Discovery of a New Electronic State in Functional Oxide: A Breakthrough Towards Next-Generation High-Performance Electronic Materials

A research group from the Graduate School of Engineering and the Graduate School of Science at The University of Tokyo, in collaboration with NTT, Inc. and Japan Atomic Energy Agency (JAEA), has, for the first time, demonstrated through synchrotron radiation-based photoelectron spectroscopy on ferromagnetic metal oxide SrRuO₃ [a chemical compound from Sr (strontium), Ru (ruthenium), and O (oxygen)] that the electronic state of the anionic O orbitals is distinct from that of the cationic Ru orbitals even in the Ru-O hybridized state due to strong electron correlation.

Functional oxides have been intensively researched as functional materials because, in addition to the electrical conductivity, magnetism, and dielectric properties of metals, semiconductors, and insulators used in conventional electronics, they exhibit a variety of properties that are expected to be useful in future electronics, such as superconductivity, giant magnetoresistance, and multiferroics. The complex motion of electrons caused by the electrostatic repulsive force (Coulomb interaction) between electrons in a material is referred to as “electron correlation”. In particular, electron correlation in transition metals, which are cations, is considered to play a crucial role in the manifestation of these physical properties. Therefore, the research focusing on the electronic state of oxygen atoms in functional oxides has been largely neglected. Additionally, the production of high-quality functional oxides necessary for precisely investigating the electronic states of oxygen atoms has been challenging. Recently, both theoretically and experimentally, the electronic correlation of oxygen has been discussed alongside that of transition metal in functional oxides. However, it is still unclear how electron correlation of the anionic O affects the properties of functional oxides.

Functional oxide SrRuO₃ is a material with properties important for electronics applications, such as metallic conduction, ferromagnetism, and perpendicular magnetic anisotropy, and has been studied for over 60 years since its discovery. In particular, in recent years, it has been demonstrated that electronic states reflecting topological properties called magnetic Weyl semimetal states exist in ultra-high-quality thin films, making it a material of interest for topological spintronics applications. SrRuO₃ forms an electronic state through the hybridization of the electronic orbitals of the Ru cation and the O anion, but it was previously unclear how the electronic state at the Fermi level, which determines the aforementioned properties, contributes to electrical conductivity. In this study, the research group performed photoelectron spectroscopy using synchrotron radiation on SrRuO₃ thin films with the world's highest crystallinity in order to precisely investigate the electronic states at the Fermi level in SrRuO₃.

In this study, the research group discovered that, in the Ru-O hybridized state near the Fermi level, the partial density of states derived from the Ru 4d orbital crosses the Fermi level and is metallic (electrons are itinerant: contributing significantly to the electrical conductivity), whereas the partial density of states derived from the O 2p orbitals is nearly zero at the Fermi level and is close to an insulating (electrons are localized: contributing little to electrical conductivity). Previously, it was thought that the hybridized electronic states of Ru and O in SrRuO₃ would have the same shape regardless of the electronic orbitals, but the results of this study differ from the previous understanding. Therefore, the research group experimentally estimated the magnitude of “electron correlation” in the O 2p orbital and found that the electron correlation of O is several times larger than that of Ru. This large electron correlation of O suggests that the electronic states derived from O orbitals are closer to insulating states and contribute little to the electrical conductivity. The observation of the influence of the electron correlation on electronic states at the Fermi energy is a world-first result and provides a new perspective for the long-standing field of functional oxide research.

It has been believed so far that the electron correlation of transition metal cations has a significant influence on the physical properties of functional oxides. The results of this study, which overturn the conventional wisdom, indicate the need to reevaluate the influence of electron correlation of O anion in functional oxides, and represent the discovery of a new concept that can be widely applied to other materials in which the electronic orbitals of transition metals and O are hybridized. Going forward, it is expected that the development of new-principle-based magnetic memory and quantum devices will be possible by improving the accuracy of material design through the construction of a theoretical framework incorporating electron correlation of O, and by utilizing functional oxide design and simulation based on property predictions derived from theoretical calculations and large-scale computations.

Conducting electrons in each electronic orbital in the functional oxide SrRuO₃: It has been clarified that the interaction between electrons (electron correlation) in the Ru 4d-O 2p hybrid orbital of SrRuO₃ depends on the electronic orbital. In the Ru 4d orbital (red clover-shape), electron correlation is weak, and electrons in this orbital mainly contribute to electrical conduction. On the other hand, in the O 2p orbital (blue dumbbell-shape), the electron correlation is strong, causing the electrons to localize in the O 2p orbitals and contribute little to electrical conduction (electrons move between the sites due to scattering between electrons).

Papers

Journal: Physical Review Letters

Title: Correlated Ligand Electrons in the Transition-Metal Oxide SrRuO₃

Authors: Yuichi Seki, Yuki K. Wakabayashi, Takahito Takeda, Kohdai Inagaki, Shin-ichi Fujimori, Yukiharu Takeda, Atsushi Fujimori, Yoshitaka Taniyasu, Hideki Yamamoto, Yoshiharu Krockenberger, Masaaki Tanaka, and Masaki Kobayashi*

DOI: 10.1103/n9qh-6739