High entropy oxides (HEOs) are a class of materials, containing equimolar portions of five or more transition metal and/or rare-earth elements. We report here about the layer-by-layer growth of HEO [(La 0.2 Pr 0.2 Nd 0.2 Sm 0.2 Eu 0.2 )NiO 3 ] thin films on NdGaO 3 substrates by pulsed laser deposition. The combined characterizations with in-situ reflection high energy electron diffraction, atomic force microscopy, and X-ray diffraction affirm the single crystalline nature of the film with smooth surface morphology. The desired +3 oxidation of Ni has been confirmed by an element sensitive X-ray absorption spectroscopy measurement. Temperature dependent electrical transport measurements revealed a first order metal-insulator transition with the transition temperature very similar to the undoped NdNiO 3 . Since both of these systems have a comparable tolerance factor, this work demonstrates that the electronic behaviors of A-site disordered perovskite-HEOs are primarily controlled by the average tolerance factor.Finding new materials and new ways to tune material's properties are essential to fulfill the demand of the constantly evolving modern technology. Transition metal oxides show various fascinating electronic and magnetic phenomena such as metal-insulator transition, superconductivity, colossal magnetoresistance, multiferroicity, skyrmions, etc., which have lots of prospect for technological applications 1-6 . Furthermore, transition metal (TM) based high entropy oxides (HEOs) are being explored in recent years to achieve tunable properties in unexplored parts of complex phase diagram 7-21 . In general, the configurational entropy of a multi-component solid solution can be enhanced by mixing a large number of cations in equiatomic proportions and a single structural phase is formed if the entropy contribution overcomes enthalpy driven phase separation (∆G mix =∆H mix -T ∆S mix ; ∆G mix , ∆H mix , ∆S mix are Gibbs free energy, enthalpy and entropy of mixing, respectively) 7,18 . After the report of the first HEO [Mg 0.2 Ni 0.2 Co 0.2 Cu 0.2 Zn 0.2 O with rocksalt structure] by Rost et al. 7 , HEOs with other structural symmetry such as perovskite 15,17 , spinel 16 have been also synthesized. However, this promising field of HEO is at a very early stage and most of the aspects of HEOs are yet to be explored experimentally. For example, it is still unknown whether the strong disorder or the average tolerance factor (t avg ) determines the electronic and magnetic behaviors of perovskite-HEOs.As a prototypical example of perovskite (ABO 3 ) series, RENiO 3 (RE= La, Pr, Nd, Sm, Eu...Lu) exhibits an interesting phase diagram as a function of tolerance factor (t= R RE +RO √ 2(RNi+RO) , where R RE , R Ni , R O are radii of RE, Ni and O, respectively) 22,23 . LaNiO 3 , the least distorted member of this series remains metallic and paramagnetic down to the lowest temperature. Bulk PrNiO 3 and NdNiO 3 (NNO) show temperature driven simultaneous transitions from an orthorhombic, paramagnetic, metallic phase to a monoclini...

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.

The discovery of skyrmions has recently sparked tremendous interest in topologically nontrivial spin textures. The signature of the noncoplanar nature of magnetic moments can be observed as topological Hall effect (THE) in electrical measurement. Realization of such nontrivial spin textures in new materials and through new routes is an ongoing endeavor due to their huge potential for future ultra‐dense, low‐power memory applications. In this work, oxygen vacancy (OV)‐induced THE and anomalous Hall effect (AHE) in a 5d0 system KTaO3 are reported. The observation of weak antilocalization behavior and THE in the same temperature range strongly implies the crucial role of spin–orbit coupling (SOC) behind the origin of THE. Ab initio calculations reveal the formation of the magnetic moment on Ta atoms around the OV and Rashba‐type spin texturing of conduction electrons. In the presence of Rashba SOC, the local moments around vacancy can form bound magnetic polarons (BMPs) with noncollinear spin texture, resulting in THE. Scaling analysis between transverse and longitudinal resistance establishes skew scattering‐driven AHE in the present case. This study opens a route to realize topological phenomena through defect engineering.

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=pm+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.

The choice of electrostatic gating over the conventional chemical doping for phase engineering of quantum materials is attributed to the fact that the former can reversibly tune the carrier density without affecting the system's level of disorder. However, this proposition seems to break down in field-effect transistors involving SrTiO 3 (STO)-based two-dimensional electron gases. Such peculiar behavior is associated with electron trapping under an external electric field. However, the microscopic nature of the trapping centers remains an open question. In this paper, we investigate electric-field-induced chargetrapping and charge-detrapping phenomena at the conducting interface between the band insulators γ -Al 2 O 3 and STO. Our transport measurements reveal that the charge trapping under a positive back-gate voltage (V g ) above the tetragonal-to-cubic structural transition temperature (T c ) of STO has a contribution from electric-field-assisted thermal escape of electrons from the quantum well, and from clustering of oxygen vacancies as well. We observe an additional source of trapping below T c , which arises from the trapping of free carriers at ferroelastic twin walls in the STO. Application of a negative V g results in charge detrapping, which vanishes above T c . This feature demonstrates the crucial role of structural domain walls in the electrical transport properties of STO-based heterostructures. The number of charges trapped (detrapped) at (from) a twin wall is controlled by the net polarity of the wall and is completely reversible with a sweep of V g .