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The local electronic structure of muons (Mu) as dilute pseudo-hydrogen in single-crystalline $\beta$-Ga$_2$O$_3$ has been studied by the muon spin rotation/relaxation ($\mu$SR). High-precision measurements over a long time range of $\sim$25 $\mu$s have clearly identified two distinct Mu states: a quasi-static Mu (Mu$_1$) and fast-moving Mu (Mu$_2$). By comparing this result with predictions from the recently established ambipolarity model, these two states are respectively attributed to the relaxed-excited states associated with the donor ($E^{+/0}$) and acceptor ($E^{-/0}$) levels predicted by density functional theory (DFT) calculations for the interstitial H. Furthermore, the local electronic structure of Mu$_1$ is found to be an OMu-bonded state with three-coordinated oxygen. The structure is almost identical with the thermal equilibrium state of H, and it is found to function as an electron donor. The other Mu$_2$ is considered to be in the hydride state (Mu$^-$) from the ambipolarity model, suggesting that it is in fast diffusion motion through the short-lived neutral state due to the charge exchange reaction with conduction electrons (Mu$^0+e^-\rightleftarrows$Mu$^-$)., Comment: 7 pages, 4 figures, plus Supplemental Material (3 pages, 6 figures)

The strain effect from a substrate is an important experimental route to control electronic and magnetic properties in transition-metal oxide (TMO) thin films. Using hard x-ray photoemission spectroscopy, we investigate the strain dependence of the valence states in LaNiO$_{3}$ thin films, strongly correlated perovskite TMO, grown on four substrates: LaAlO$_{3}$, (LaAlO$_{3}$)$_{0.3}$(SrAl$_{0.5}$Ta$_{0.5}$O$_{3}$)$_{0.7}$, SrTiO$_{3}$, and DyScO$_{3}$. A Madelung potential analysis of core-level spectra suggests that the point-charge description is valid for the La ions while it breaks down for Ni and O ions due to a strong covalent bonding between the two. A clear x-ray photon-energy dependence of the valence spectra is analyzed by the density functional theory, which points to a presence of the La 5$p$ state near the Fermi level., Comment: 6 pages, 4 figures, and 2 tables

We report on the local electronic structure of interstitial muon (Mu) as pseudo-hydrogen in In-Ga-Zn oxide (IGZO) semiconductor studied by muon spin rotation/relaxation ($\mu$SR) experiment. In polycrystalline (c-) IGZO, it is inferred that Mu is in a diamagnetic state, where the $\mu$SR time spectra under zero external field is perfectly described by the Gaussian Kubo-Toyabe relaxation function with the linewidth $\Delta$ serving as a sensitive measure for the random local fields from In/Ga nuclear magnetic moments. The magnitude of $\Delta$ combined with the density functional theory calculations for H (to mimic Mu) suggests that Mu occupies Zn-O bond-center site (Mu$_{\rm BC}$) similar to the case in crystalline ZnO. This implies that the diamagnetic state in c-IGZO corresponds to Mu$_{\rm BC}^+$, thus serving as an electron donor. In amorphous (a-) IGZO, the local Mu structure in as-deposited films is nearly identical with that in c-IGZO, suggesting Mu$_{\rm BC}^+$ for the electronic state. In contrast, the diamagnetic signal in heavily hydrogenated a-IGZO films exhibits the Lorentzian Kubo-Toyabe relaxation, implying that Mu accompanies more inhomogeneous distribution of the neighboring nuclear spins that may involve Mu$^-$H$^-$-complex state in an oxygen vacancy., Comment: 5 pages (in double column), 3 figures

A new diagnosis method for high energy ions utilizing a single CR-39 detector mounted on plastic plates is demonstrated to identify the presence of the high energy component beyond the CR-39's detection threshold limit. On irradiation of the CR-39 detector unit with a 25 MeV per nucleon He ion beam from conventional rf-accelerators, a large number of etch pits having elliptical opening shapes are observed on the rear surface of the CR-39. Detailed investigations reveal that these etch pits are created by heavy ions inelastically backscattered from the plastic plates. This ion detection method is applied to laser-driven ion acceleration experiments using cluster-gas targets, and ion signals with energies up to 50 MeV per nucleon are identified.