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We report on the successful fabrication of (111) oriented thin films of pyrochlore iridate Y2Ir2O7 by means of solid phase epitaxy. Essentially, we developed an entirely in situ annealing protocol, which is superior to the conventional ex-situ routine that requires multi-hour annealing to stabilize the proper pyrochlore structure. The morphological quality of the films has been confirmed by x-ray diffraction, reflectivity, and reciprocal space mapping, demonstrating their high crystallinity with a pure pyrochlore phase and an expected epitaxial relation to the substrate. Strikingly, below the magnetic phase transition, the temperature dependence of resistivity shows a power-law behavior with the power exponent of ∼3/2, characteristic of a Weyl semimetal in the presence of impurities and electron–electron correlations. Below 10 K, the magneto-transport measurements reveal the emergence of a non-saturated negative magnetoresistance up to 9 T with a small "valley" around zero field. These findings are further interpreted in relation to the weak anti-localization effect and the non-collinear antiferromagnetic ordering on the Ir sublattice. [ABSTRACT FROM AUTHOR]

Heterostructure engineering provides an efficient way to obtain several emergent phases of LaNiO3, as demonstrated in recent studies. In this work, a new class of short-periodic superlattice, consisting of LaNiO3 and EuNiO3, has been grown by pulsed laser interval deposition to investigate the effect of structural symmetry mismatch on the electronic and magnetic behaviors. Using synchrotron-based soft and hard x-ray resonant scattering experiments, we have found that these heterostructures undergo simultaneous electronic and magnetic transitions. Most importantly, LaNiO3 within these artificial structures exhibits a new antiferromagnetic, charge ordered insulating phase, which may be a potential candidate to achieve high temperature superconductivity. [ABSTRACT FROM AUTHOR]

We report on the unusual nanoscale kinetics of the layer-by-layer deposition of complex oxide heterostructures composed of a unit-cell thick correlated metal LaNiO3 and dielectric LaAlO3. The ultrathin multilayers demonstrate extremely good crystallinity and surface morphology maintained over the large number of layers, as confirmed by combination of AFM, RHEED, and synchrotron x-ray diffraction. To elucidate the physics behind the growth, the temperature of the substrate and the deposition rate are varied over a wide range and the results are treated in the framework of a two-layer model. The results demonstrate that, in addition to temperature and vapor supersaturation, modulated flux can be employed as a powerful kinetic handle for the fine-tuned manipulation of sub-monolayer growth regimes of multicomponent materials. [ABSTRACT FROM AUTHOR]

We report on the electronic and magnetic properties of a series of [m EuNiO3/p LaNiO3] superlattices (thickness m and/or p = 1 unit cell) epitaxially grown on single crystalline NdGaO3 substrates. The structural symmetry of these films has been investigated by the combination of in-situ reflection high energy electron diffraction and X-ray diffraction measurements. The metal-insulator transition and the magnetic transition temperatures of the short-period superlattices with m ≥ p are modified from the corresponding bulk Eu1–xLaxNiO3 (x = p m + p ) composition. In contrast to the corresponding bulk doped compound with x = 0.67, the [1 EuNiO3/2 LaNiO3] film remains metallic down to at least 2 K without signs of electronic or magnetic transitions. These findings demonstrate the power of the digital synthesis approach to realize electronic and magnetic phases of perovskite nickelates, unattainable in bulk. [ABSTRACT FROM AUTHOR]

Unlike widely explored complex oxide heterostructures grown along [001], the study of [111]- oriented heterointerfaces is very limited thus far. One of the main challenges is to overcome the polar discontinuity that hinders the epitaxy of atomically sharp interfaces. Here, by taking LaCoO3/ SrTiO3(111) as a prototype, we show that the reconstruction, which effectively compensates the surface polarity, can stabilize the epitaxy of the heterostructure with polar discontinuity. A reconstructed substrate surface is prepared, while the growth is controlled to form reconstruction on the film surface. To suppress the chemical diffusion across the interface, the growth is interrupted between each unit cell layer to allow the lattice relaxation at a lowered temperature. In this way, high quality two-dimensional growth is realized and the heterointerfaces exhibit sharpness at the atomic scale. Our work provides a path to precisely control the growth of complex oxide heterostructures along polar orientations that exhibit emergent quantum phenomena. [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/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]

Ultra-thin films of the electron doped manganite La0.8Ce0.2MnO3 were grown in a layer-by-layer growth mode on SrTiO3 (001) substrates by pulsed laser interval deposition. High structural quality and surface morphology were confirmed by a combination of synchrotron based x-ray diffraction and atomic force microscopy. Resonant X-ray absorption spectroscopy measurements confirm the presence of Ce4þ and Mn2þ ions. In addition, the electron doping signature was corroborated by Hall effect measurements. All grown films show a ferromagnetic ground state as revealed by both dc magnetization and x-ray magnetic circular dichroism measurements and remain insulating contrary to earlier reports of a metal-insulator transition. Our results hint at the possibility of electron-hole asymmetry in the colossal magnetoresistive manganite phase diagram akin to the high-Tc cuprates. [ABSTRACT FROM AUTHOR]

We have performed comparative depth-dependent x-ray excited optical luminescence (XEOL) and depth resolved cathodoluminescence spectroscopy measurements in order to understand the native point defect distribution in three SrTiO3 samples. Both techniques found surface segregation of Ti3+ defects, but apparent differences in the oxygen vacancy distribution. Due to the lower excitation flux densities employed in XEOL, there is a delayed onset ('dead layer') revealed in the oxygen defect depth distribution, which results from band bending near the surface. By modeling the data, we are able to estimate the Ti3+ depth distribution and the depletion layer width. [ABSTRACT FROM AUTHOR]

We studied an electron-induced metal-insulator transition in a two-terminal device based on oxide NdNiO3. In our device, the NdNiO3 is electrostatically doped by the voltage applied between the terminals, resulting in an asymmetric conductivity with respect to the bias polarity. The asymmetry is temperature-dependent and is most significant near the metal-insulator transition. The I-V characteristics exhibit a strong dependence both on the thermal history and the history of the applied voltage bias. Our two-terminal device represents a simple and efficient route for studies of the effect of electron doping on the metal-insulator transition. [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]

Ultra-thin tri-color (tri-layer) titanate superlattices ([3 u.c. LaTiO3/2 u.c. SrTiO3/3 u.c. YTiO3], u.c. = unit cells) were grown in a layer-by-layer way on single crystal TbScO3 (110) substrates by pulsed laser deposition. High sample quality and electronic structure were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [3 u.c. LaTiO3/2 u.c. SrTiO3] bi-layers and all the tri-color structures, whereas a [3 u.c. YTiO3/2 u.c. SrTiO3] bi-layer shows insulating behavior. Considering that in the bulk YTiO3 is ferromagnetic below 30 K, the tri-color titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas with Mott carriers. [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

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]