We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular ${\mathrm{Cu}}^{2+}$ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies ($V_{\mathrm{O}}$) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic “indirect double-exchange” model, in which alignments of localized large moments of Cu in the vicinity of the $V_{\mathrm{O}}$ are mediated by the large-sized vacancy orbitals.

Synchrotron-based in situ photoelectron spectroscopy investigations demonstrate effective surface transfer p-type doping of epitaxial graphene (EG) thermally grown on 4H–SiC(0001) via the deposition of MoO3 thin film on top. The large work function difference between EG and MoO3 facilitates electron transfer from EG to the MoO3 thin film. This leads to hole accumulation in the EG layer with an areal hole density of about 1.0×1013 cm−2, and places the Fermi level 0.38 eV below the graphene Dirac point.

In condensed matter physics the quasi two-dimensional electron gas at the interface of two different insulators, polar LaAlO₃ on nonpolar SrTiO₃ (LaAlO₃/SrTiO₃) is a spectacular and surprising observation. This phenomenon is LaAlO₃ film thickness dependent and may be explained by the polarization catastrophe model, in which a charge transfer of 0.5e⁻ from the LaAlO₃ film into the LaAlO₃/SrTiO₃ interface is expected. Here we show that in conducting samples (≥4 unit cells of LaAlO₃) there is indeed a ~0.5e⁻ transfer from LaAlO₃ into the LaAlO₃/SrTiO₃ interface by studying the optical conductivity in a broad energy range (0.5–35 eV). Surprisingly, in insulating samples (≤3 unit cells of LaAlO₃) a redistribution of charges within the polar LaAlO₃ sublayers (from AlO₂ to LaO) as large as ~0.5e⁻ is observed, with no charge transfer into the interface. Hence, our results reveal the different mechanisms for the polarization catastrophe compensation in insulating and conducting LaAlO₃/SrTiO₃ interfaces.