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Depth-resolved cathodoluminescence spectroscopy of atomically flat TiO2-terminated SrTiO3 single crystal surfaces reveals dramatic differences in native point defects produced by conventional etching with buffered HF (BHF) and an alternative procedure using HCl–HNO3 acidic solution (HCLNO), which produces three times fewer oxygen vacancies before and nearly an order of magnitude fewer after pure oxygen annealing. BHF-produced defect densities extend hundreds of nanometers below the surface, whereas the lower HCLNO-treated densities extend less than 50 nm. This “Arkansas” HCLNO etch and anneal method avoids HF handling and provides high-quality SrTiO3 surfaces with low native defect density for complex oxide heterostructure growth. [ABSTRACT FROM AUTHOR]

By using an alternative wet-etch procedure, we have obtained high-quality atomically flat TiO2 terminated surfaces of SrTiO3 single crystals with the morphology equivalent to that of the conventional wet-etch methods. By applying a combined power of photoluminescence (PL) spectroscopy, reflection high-energy electron diffraction, atomic force microscopy imaging, and soft x-ray absorption (XAS), we were able to identify and monitor the complex evolution of oxygen defect states and Ti valency at the surface and near-surface layers. Our experiments revealed a high level of local defects resulting in the presence of the Ti3+ states at the surface. We have developed a method to control the defect states capable of a marked reduction of the defect concentration. We have demonstrated that the PL and XAS are able to distinguish the surface-related Ti3+ states from oxygen vacancies trapping charge transfer vibronic excitons that define the PL intensity. The experimental findings will have important implications for the growth of high-quality ultrathin complex oxide heterostructures. [ABSTRACT FROM AUTHOR]

Understanding, creating, and manipulating spin polarization of two-dimensional electron gases at complex oxide interfaces present an experimental challenge. For example, despite almost a decade long research effort, the microscopic origin of ferromagnetism in LaAlO3/SrTiO3 heterojunctions is still an open question. Here, by using a prototypical two-dimensional electron gas (2DEG) which emerges at the interface between band insulator SrTiO3 and antiferromagnetic Mott insulator LaTiO3, the experiment reveals the evidence for magnetic phase separation in a hole-doped Ti d1 t2g system, resulting in spin-polarized 2DEG. The details of electronic and magnetic properties of the 2DEG were investigated by temperature-dependent d.c. transport, angle-dependent X-ray photoemission spectroscopy, and temperature-dependent magnetoresistance. The observation of clear hysteresis in magnetotransport at low magnetic fields implies spin-polarization from magnetic islands in the hole rich LaTiO3 near the interface. These findings emphasize the role of magnetic instabilities in doped Mott insulators, thus providing another path for designing all-oxide structures relevant to spintronic applications. [ABSTRACT FROM AUTHOR]

On a road towards applications and devices based on functional oxides with correlated electrons, the crucial element is uncovering the effects of the reduced dimensionality on the electronic phase transition into a multiordered ground state. Towards this goal, we present a study of reduced dimensionality on charge and antiferromagnetic orderings in ultrathin EuNiO3 films on NdGaO3 substrates using hard and soft resonant x-ray scattering to investigate the presence of electronic and magnetic orderings. Despite the ultrathin nature of the films, they exhibit the bulklike order parameters up to room temperature, suggesting that the spontaneously coherent Mott ground state in the highly distorted rare-earth nickelates can be successfully sustained even when constrained towards two-dimensionality. The presence of charge ordering at room temperature and below opens prospects for their use in novel electric-field-controlled devices. [ABSTRACT FROM AUTHOR]

A series of ultrathin epitaxial films of EuNiO3 (ENO) were grown on a set of substrates traversing from compressive (−2.4%) to tensile (+2.5%) lattice mismatch. On moving from tensile to compressive strain, transport measurements demonstrate a successively suppressed Mott insulating behavior eventually resulting in a complete suppression of the insulating state at high compressive strain. Corroborating these findings, resonant soft x-ray absorption spectroscopy at the Ni L3,2 edge reveals the presence of a strong multiplet splitting in the tensile strained samples that progressively weakens with increasing compressive strain. Combined with cluster calculations, the results show how cumulatively enhanced covalency (i.e., bandwidth) between Ni d and O p orbital derived states leads to the emergent metallic ground state not attainable in the bulk ENO. [ABSTRACT FROM AUTHOR]

