Unexpected, yet useful functionalities emerge when two or more materials merge coherently. Artificial oxide superlattices realize atomic and crystal structures that are not available in nature, thus providing controllable correlated quantum phenomena. This review focuses on 4d and 5d perovskite oxide superlattices, in which the spin–orbit coupling plays a significant role compared with conventional 3d oxide superlattices. Modulations in crystal structures with octahedral distortion, phonon engineering, electronic structures, spin orderings, and dimensionality control are discussed for 4d oxide superlattices. Atomic and magnetic structures, Jeff = 1/2 pseudospin and charge fluctuations, and the integration of topology and correlation are discussed for 5d oxide superlattices. This review provides insights into how correlated quantum phenomena arise from the deliberate design of superlattice structures that give birth to novel functionalities. 

Correlated Quantum Phenomena of Spin–Orbit Coupled Perovskite Oxide Heterostructures: Cases of SrRuO
 3
 and SrIrO
 3
 Based Artificial Superlattices

LaMnO3 is an insulator with perovskite structure, which attracts the intense attention due to its promising properties. However, the previous studies mainly focused on the p-type doping of LaMnO3, while the n-type doping is under investigated. The effects of doping with various concentrations on the lattice structure, energy band and optical absorption of La1-<italic>x</italic> Hf <italic>x</italic> MnO3, as well as the spin valve device consisted of La1-<italic>x</italic> Hf <italic>x</italic> MnO3 scattering region sandwiched by two Ba doped LaMnO3 probes, are studied by the first-principles calculations based on the density functional theory. The results show that the antiferromagnetic or ferrimagnetic structure rather than the ferromagnetic structure is the ground state for La1-<italic>x</italic> Hf <italic>x</italic> MnO3. Hf elements exist in La1-<italic>x</italic> Hf <italic>x</italic> MnO3 as stable Hf 4+ ions, showing the properties of n-type doping. When the HF doping concentration x is small (i.e. <italic>x</italic> ≤ 0.031), the bandgap of La1-<italic>x</italic> Hf <italic>x</italic> MnO3 is close to that of the undoped intrinsic phase LaMnO3, and an isolated impurity level due to Hf doping is formed in the bandgap. The optical absorption spectrum of La1-<italic>x</italic> Hf <italic>x</italic> MnO3 shows that its optical absorption edge appears a red shift and the optical bandgap decreases with the increase of <italic>x</italic>. Meanwhile, the spin valve device composed of La3/4Ba1/4MnO3 as the left and right electrodes and La3/4Hf1/4MnO3 as the center layers is calculated, and the magnetoresistance near the Fermi level reaches about ~ (2.8 × 105)%. Through the gate voltage manipulation, the magnetoresistance can reach up to ~ (5.1 × 105)%. Our work provides a theoretical guideline for design of novel spintronic devices.

First-principles study on Hf doped LaMnO3

(111)‐oriented [(SrMnO3)1/(LaMnO3)2]n superlattices with artificial A‐site cation ordering with respect to the compositionally equivalent La2/3Sr1/3MnO3 films with a random distribution of the A‐site dopants are fabricated. It is found that the ferromagnetism and electrical transport properties of La2/3Sr1/3MnO3 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 La2/3Sr1/3MnO3 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.

Effect of A‐Site Cation Ordering on the Electrical and Magnetic Properties of Manganite Films

Due to a combination of the interplay between electron–electron correlation (EEC) and spin–orbit coupling, 3d–5d transition-metal oxide interfaces have been an intriguing platform for the exploration of emergent quantum phenomena. In this work, we investigate the 3d–5d electron coupling by designing SrIrO3/CaMnO3 superlattices and studying their electrical transport behaviors. The [(SrIrO3)n/(CaMnO3)n]m superlattices 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–5d electron coupling. 

Modulation of the Metal–Nonmetal Crossover in SrIrO3/CaMnO3 Superlattices


 We report comprehensive investigations on the structure of high-quality (111)-oriented SrRuO3films on SrTiO3 substrates to elucidate the effect of (111) heteroepitaxial strain. It is found that SrRuO3 film 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 its 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 the analyses of the scanning transmission electron microscopy images, we discovered that the heteroepitaxial strain is accommodated via the significant suppression of the degree of c
 + octahedral tilting, and the formation of three equivalent domain structures on (111) SrTiO3 substrate. This anomalous effect shed light on the understanding of unconventional paradigm of film-substrate coupling for the (111) heteroepitaxial strain.

Anomalous strain effect in heteroepitaxial SrRuO3 films on (111) SrTiO3 substrates

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.

Engineered Kondo screening and nonzero Berry phase in SrTiO3/LaTiO3/SrTiO3 heterostructures

Controlling the interplay between localized spins and itinerant electrons at the oxide interfaces can lead to exotic magnetic states. Here we devise SrTiO3/LaTiO3/SrTiO3 heterostructures with varied thickness of the LaTiO3 layer (n monolayers) to investigate the magnetic interactions in the two-dimensional electron gas system. The heterostructures exhibit significant Kondo effect when the LaTiO3 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 {\pi} is extracted when the LaTiO3 layer is rather thin (n = 2, 10), which is absent in the heterostructure with thicker LaTiO3 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. 

/pdf/engineered-kondo-screening-and-nonzero-berry-phase-in-mml-6bck4waf.pdf

Engineered Kondo screening and nonzero Berry phase in 
<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>SrTiO</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>LaTiO</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>SrTiO</mml:m

Zhenzhen Wang

Papers

Effect of A‐Site Cation Ordering on the Electrical and Magnetic Properties of Manganite Films

Modulation of the Metal–Nonmetal Crossover in SrIrO3/CaMnO3 Superlattices

Anomalous strain effect in heteroepitaxial SrRuO3 films on (111) SrTiO3 substrates

Engineered Kondo screening and nonzero Berry phase in SrTiO3/LaTiO3/SrTiO3 heterostructures

Engineered Kondo screening and nonzero Berry phase in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>SrTiO</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>LaTiO</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:msub><mml:mi>SrTiO</mml:m