Thin films of the pyrochlore iridates along the [111] direction have drawn significant attention to investigate exotic correlated topological phenomena. Here, we report the fabrication of Eu2Ir2O7 thin films via reactive solid phase epitaxy using the pulsed laser deposition technique. We mainly focus on the transport properties of the films below the magnetic phase transition at 105 K. Analyses on the temperature and the field dependences of resistivity unveil the presence of weak antilocalization, a characteristic signature of the Weyl semimetallic state that has been “buried” by magnetism. Moreover, it is noteworthy that the contribution from many-body interactions in Eu2Ir2O7 thin films is enhanced at lower temperatures and competes with the weak antilocalization effect, and eventually drives the crossover to weak localization at 2 K.

A uniform one-unit-cell-high step on the SrTiO3 substrate is a prerequisite for growing high-quality epitaxial oxide heterostructures. However, it is inevitable that defects induced by mixed substrate surface termination exist at the interface, significantly impacting the properties of ultrathin films. In this study, we microscopically identify the origin for the lateral inhomogeneity in the growth of ultrathin SrRuO3 films due to the step effects of SrTiO3(001). By using atomic-resolved scanning transmission electron microscopy, we observe two distinct types of step propagation along the [011] and [0-11]crystallographic direction in SrTiO3-SrRuO3 heterostructures, respectively. In particular, the type-II [0-11] step results in lateral discontinuity of monolayer SrRuO3 and originates from the SrO-terminated regions along the TiO2-terminated step edge. Such an induced lateral discontinuity should be responsible for the distinct electronic and magnetic properties of monolayer SrRuO3. Our findings underscore the critical importance of using single termination STO substrate to achieve high-quality termination selective films and to unveil the intrinsic properties of epitaxial films in the atomic limit.

The interplay among symmetry of lattices, electronic correlations, and Berry phase of the Bloch states in solids has led to fascinating quantum phases of matter. A prototypical system is the magnetic Weyl candidate SrRuO3, where designing and creating electronic and topological properties on artificial lattice geometry is highly demanded yet remains elusive. Here, we establish an emergent trigonal structure of SrRuO3 by means of heteroepitaxial strain engineering along the [111] crystallographic axis. Distinctive from bulk, the trigonal SrRuO3 exhibits a peculiar XY-type ferromagnetic ground state, with the coexistence of high-mobility holes likely from linear Weyl bands and low-mobility electrons from normal quadratic bands as carriers. The presence of Weyl nodes are further corroborated by capturing intrinsic anomalous Hall effect, acting as momentum-space sources of Berry curvatures. The experimental observations are consistent with our first-principles calculations, shedding light on the detailed band topology of trigonal SrRuO3 with multiple pairs of Weyl nodes near the Fermi level. Our findings signify the essence of magnetism and Berry phase manipulation via lattice design and pave the way towards unveiling nontrivial correlated topological phenomena.

High entropy oxides (HEOs), which contain multiple elements in the same crystallographic site, are a promising platform for electrocatalysis in oxygen evolution reaction (OER). Investigating these materials in epitaxial thin film form expands the possibility of tuning OER activity by several means, which are not realizable in polycrystalline samples. To date, very few such studies have been reported. In this work, the OER activity of single-crystalline thin films of (La0.2Pr0.2Nd0.2Sm0.2Eu0.2)NiO3, grown on NdGaO3 substrates have been investigated in 0.1 M KOH electrolyte as a function of film thickness. The OER activity increases with the thickness of the film. X-ray absorption spectroscopy measurements find an increase in Ni d-O p covalency and a decrease in charge transfer energy with the increase in film thickness. These facilitate higher charge transfer between Ni and surface adsorbates, increasing OER activity. However, the OER process leads to excessive leaching of thicker films and the OER activity of a 75 unit cell thick film is found to be optimal in the present study. This work demonstrates that the thickness of perovskite oxides can be used as a parameter to enhance OER activity.