An optical study of NdNiO3 ultrathin films with insulating and metallic ground states reveals new aspects of the insulator-to-metal transition that point to Mott physics as the driving force. In contrast with the behavior of charge-ordered systems, we find that the emergence of the Drude resonance across the transition is linked to a spectral weight transfer over an energy range of the order of the Coulomb repulsion U, as the energy gap is filled with states instead of closing continuously. [ABSTRACT FROM AUTHOR]

We explore the electrical transport and magneto-conductance (MC) in quasi-two-dimensional strongly correlated ultra-thin films of LaNiO3 (LNO) to investigate the effect of hetero-epitaxial strain on electron-electron and electron-lattice interactions from the low to intermediate temperature range (2-170 K). The fully epitaxial 10 unit cell thick films spanning tensile strain up to ~4% are used to investigate the effects of enhanced carrier localization driven by a combination of weak localization (WL) and electron-electron interactions at low temperatures. The MC data show the importance of the increased contribution of WL to low-temperature quantum corrections. The obtained results demonstrate that with increasing tensile strain and reduced temperature, the quantum-confined LNO system gradually evolves from the Mott into the Mott-Anderson regime. [ABSTRACT FROM AUTHOR]

We report on the phase diagram of competing magnetic interactions at the nanoscale in engineered ultrathin trilayer heterostructures of LaTiO3/SrTiO3/YTiO3, in which the interfacial inversion symmetry is explicitly broken. Combined atomic layer resolved scanning transmission electron microscopy with electron energy loss spectroscopy and electrical transport have confirmed the formation of a spatially separated two-dimensional electron liquid and high density two-dimensional localized magnetic moments at the LaTiO3/SrTiO3 and SrTiO3/YTiO3 interfaces, respectively. Resonant soft x-ray linear dichroism spectroscopy has demonstrated the presence of orbital polarization of the conductive LaTiO3/SrTiO3 and localized SrTiO3/YTiO3 electrons. Our results provide a route with prospects for exploring new magnetic interfaces, designing a tunable two-dimensional d-electron Kondo lattice, and potential spin Hall applications. [ABSTRACT FROM AUTHOR]

Despite its use as a constituent layer for realization of a polar metal and interfacial conductivity, the microscopic study of electronic structure of CaTiO3 is still very limited. Here, we epitaxially stabilized CaTiO3 films on NdGaO3 (110) substrates in a layer-by-layer way by pulsed laser deposition. The structural and electronic properties of the films were characterized by reflection high-energy-electron-diffraction, X-ray diffraction, and element-specific resonant X-ray absorption spectroscopy. To reveal the orbital polarization and the crystal field splitting of the titanium 3d state, X-ray linear dichroism was carried out on CaTiO3 films, demonstrating the orbital configuration of dxz/dyz

A correction to the article "Strain-Modulated Mott Transition in EuNiO3 Ultrathin Films" in the August 16, 2013 is presented.

We report on a systematic study of the temperature-dependent Hall coefficient and thermoelectric power in ultrathin metallic LaNiO3 films that reveal a strain-induced, self-doping carrier transition that is inaccessible in the bulk. As the film strain varies from compressive to tensile at fixed composition and stoichiometry, the evolution of the transport coefficients is strikingly similar to those of bulk hole-doped superconducting cuprates with varying doping level. Density functional calculations reveal that the strain-induced changes in transport properties arise from changes in the low-energy electronic band structure that induce self-doping, a transfer of charge between O p and Ni d states. The results suggest that thin-film epitaxy can serve as a means to vary the charge-carrier concentration in other (negative) charge-transfer gap transition-metal oxides without resorting to chemical substitution. [ABSTRACT FROM AUTHOR]

