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Electrical resistivity of nanostructured granular alloys ${\mathrm{Co}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}x}$ ($x\ensuremath{\sim}0.01$--$0.76$) prepared by the chemical reduction method is investigated in the temperature range 2--300 K. The samples with a low cobalt content of $x\ensuremath{\le}0.1$ show a metallic resistivity behavior. For samples with a higher cobalt content, $x\ensuremath{\ge}0.17$, the resistivity shows a minimum. The minimum becomes more pronounced as Co content ($x$) increases and also as the temperature of minimum resistivity, ${T}_{\text{min}}$, increases with $x$. The resistivity minimum is obtained in this magnetic alloy system even for a cobalt concentration as high as $\ensuremath{\sim}76%$. Application of an external magnetic field has a negligible effect on the resistivity behavior. Detailed analysis suggests that the low-temperature upturn in resistivity most probably arises due to elastic electron-electron interaction (the quantum-interference effect). Magnetic measurements at 4 K on the same samples show the absence of long-range magnetic interaction and evidence of increasing magnetic disorder as $x$ increases beyond $\ensuremath{\sim}10%$. Combining the results of the two types of measurements, a model of formation of these alloy particles involving random clusters of Co atoms within the Cu matrix has been proposed.

The topologically nontrivial band structure of A15 compounds has drawn a significant amount of attention owing to the possible realization of topological superconductivity. The superconducting ground state of the A15 alloys ${\mathrm{Ti}}_{3}X$ ($X=\mathrm{Ir},\mathrm{Sb}$), by muon spectroscopy, has been investigated. Zero-field muon spin relaxation measurements have shown that time-reversal symmetry is preserved in these materials. Furthermore, specific-heat and transverse-field muon rotation measurements rule out any possibility of a nodal or anisotropic superconducting gap but reveal a conventional isotropic $s$-wave gap structure. This supports the idea that these A15 superconductors are time-reversal-symmetry-preserved topological superconducting candidate compounds.