Research progress of novel quantum states and charge-spin interconversion in transition metal oxides

For satisfying the efficient storage and processing of massive data in the information technology era, spintronic device attracts tremendous attention due to its low power consumption and non-volatile feature. Spin source material, which can efficiently generates spin current, is an important constituent of novel spin-orbit torque device. The efficiency of spin current generation in spin source materials directly determines the performance of various spintronic devices. In the nearly two decades, great progress has been achieved in exploring high-efficient spin source material systems, as well as understanding the relevant physical mechanisms. A wide variety of materials are explored ranging from the traditional heavy metal and semiconductors to topological insulators and 2D materials. Recently, the material family of transition metal oxides attracts tremendous attention due to its efficient and highly tunable charge-spin conversion intimately related to its emerging novel quantum states and electronic structure. The mechanism of charge-spin conversion generally have two contributions from the bulk spin Hall effect and the spin-momentum locked interface with inversion symmetry breaking. Novel electronic structure such as the topological band structure and spin-momentum locked surface states can realize efficient charge-spin conversion. For example, the Weyl points in SrRuO3 and the topological Dirac nodal line in SrIrO3 are predicted to give rise to large Berry curvature and the corresponding spin Hall conductance; the topological surface states can generate spin accumulation due to spin-momentum locking; the Rashba states at the oxide interface such as the 2D electron gas in SrTiO3 and KTaO3 can generate spin current by Rashba-Edelstein effect. Furthermore, the entanglement of various degrees of freedom, including spin, charge, lattice and orbit in transition metal oxides lead to the electronic structure highly tunable by various methods including gate voltage, substrate constraint, thickness, interface engineering etc. Therefore, the charge-spin conversion in transition metal oxides is of great interest for both fundamental research in modulation of novel electronic structure and exploring its promising potential in future spintronic devices. In this review, we focus on introducing the aspects of exotic electronic structures, spin transport mechanism, charge-spin interconversion characterization, efficiency and manipulation in TMOs, and giving a perspective about the future development trend.