We investigate the electronic reconstruction in (LaNiO3)n/(LaAlO3)3 (n=3, 5, and 10) superlattices due to the quantum confinement (QC) by dc transport and resonant soft x-ray absorption spectroscopy. In proximity to the QC limit, a Mott-type transition from an itinerant electron behavior to a localized state is observed. The system exhibits tendency toward charge-order during the transition. Ab initio cluster calculations are in good agreement with the absorption spectra, indicating that the apical ligand hole density is highly suppressed, resulting in a strong modification of the electronic structure. At the dimensional crossover cellular dynamical-mean-field calculations support the emergence of a Mott insulator ground state in the heterostructured ultrathin slab of LaNiO3. [ABSTRACT FROM AUTHOR]

An optical study of fully strained ultrathin LaNiO3 films is presented and compared with LDA?+?DMFT calculations. LaNiO3 films were grown by pulsed laser deposition on LaAlO3 and SrTiO3 substrates which provide compressive and tensile strain, respectively. Optical conductivity data show a Drude peak with a spectral weight that is significantly reduced compared to that obtained from LDA calculations. The extended Drude analysis reveals the presence of a pseudogap around 80 meV for the film on SrTiO3 and near 40 meV, at low temperature only, for the film on LAO. An unusual temperature dependence of the optical conductivity is observed, with the Drude plasma frequency increasing by up to 20% at low temperature due to spectral weight transfer from bands lying 2-4 eV below the Fermi energy. Such a strong temperature dependence of the Drude spectral weight has previously been reported for correlated electron systems in which a phase transition is present. In LaNiO3, however, no phase transition is observed. [ABSTRACT FROM AUTHOR]

Depth-resolved cathodoluminescence spectroscopy of atomically flat TiO2-terminated SrTiO3 single crystal surfaces reveals dramatic differences in native point defects produced by conventional etching with buffered HF (BHF) and an alternative procedure using HCl–HNO3 acidic solution (HCLNO), which produces three times fewer oxygen vacancies before and nearly an order of magnitude fewer after pure oxygen annealing. BHF-produced defect densities extend hundreds of nanometers below the surface, whereas the lower HCLNO-treated densities extend less than 50 nm. This “Arkansas” HCLNO etch and anneal method avoids HF handling and provides high-quality SrTiO3 surfaces with low native defect density for complex oxide heterostructure growth. [ABSTRACT FROM AUTHOR]

Understanding, creating, and manipulating spin polarization of two-dimensional electron gases at complex oxide interfaces present an experimental challenge. For example, despite almost a decade long research effort, the microscopic origin of ferromagnetism in LaAlO3/SrTiO3 heterojunctions is still an open question. Here, by using a prototypical two-dimensional electron gas (2DEG) which emerges at the interface between band insulator SrTiO3 and antiferromagnetic Mott insulator LaTiO3, the experiment reveals the evidence for magnetic phase separation in a hole-doped Ti d1 t2g system, resulting in spin-polarized 2DEG. The details of electronic and magnetic properties of the 2DEG were investigated by temperature-dependent d.c. transport, angle-dependent X-ray photoemission spectroscopy, and temperature-dependent magnetoresistance. The observation of clear hysteresis in magnetotransport at low magnetic fields implies spin-polarization from magnetic islands in the hole rich LaTiO3 near the interface. These findings emphasize the role of magnetic instabilities in doped Mott insulators, thus providing another path for designing all-oxide structures relevant to spintronic applications. [ABSTRACT FROM AUTHOR]

Rare-earth nickelates exhibit several temperature-driven phase transitions that are tunable by the size of the rare-earth ions, pressure, epitaxial strain in ultrathin films, etc. We investigate the metal-insulator and Neel transitions in a series of NdNiO3 thin films with varying degrees of lattice mismatch using ultralow frequency electrical noise measurements. The noise magnitude follows a 1/f behavior and is Gaussian in the high temperature paramagnetic metallic phase of the films, however deviations are seen in the low temperature paramagnetic and antiferromagnetic insulating phases. The noise magnitude increases by orders of magnitude at temperatures below the metal-insulator transition and is indicative of inhomogeneous electrical conduction arising from phase separation. The inhomogeneous nature of conduction is corroborated by the presence of a large non-Gaussian noise signature in the low temperature phases. Well below the Neel temperature, the noise behavior evolves between Gaussian and non-Gaussian over several hours pointing to dynamically competing ground states with subtle variations within the antiferromagnetic insulating phase. [ABSTRACT FROM AUTHOR]