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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/dyzxy3z2-r2 x2-y2 . To further explore the origin of this configuration, we performed the first-principles density function theory calculations, which linked the orbital occupation to the onsite energy of Ti 3d orbitals. These findings can be important for understanding and designing exotic quantum states in heterostructures based on CaTiO3. [ABSTRACT FROM AUTHOR]

High-quality (001)-oriented (pseudo-cubic notation) ferromagnetic YTiO3 thin films were epitaxially synthesized in a layer-by-layer way by pulsed laser deposition. Structural, magnetic, and electronic properties were characterized by reflection-high-energy-electron-diffraction, X-ray diffraction, vibrating sample magnetometry, and element-resolved resonant soft X-ray absorption spectroscopy. To reveal ferromagnetism of the constituent titanium ions, X-ray magnetic circular dichroism spectroscopy was carried out using four detection modes probing complementary spatial scale, which overcomes a challenge of probing ferromagnetic titanium with pure Ti3+(3d¹). Our work provides a pathway to distinguish between the roles of titanium and A-site magnetic rareearth cations in determining the magnetism in rare-earth titanates thin films and heterostructures. [ABSTRACT FROM AUTHOR]

High quality (111)-oriented CoCr2O4/Al2O3 heterostructures were synthesized on the sapphire (0001) single crystal substrates by pulsed laser deposition. The structural properties are demonstrated by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray reflectivity, and X-ray diffraction. X-ray photoemission spectroscopy confirms that the films possess the proper chemical stoichiometry. This work offers a pathway to fabricating spinel type artificial quasi-two-dimensional frustrated lattices by means of geometrical engineering. [ABSTRACT FROM AUTHOR]

High quality (111)-oriented CoCr2O4/Al2O3 heterostructures were synthesized on the sapphire (0001) single crystal substrates by pulsed laser deposition. The structural properties are demonstrated by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray reflectivity, and X-ray diffraction. X-ray photoemission spectroscopy confirms that the films possess the proper chemical stoichiometry. This work offers a pathway to fabricating spinel type artificial quasi-two-dimensional frustrated lattices by means of geometrical engineering. [ABSTRACT FROM AUTHOR]

We report on the fabrication of high quality (1 1 1)-oriented ferrimagnetic normal spinel CoCr2O4 epitaxial thin films on single crystal Al2O3 substrates. The structural, electronic, and magnetic properties were characterized by in-situ reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, dc magnetization measurement, and element resolved resonant X-ray magnetic scattering. The comprehensive characterization reveals that no disorder in the cation distribution or multivalency issue is present in the samples. As a result, Kagome and triangular layers are naturally formed via this specific growth approach. These findings offer a pathway to fabricate two dimensional Kagomé heterostructures with exotic quantum many-body phenomena by means of geometrical design. [ABSTRACT FROM AUTHOR]

The metal-insulator transition and the underlying electronic and orbital structure in ej, quantum wells based on NdNiO3 were investigated by dc transport and resonant soft x-ray absorption spectroscopy. By comparing quantum wells of the same dimension but with two different confinement structures, we explicitly demonstrate that the quantum well boundary condition of correlated electrons is critical to selecting the many-body ground state. In particular, the long-range orderings and the metal-insulator transition are found to be strongly enhanced under quantum confinement by sandwiching NdNiO3 with the wide-gap dielectric LaAlO3, while they are suppressed when one of the interfaces is replaced by a surface (interface with vacuum). Resonant spectroscopy reveals that the reduced charge fluctuations in the sandwich structure are supported by the enhanced propensity to charge ordering due to the suppressed eg orbital splitting when interfaced with the confining LaAlO3 layer. [ABSTRACT FROM AUTHOR]

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single-unit-cell superlattices of the correlated metal LaNiO3 and the band insulator LaAlO3. Polarized X-ray absorption spectra show a distinct asymmetry in the orbital response under strain. A splitting of orbital energies consistent with octahedral distortions is found for the case of compressive strain. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is present. Density functional theory calculations including a Hubbard U-term reveal that this asymmetry is a result of the interplay of strain and confinement that induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disproportionation at the Ni sites for tensile strain. [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 fabrication of all perovskite Mott insulator/band insulator YTiO3/CaTiO3 superlattices by pulsed laser deposition. The combination of in situ reflection high energy electron diffraction, X-ray diffraction, and X-ray reflectivity confirms the high quality of the films grown in a layer-by-layer mode. Electrical transport measurements reveal that a non-SrTiO3 based twodimensional electron gas system has formed at the YTiO3/CaTiO3 interface. These studies offer another route in the pursuit of complex oxide two-dimensional electron gas systems, which allows to obtain greater insights into the exotic many-body phenomena at such interfaces. [ABSTRACT FROM AUTHOR]

The epitaxial stabilization of a single layer or superlattice structures composed of complex oxide materials on polar (111) surfaces is severely burdened by the reconstructions at the interface that commonly arise to neutralize the polarity. We report on the synthesis of high quality LaNiO3/mLaAlO3 pseudo cubic (111) superlattices on polar (111)-oriented LaAlO3, the proposed complex oxide candidate for a topological insulating behavior. Comprehensive X-Ray diffraction measurements, reflection high energy electron diffraction, and element specific resonant X-ray absorption spectroscopy affirm their high structural and chemical quality. The study offers an opportunity to fabricate interesting interface and topology controlled (111)-oriented superlattices based on ortho-nickelates. [ABSTRACT FROM AUTHOR]

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior. [ABSTRACT FROM AUTHOR]

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior. [ABSTRACT FROM AUTHOR]

We have conducted a detailed microscopic investigation of [LaNiO3(1 u.c.)/LaAlO3(1 u.c.)]N superlattices grown on (001) SrTiO3 and LaAlO3 to explore the influence of polar mismatch on the resulting electronic and structural properties. Our data demonstrate that the initial growth on the nonpolar SrTiO3 surface leads to a rough morphology and unusual 2+ valence of Ni in the initial LaNiO3 layer, which is not observed after growth on the polar surface of LaAlO3. A devised model suggests that the polar mismatch can be resolved if the perovskite layers grow with an excess of LaO, which also accounts for the observed electronic, chemical, and structural effects. [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]

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]