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Suppression of Superconductivity in Lu$_x$Zr$_{1-x}$B$_{12}$ : Evidence of Static Magnetic Moments Induced by Non-Magnetic Impurities

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 Added by Nikolay Sluchanko E
 Publication date 2015
  fields Physics
and research's language is English




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Based on low temperature resistivity, heat capacity and magnetization investigations we show that the unusually strong suppression of superconductivity in Lu$_x$Zr$_{1-x}$B$_{12}$ BSC-type superconductors in the range $x$$<$0.08 is caused by the emergence of static spin polarization in the vicinity of non-magnetic lutetium impurities. The analysis of received results points to a formation of static magnetic moments with $mu_{eff}$$approx$$3mu_B$ per Lu-ion. The size of these spin polarized nanodomains was estimated to be about 5 ${AA}$.



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We have determined the superconducting and magnetic properties of four samples of Lu$_x$Zr$_{1-x}$B$_{12}$ ($x=0.04$, $0.07$, $0.17$, and $0.8$) using muon spin rotation ($mu$SR) and magnetometry measurements. We observed a strong magnetic signal in both the $mu$SR and magnetometry data in one sample ($x=0.07$), likely caused by the formation of static moments of size $approx 1,mu_{rm B}$ due to a clustering effect of the Lu$^{3+}$ ions. In all other samples, we find only a small magnetic signal in the $mu$SR data thought to originate from boron nuclei in the B$_{12}$ cages. The superconductivity is found to evolve with $x$, with a decrease in $x$ resulting in an increase in critical temperature and a decrease of the penetration depth. Most remarkably, we find the formation of nodes in the superconducting gap for $x leq 0.17$, providing a new example of an $s$-to-$d$-wave crossover in a superconductor.
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We report unusual effects of nonmagnetic impurities on the spin-triplet superconductor Sr2RuO4. The substitution of nonmagnetic Ti4+ for Ru4+ induces localized-moment magnetism characterized by unexpected Ising anisotropy with the easy axis along the interlayer c direction. Furthermore, for x(Ti) > 0.03 magnetic ordering occurs in the metallic state with the remnant magnetization along the c-axis. We argue that the localized moments are induced in the Ru4+ and/or oxygen ions surrounding Ti4+ and that the ordering is due to their interaction mediated by itinerant Ru-4d electrons with strong spin fluctuations.
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