Polar metals, a family of materials with exclusive but coexisting electric polarization and metallicity, have attracted plenty of studies recently. Experimentally, how these two exclusive states interact with each other is still an open question thus far. Here, we report on the existence of ferroelectric metal states with switchable electric polarization and unexpected high carrier density in Ba0.5La0.5TiO3 films. A combination of atomic resolution scanning transmission electron microscopy, high-resolution x-ray diffraction, piezoresponse force microscopy, optical second harmonic generation, and electrical transport was utilized to investigate the crystal and electronic structures of Ba0.5La0.5TiO3 films. Unexpectedly, with the modulation of ferroelectricity, the density of conduction electrons can be tuned from 1019 to 1021 cm−3. Our results provide a way to design polar metals with coexisting switchable electric polarization and high-density conduction electrons.

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.

We report comprehensive investigations into the structure of high-quality (111)-oriented SrRuO 3 films on SrTiO 3 substrates to elucidate the effect of (111) heteroepitaxial strain. We found that SrRuO 3 film with a thickness of ∼ 40 nm is compressively strained in plane on the substrate with full coherency. Nevertheless, the out-of-plane spacing is almost the same as in the bulk, which is at odds with the conventional paradigm. By probing a series of half-order Bragg reflections using synchrotron-based x-ray diffraction combined with analyses of the scanning transmission electron microscopy images, we discovered that the heteroepitaxial strain is accommodated via significant suppression of the degree of c + octahedral tilting and the formation of three equivalent domain structures on the (111) SrTiO 3 substrate. This anomalous effect sheds light on the understanding of an unconventional paradigm of film–substrate coupling for the (111) heteroepitaxial strain.

Due to a combination of the interplay between electron- electron correlation (EEC) and spin-orbit coupling, 3d-Sd transition-metal oxide interfaces have been an intriguing platform for the exploration of emergent quantum phenomena. In this work, we investigate the 3d-Sd electron coupling by designing SrIrO3/CaMnO3 superlattices and studying their electrical transport behaviors. The [(SrIrO3)(n)/(CaMnO3)(n)](m) super-lattices show a metal-nonmetal crossover (MNC) with the critical temperature increasing monotonically with decreasing n, until n = 1, a nonmetallic behavior is observed up to room temperature. Detailed analyses reveal that the MNC is the consequence of the weak localization modified by the enhanced EEC in SrIrO3 competing with the effective field of spin- orbital relaxation, with the interfaces with CaMnO3 being introduced to the superlattices. This study provides a framework for understanding the microscopic picture of 3d-Sd electron coupling.

Controlling the interplay between localized spins and itinerant electrons at the oxide interfaces can lead to exotic magnetic states. Here we devise SrTiO 3 /LaTiO 3 /SrTiO 3 heterostructures with varied thickness of the LaTiO 3 layer ( n monolayers) to investigate the magnetic interactions in the two-dimensional electron gas system. The heterostructures exhibit significant Kondo effect when the LaTiO 3 layer is rather thin ( n = 2, 10), manifesting the strong interaction between the itinerant electrons and the localized magnetic moments at the interfaces, while the Kondo effect is greatly inhibited when n = 20. Notably, distinct Shubnikov-de Haas oscillations are observed and a nonzero Berry phase of π is extracted when the LaTiO 3 layer is rather thin ( n = 2, 10), which is absent in the heterostructure with thicker LaTiO 3 layer ( n = 20). The observed phenomena are consistently interpreted as a result of sub-band splitting and symmetry breaking due to the interplay between the interfacial Rashba spin-orbit coupling and the magnetic orderings in the heterostructures. Our findings provide a route for exploring and manipulating nontrivial electronic band structures at complex oxide interfaces.

(111)‐oriented [(SrMnO 3 ) 1 /(LaMnO 3 ) 2 ] n superlattices with artificial A‐site cation ordering with respect to the compositionally equivalent La 2/3 Sr 1/3 MnO 3 films with a random distribution of the A‐site dopants are fabricated. It is found that the ferromagnetism and electrical transport properties of La 2/3 Sr 1/3 MnO 3 films are primarily independent of the degree of A‐site cation ordering, as evidenced by their identical critical temperatures of phase transition. These results may reveal the spontaneous nature of the electronic phase separation, which is key to the colossal magnetoresistance effect in manganites. In contrast, compared to the La 2/3 Sr 1/3 MnO 3 films, the A‐site ordered (111) superlattices have a tendency to preserve oxygen vacancies inside, making them extremely difficult to be compensated by the post‐annealing process.

The d-band-filling of transition metals in complex oxides plays an essential role in determining their structural, electronic and magnetic properties. Traditionally, at the oxide heterointerface, band-filling control has been achieved via electrostatic modification in the structure of field-effect transistors or electron transfer, which is limited to the quasi-two-dimension at the interface. Here we report a three-dimensional (3D) band-filling control by changing the local lattice coordination in a designed oxide heterostructure. At the LaCoO3/LaTiO3 heterointerface, due to the Fermi level mismatch, electrons transfer from LaTiO3 to LaCoO3. This triggers destabilisation of the CoO6 octahedrons, i.e. the formation of lattice configurations with a reduced Co valence. The associated oxygen migration results in the 3D topotactic phase transition of LaCoO3. Tuned by the thickness of LaTiO3, different crystalline phases and band-fillings of Co occur, leading to the emergence of different magnetic ground states.

Oxygen vacancies (Vo) play significant roles in determining the properties of transition-metal oxides. However, the concentration of Vo cannot be tuned quantitatively by optimizing the preparation conditions, and the precise control of Vo distribution at the atomic scale is even more challenging. Here, by controlling the reversible phase transitions between perovskite LaCoO3 (PV-LCO) and brownmillerite LaCoO2.5, we realize the tuning of Vo in PV-LCO, including the concentration with quantitative precision and the spatial distribution at the atomic scale. With the first principles calculations, we clarify that two thirds of Vo in PV-LCO can be eliminated after a cycle of the reversible phase transitions, and all the residual Vo are confined in specific lattice sites in PV-LCO. Such an ordered distribution of Vo can help to enhance the ferromagnetism of PV-LCO.

Compared to their parent materials, oxygen vacancy ( V O )–ordered phases of complex oxides exhibit distinct physical properties and show great potential in multifunctional devices. With the expected unique behaviors, however, most of the V O ‐ordered oxide films are extremely difficult to synthesize due to their thermodynamic instabilities. Herein, using LaCoO x as an example, the synthesis of large‐scale monophased V O ‐ordered LaCoO 2.67 and LaCoO 2.5 films is realized by annealing as‐grown perovskite LaCoO 3 films in vacuum with a special heating method. Their macroscopic properties, including the magnetic and electric transport and optical properties, are measured. It is demonstrated that the functionalities of LaCoO x films can be manipulated by tuning the concentration and ordering of the V O . The method in this work can be applied to fabricate a wide range of V O ‐ordered oxide films.