We also report the coexistence of spin fluctuations and superconductivity for ${\mathrm{Y}}_{5}{\mathrm{Rh}}_{6}{\mathrm{Sn}}_{18}$ doped with Pd and Co.

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