Straining quantum materials even further

Oxidic 2D Materials

The possibility of producing stable thin films, only a few atomic layers thick, from a variety of materials beyond graphene has led to two-dimensional (2D) materials being studied intensively in recent years. By reducing the layer thickness and approaching the crystallographic monolayer limit, a variety of unexpected and technologically relevant property phenomena were observed, which also depend on the subsequent arrangement and possible combination of individual layers to form heterostructures. These properties can be specifically used for the development of multifunctional devices, meeting the requirements of the advancing miniaturization of modern manufacturing technologies and the associated need to stabilize physical states even below critical layer thicknesses of conventional materials in the fields of electronics, magnetism and energy conversion. Differences in the structure of potential two-dimensional materials result in decisive influences on possible growth methods and possibilities for subsequent transfer of the thin films. In this review, we focus on recent advances in the rapidly growing field of two-dimensional materials, highlighting those with oxidic crystal structure like perovskites, garnets and spinels. In addition to a selection of well-established growth techniques and approaches for thin film transfer, we evaluate in detail their application potential as free-standing monolayers, bilayers and multilayers in a wide range of advanced technological applications. Finally, we provide suggestions for future developments of this promising research field in consideration of current challenges regarding scalability and structural stability of ultra-thin films.

Eu-Mn Charge Transfer and the Strong Charge-Spin-Electronic Coupling Behavior in EuMnO3

Based on first-principles calculations with the DFT + U method, the couplings of lattice, charge, spin, and electronic behaviors underlying the Eu-Mn charge transfer in a strongly correlated system of EuMnO₃ were investigated. The potential valence transition from Eu³⁺/Mn³⁺ to Eu²⁺/Mn⁴⁺ was observed in a compressed lattice with little distortions, which is achieved under hydrostatic pressure and external strain. The intraplane antiferromagnetism (AFM) of Mn is proved to be instrumental in the emergence of Eu²⁺. Furthermore, we calculated the magnetic exchange interactions within two equilibrium structures of Eu³⁺Mn³⁺O₃ and Eu²⁺Mn⁴⁺O₃. Mn-Mn ferromagnetic exchange in the ab-plane is enhanced strongly in the Eu²⁺Mn⁴⁺O₃ structure, contributing to the existence of mixed states. The versatile electronic structures were obtained within the Eu²⁺Mn⁴⁺O₃ phase by imposing different magnetic configurations on the Eu and Mn sublattice, attributed to the coupling of charge transfer and magnetic orderings. It is found that the intraplane ferromagnetic ordering of Mn leads to a metallic electronic structure with the coexistence of Eu²⁺ and Eu³⁺, while the intraplane AFM Mn spin ordering leads to insulating states only with Eu²⁺. Notably, a half-metallic characteristic emerges at the magnetic ground state of CF ordering (C-type AFM for the Eu sublattice and ferromagnetic for the Mn sublattice), which makes such a supposed phase more intriguing than the conventional experimental phase. Additionally, the mixture of delocalized 4f with 5d states of Eu in the background of Mn 3d and O 2p orbitals implies a pathway of Eu 4f 5d ↔ O 2p ↔ Mn 3d for charge transfer between Eu and Mn. Our calculation shows that the Eu-Mn charge transfer could be expected in compressed EuMnO₃ and the introduction of Eu²⁺ 4f states near the Fermi level plays an important role in manipulating the interlinks of charge and spin together with electronic behaviors.