Functional Heterointerfaces of Quantum Materials by Design

The concept of ‘Quantum Materials’, of which the physical properties are beyond a simple description of the laws of classical physics, has been gaining widespread attention across various disciplines in recent years. This area of research encompasses a broad range of materials, including but not limited to two-dimensional materials, unconventional superconductors, multiferroics, complex oxide interfaces and topological quantum materials. As a result, the research of quantum materials provides a vivid platform that brings both scientists and engineers to explore the frontiers of materials science and applications. To fully realize the potential of those quantum materials, it is essential to design them on demand. Fortunately, due to the rapid development of modern techniques, the design strategy of quantum materials has become highly sophisticated and efficient. Approaches that are associated with dimensional confinement, doping, strain, interface engineering and electrical gating are all applicable to the design of quantum materials. Based on their strongly correlated degrees of freedom, the designed heterointerfaces between quantum materials are expected to have great application potentials in various fields, as sketched in Figure 1. Therefore, there is an urgent need for timely reports on connecting the designs and applications of quantum material interfaces. In this special issue, a collection of articles sheds light on the design of quantum materials to offer greater flexibility in functional heterostructures. In article 2200441, Liu et al. demonstrated the correlation between spatial confinement and Rashba spin-orbit coupling at the LaAlO3/KTaO3 heterointerface, of which the large spin-splitting energy and enhanced spin-orbit coupling are essential in exploring Majorana fermions. For the heterostructure that consists of different materials and lattices, strain is another effective way to modify the crystal symmetry and lattice structure in controlling physical properties. In article 2200398, the formation mechanisms of misfit dislocations networks in the quantum-well-type heterostructures were investigated theoretically, allowing a better understanding of the defect formation and local strain relaxation in practical devices. Furthermore, article 2200491 experimentally presented the anisotropic behaviour and sign reversal of magnetoresistance, which is intrinsically coupled to the crystal symmetry in the epitaxially strained SrRuO3/SrTiO3 heterostructures. Besides the lattice structure, the electronic structure of quantum material heterostructures can be significantly modified by doping. article 2200348 showed an example of co-doping via ion implantation, where the artificially induced Co and Nd ions cause the high magnetization in two-dimensional MoS2 at both room and low temperatures. Moreover, as an example of electrical gating, article 2200378 explored an emerging modulation mean of electrochemical gating on quantum functionalities and demonstrated significant changes in both conductivity and magnetism in a ferromagnetic metal system. In the same study, the author further fabricated a functional synaptic transistor, linking the design of interfaces and their functional applications, which is artificial intelligence in this case. Furthermore, in article 2200272, Huang et al. adopted interfacial engineering using different growth parameters to control the transport properties of the LaAlO3/SrTiO3 bilayer, in which the carrier scatting mechanism is different from the conventional SrTiO3 interface with the lower carrier densities. On the side of potential applications, carefully-designed quantum materials and heterointerfaces hold vast promise. article 2200493 provided a review of recent progress in various types of spintronic heterostructures, in which the magnetization is controlled by spin-orbit torque. This article also addressed the