The perovskite ruthenates (ARuO3, A = Ca, Ba, or Sr) exhibit unique properties owing to a subtle interplay of crystal structure and electronic-spin degrees of freedom. Here, we demonstrate an intriguing continuous tuning of crystal symmetry from orthorhombic to tetragonal (no octahedral rotations) phases in epitaxial SrRuO3 achieved via Ba substitution (Sr1-xBaxRuO3 with 0 x 0.7). An initial Ba substitution to SrRuO3 not only changes the ferromagnetic properties, but also tunes the perpendicular magnetic anisotropy via flattening the Ru-O-Ru bond angle (to 180 degrees), resulting in the maximum Curie temperature and an extinction of RuO6 rotational distortions at x approximate to 0.20. For x 0.2, the reduction of RuO6 octahedral rotational distortion dominantly enhances the ferromagnetism in the system, though competing with the effect of the RuO6 tetragonal distortion. Further increasing Ba substitution (x > 0.2) gradually enhances the tetragonal-type distortion, resulting in the tuning of Ru-4d orbital occupancy and suppression of ferromagnetism. Our results demonstrate that isovalent substitution of the A-site cations significantly and controllably impacts both electronic and magnetic properties of perovskite oxides.

The electromagnetic properties of ultrathin epitaxial ruthenate films have long been the subject of debate. Here we combine experimental with theoretical investigations of (SrTiO3)(5)-(SrRuO3)n-(SrTiO3)(5) (STO5-SROn-STO5) heterostructures with n = 1 and 2 unit cells, including extensive atomic-resolution scanning-transmission electron-microscopy imaging, electron-energy-loss spectroscopy chemical mapping, as well as transport and magnetotransport measurements. The experimental data demonstrate that the STO5-SRO2-STO5 heterostructure is nearly stoichiometric, metallic, and ferromagnetic with T-C similar to 128 K, even though it lacks the characteristic bulk-SRO octahedral tilts and matches the cubic STO structure. In contrast, the STO5-SRO1-STO5 heterostructure features Ru-Ti intermixing in the RuO2 layer, also without octahedral tilts, but is accompanied by a loss of metallicity and ferromagnetism. Density-functional theory calculations show that stoichiometric n = 1 and n = 2 heterostructures are metallic and ferromagnetic with no octahedral tilts, while nonstoichiometry in the Ru sublattice in the n = 1 case opens an energy gap and induces antiferromagnetic ordering. Thus, the results indicate that the observed nonstoichiometry is the cause of the observed loss of metallicity and ferromagnetism in the n = 1 case.

The new Eu(5)Al(3)Sh(6) phase has been successfully synthesized as a pure phase through Sn flux methods yielding large, high-quality crystals. This structure type features disordered Al clusters that appear in the form of dual tetrahedra. It crystallizes in the monoclinic C2/m space group exhibiting a rock-salt-like Eu-Sb framework with [Al-4] tetrahedra replacing some of the cationic Eu atoms (space group: C2/m, a = 8.151(1) angstrom, b = 14.181(2) angstrom, c = 8.145(1) angstrom, beta = 109.577(2)degrees). The structure models the [Al-4] as dual tetrahedra with the Al atom sites 37.5% occupied along with Eu present on the central site at 8% occupancy and the remainder of the site being vacant. The presence of the [Al-4] cluster is further supported by HRTEM. Electronic structure calculations show that this material is a semimetal with observed band crossings close to the Fermi level. Strong Al-Sb antibonding interactions were found from COHP calculations close to the Fermi level and provide the rationale for the deficiency of the Al cluster. Mossbauer spectroscopy on Eu-151 and Sb-121 provides oxidation states of 2+ and 3- along with the local environment. Magnetic susceptibility measurements can be described well with a Curie-Weiss law where an effective moment of 7.80 mu(B)/mol Eu is obtained, consistent with Eu2+, and show canted antiferromagnetic behavior below 10 K. Temperature dependent resistivity shows a Kondo-like low-temperature upturn caused by enhanced scattering of the itinerant electrons with the 4f orbitals of Eu.

