The Meissner effect has been directly demonstrated by depth-resolved muon spin rotation measurements in high-quality thin films of the T-structured cuprate, T-La$_{1.9}$Y$_{0.1}$CuO$_4$, to confirm bulk superconductivity ($T_csimeq21$ K) in its {sl u
ndoped} state. The gradual expelling of an external magnetic field is observed over a depth range of $sim$100 nm in films with a thickness of 275(15) nm, from which the penetration depth is deduced to be 466(22) nm. Based on this result, we argue that the true ground state of the parent compound of the $n$-type cuprates is not a Mott insulator but a strongly correlated metal with colossal sensitivity to apical oxygen impurities.
We demonstrate via a muon spin rotation experiment that the electronic ground state of the iridium spinel compound, CuIr$_2$S$_4$, is not the presumed spin-singlet state but a novel paramagnetic state, showing a quasistatic spin glass-like magnetism
below ~100 K. Considering the earlier indication that IrS$_6$ octahedra exhibit dimerization associated with the metal-to-insulator transition below 230 K, the present result suggests that a strong spin-orbit interaction may be playing an important role in determining the ground state that accompanies magnetic frustration.
