Research on the electronic phase transitions in strongly correlated oxides and multi-field regulation

Abstract

External-field-triggered multiple electronic phase transitions within correlated oxides open up a new paradigm to explore exotic physical functionalities and new quantum transitions via regulating the electron correlations and the interplay in the degrees of freedom, which makes the multidisciplinary fields have the promising application prospects, such as neuromorphic computing, magnetoelectric coupling, smart windows, bio-sensing, and energy conversion. This review presents a comprehensive picture of regulating the electronic phase transitions for correlated oxides via multi-field covering the VO<sub>2</sub> and <i>Re</i>NiO<sub>3</sub>, thus highlighting the critical role of external field in exploring the exotic physical property and designing new quantum states. Beyond conventional semiconductors, the complex interplay in the charge, lattice, orbital and spin degrees of freedom within correlated oxides triggers abundant correlated physical functionalities that are rather susceptible to the external field. For example, hydrogen-related electron-doping Mottronics makes it possible to discover new electronic phase and magnetic ground states in the hydrogen-related phase diagram of correlated oxides. In addition, filling-controlled Mottronics by using hydrogenation triggers multiple orbital reconfigurations for correlated oxides away from the correlated electronic ground state that results in new quantum transitions via directly manipulating the d-orbital configuration and occupation, such as unconventional Ni-based superconductivity. The transition metals of correlated oxides are generally substituted by dopants to effectively adjust the electronic phase transitions via introducing the carrier doping and/or lattice strain. Imparting an interfacial strain to correlated oxides introduces an additional freedom to manipulate the electronic phase transition via distorting the lattice framework, owing to the interplay between charge and lattice degrees of freedom. In recent years, the polarization field associated with BiFeO<sub>3</sub> or PMN-PT material triggered by a cross-plane electric field has been used to adjust the electronic phase transition of correlated oxides that enriches the promising correlated electronic devices. The exotic physical phenomenon as discovered in the correlated oxides originates from the non-equilibrium states that are triggered by imparting external fields. Nevertheless, the underneath mechanism as associated with the regulation in the electronic phase transitions of correlated oxides is still in a long-standing puzzle, owing to the strong correlation effect. As a representative case, hydrogen-associated Mottronic transition introduces an additional ion degree of freedom into the correlated oxides that is rather difficult to decouple from the correlated system. In addition, from the perspective of material synthesis, the above-mentioned correlated oxides are expected to be compatible with conventional semiconducting process, by which the prototypical correlated electronic devices can be largely developed. The key point that accurately adjusts and designs the electronic phase transitions for correlated oxides via external fields is presented to clarify the basic relationship between the microscopic degrees of freedom and macroscopic correlated physical properties. On the basis, the multiple electronic phase transitions as triggered by external field within correlated oxides provide new guidance for designing new functionality and interdisciplinary device applications.