Dislocations often occur in thin films with large misfit strain as a result of strain energy accumulation and can drastically change the film properties. Here the structure and dislocations in oxide heterostructures with large misfit strain are investigated on atomic scale. When grown on SrTiO3 (001), the dislocations in both the monolithic BaTiO3 thin film and its superlattices with SrIrO3 appear above a critical thickness around 6 nm. The edge component of the dislocations is seen in both cases with the Burgers vector of a ⟨100⟩. However, compared to monolithic BaTiO3, the dislocation density is slightly lower in BaTiO3/SrIrO3 superlattices. In the superlattice, when considering the SrTiO3 lattice constant as the reference, BaTiO3 has a larger misfit strain comparing with SrIrO3. It is found that in both cases, the formation of dislocation is only affected by the critical thickness of the film with larger lattice misfit (BaTiO3), regardless of the existence of a strong octahedral tilt/rotation mismatch at BaTiO3/SrIrO3 interface. Our findings suggest that it is possible to control the position of dislocations, an important step toward defect engineering.

Spin glass (SG) is a magnetic state with spin structure incommensurate with lattice and charge. Fundamental understanding of its behavior has a profound impact on many technological problems. Here, we present a novel case of interface-induced spin glass behavior via self-assembly of single-crystalline NiO microcolumns in a single-crystalline NiFe2O4 matrix. Scanning transmission electron microscopy indicates that the hexagonal-shaped NiO columns are along their [211] direction and oriented along the [111] direction of the NiFe2O4 matrix. Magnetic force microscopy reveals magnetic anisotropy between NiO columns (antiferromagnetic transition temperature TN ∼ 523 K) and NiFe2O4 matrix (ferrimagnetic transition temperature TFI ∼ 860 K). This leads to spin disorder/frustration at atomically sharp NiFe2O4/NiO interfaces responsible for spin glass behavior below TSG ∼ 28 K. Our results demonstrate that self-assembly of magnetically distinct microstructures into another crystalline and magnetically ordered matrix is an effective way to create novel spin states at interfaces.

Complex oxide interfaces have been one of the central focuses in condensed matter physics and material science. Over the past decade, aberration corrected scanning transmission electron microscopy and spectroscopy has proven to be invaluable to visualize and understand the emerging quantum phenomena at an interface. In this paper, we briefly review some recent progress in the utilization of electron microscopy to probe interfaces. Specifically, we discuss several important challenges for electron microscopy to advance our understanding on interface phenomena, from the perspective of variable temperature, magnetism, electron energy loss spectroscopy analysis, electronic symmetry, and defects probing.

Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report intriguing magnetic phenomena for La2/3Sr1/3MnO3 films on an SrTiO3 (001) substrate (LSMO/STO), where the interface governs the macroscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, subtle lattice distortions, and oxidation-state changes. Magnetic measurements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, persisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial oxidation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved electron microscopy and spectroscopy for interface studies. Theory demonstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal-insulator polar-nonpolar interfaces.

The origin of simultaneous electronic, structural and magnetic transitions in bulk rare-earth nickelates (RENiO_3) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple metal-insulator transition (MIT) from the magnetic transition. However, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have measured Hall effect on a series of epitaxial NdNiO_3 films, spanning a variety of electronic and magnetic phases. We find that the MIT results in only partially gapped Fermi surface, whereas full insulating phase forms below the magnetic transition. In addition, we also find a systematic reduction of the Hall coefficient (R_H) in the metallic phase of these films with epitaxial strain and also a surprising transition to negative value at large compressive strain. Partially gapped weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high T_c cuprates. The precursor metallic phase, which undergoes transition to insulating phase is a non-Fermi liquid with the temperature exponent (n) of resistivity of 1, whereas the exponent increases to 4/3 in the non-insulating samples. Such nickelate phase diagram with sign-reversal of R_H, pseudo-gap phase and non Fermi liquid behavior are intriguingly similar to high T_c cuprates, giving important guideline to engineer unconventional superconductivity in oxide heterostructure.

Metallic oxide SrVO3 represents a prototype system for the study of the mechanism behind thickness-induced metal-to-insulator transition (MIT) or crossover in thin films due to its simple cubic symmetry with one electron in the 3d state in the bulk. Here we report a deviation of chemical composition and distortion of lattice structure existing in the initial 3 unit cells of SrVO3 films grown on SrTiO3 (001) from its bulk form, which shows a direct correlation to the thickness-dependent MIT. In-situ photoemission and scanning tunneling spectroscopy indicate a MIT at the critical thickness of 3 unit cell (u.c.), which coincides with the formation of a (root2Xroot2)R45 surface reconstruction. However, atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy show depletion of Sr, change of V valence, thus implying the existence of a significant amount of oxygen vacancies in the 3 u.c. of SrVO3 near the interface. Transport and magneto-transport measurements further reveal that disorder, rather than electron correlations, is likely to be the main cause for the MIT in the SrVO3 ultrathin films.

Polar metals are commonly defined as metals with polar structural distortions. Strict symmetry restrictions make them an extremely rare breed as the structural constraints favor insulating over metallic phase. Moreover, no polar metals are known to be magnetic. Here we report on the realization of a magnetic polar metal phase in a BaTiO 3 /SrRuO 3 /BaTiO 3 heterostructure. Electron microscopy reveals polar lattice distortions in three-unit-cells thick SrRuO 3 between BaTiO 3 layers. Electrical transport and magnetization measurements reveal that this heterostructure possesses a metallic phase with high conductivity and ferromagnetic ordering with high saturation moment. The high conductivity in the SrRuO 3 layer can be attributed to the effect of electrostatic carrier accumulation induced by the BaTiO 3 layers. Density-functional-theory calculations provide insights into the origin of the observed properties of the thin SrRuO 3 film. The present results pave a way to design materials with desired functionalities at oxide interfaces.

Interface-driven magnetic properties such as exchange bias and inverted hysteresis are highly sought after in modern functional materials, where at least two magnetically active layers are required. The ability to achieve these functionalities in a single-layer thin film (monolithic) reduces the dimensionality while enriching the magnetism. Here we uncover a previously unseen part of the phase diagram of a monolithic epitaxial thin film of La0.67Sr0.33MnO3 on SrTiO3 which exhibits inverted hysteresis, spontaneous magnetic reversal, and exchange bias due to a structural gradient in the oxygen-octahedral network. Varying the growth conditions, we have mapped the phase diagram of this material and discovered that at a specific oxygen pressure and above a critical thickness, a complex magnetic behavior appears. Atomic-scale characterization shows that this peculiar magnetism is closely linked to a continuous structural gradient that creates three distinct regions within the monolithic film, each with a different magnetism onset. Extracting oxygen-octahedral geometry by electron microscopy, we found that the Curie temperature is directly correlated with the metal-oxygen bond angle. This study illustrates the importance of octahedral geometry in shaping the physical properties of the materials.

Transition-metal (TM)-doped solids are one of the most extensively studied compounds in the fields of catalysis, magnetism, solar cells, etc., due to their tunable optoelectronic properties that stem from TM energy-level hybridization. In this work, the hybridization of the Ni-O bond in TiO2:Ni films was controlled in a stable, reversible manner via surface functionalization with polarized molecules. The Ni-doped TiO2 surface was functionalized with parabenzoic acid groups to modify the electron density distribution within the film. The dopant distribution and elemental composition at the interface are probed via high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy mapping. The effect of the surface modification on the dopant, Ni2+ is studied via surface-sensitive electronic characterization techniques, such as X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy (XAS). The electron density in the valence orbitals of the dopant was observed to be a function of the dipole moment of the para-substituted benzoic acid. The resulting XAS spectra of the Ni2+ after surface modification of TiO2:Ni films were modeled (CTM4XAS) and indicated ligand-dependent symmetry breaking around the Ni2+ at the functionalized interface. Therefore, the modified electron density at the interface due to the polarized molecules is observed to impact the hybridization of the TM dopant energy levels in solid hosts. This phenomenon of adaptive dopant hybridization in a solid host (TiO2) can be exploited to obtain tunable optical responses from TM-doped inorganic phosphors, which have an impact in various fields, such as luminescent displays, solar cells, sensors, telecommunications, counterfeit technologies, and biodetection.

Understanding oxide interface-induced effects requires controlled epitaxial growth of films on well-defined substrate surfaces. While conventional film growth on ex situ prepared substrates has proven to be a successful route, the choices of appropriate substrates with atomically defined surfaces are limited. Here, by depositing La2/3Sr1/3MnO3 on Sr2RuO4 (001), we present an alternative method of growing oxide thin films on in situ cleaved surfaces of layered-structured substrates. Cleaving Sr2RuO4 at low temperature in ultrahigh vacuum exposes an atomically flat, solely SrO-terminated surface with up to micrometer-scale terraces. The deposition of La2/3Sr1/3MnO3 spontaneously diminishes the surface RuO6 in-plane rotational distortion of the substrate and results in a cubic-like perovskite film structure with (La/Sr)-O layer termination. The interface is atomically sharp without obvious deviation of lattice spacing and chemical valence, except in the first unit cell where Ru-Mn intermixing is observed. These results demonstrate that film growth on a cleaved substrate can be an alternative route to obtain well-defined interfaces and in addition increase the availability of substrates for future oxide films.

Extended defects are known to have critical influences in achieving desired material performance. However, the nature of extended defect generation is highly elusive due to the presence of multiple nucleation mechanisms with close energetics. A strategy to design extended defects in a simple and clean way is thus highly desirable to advance the understanding of their role, improve material quality, and serve as a unique playground to discover new phenomena. In this work, we report an approach to create planar extended defects-antiphase boundaries (APB) -with well-defined origins via the combination of advanced growth, atomic-resolved electron microscopy, first-principals calculations, and defect theory. In La2/3Sr1/3MnO3 thin film grown on Sr2RuO4 substrate, APBs in the film naturally nucleate at the step on the substrate/film interface. For a single step, the generated APBs tend to be nearly perpendicular to the interface and propragate toward the film surface. Interestingly, when two steps are close to each other, two corresponding APBs communicate and merge together, forming a unique triangle-shaped defect domain boundary. Such behavior has been ascribed, in general, to the minimization of the surface energy of the APB. Atomic-resolved electron microscopy shows that these APBs have an intriguing antipolar structure phase, thus having the potential as a general recipe to achieve ferroelectric-like domain walls for high-density nonvolatile memory.

Polar metals, commonly defined by the coexistence of polar crystal structure and metallicity, are thought to be scarce because the long-range electrostatic fields favoring the polar structure are expected to be fully screened by the conduction electrons of a metal. Moreover, reducing from three to two dimensions, it remains an open question whether a polar metal can exist. Here we report on the realization of a room temperature two-dimensional polar metal of the B-site type in tri-color (tri-layer) superlattices BaTiO3/SrTiO3/LaTiO3. A combination of atomic resolution scanning transmission electron microscopy with electron energy-loss spectroscopy, optical second harmonic generation, electrical transport, and first-principles calculations have revealed the microscopic mechanisms of periodic electric polarization, charge distribution, and orbital symmetry. Our results provide a route to creating all-oxide artificial non-centrosymmetric quasi-two-dimensional metals with exotic quantum states including coexisting ferroelectric, ferromagnetic, and superconducting phases.

Interfaces between materials present unique opportunities for the discovery of intriguing quantum phenomena. Here, we explore the possibility that, in the case of superlattices, if one of the layers is made ultrathin, unexpected properties can be induced between the two bracketing interfaces. We pursue this objective by combining advanced growth and characterization techniques with theoretical calculations. Using prototype La2/3Sr1/3MnO3 (LSMO)/BaTiO3 (BTO) superlattices, we observe a structural evolution in the LSMO layers as a function of thickness. Atomic-resolution EM and spectroscopy reveal an unusual polar structure phase in ultrathin LSMO at a critical thickness caused by interfacing with the adjacent BTO layers, which is confirmed by first principles calculations. Most important is the fact that this polar phase is accompanied by reemergent ferromagnetism, making this system a potential candidate for ultrathin ferroelectrics with ferromagnetic ordering. Monte Carlo simulations illustrate the important role of spin-lattice coupling in LSMO. These results open up a conceptually intriguing recipe for developing functional ultrathin materials via interface-induced spin-lattice coupling.