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Nature Materials reports ICQM faculty members Wei Han and Xin-Cheng Xie’s invited review article titled “spin current as a probe of quantum materials”

Recently, ICQM faculty members Wei Han and Xin-Cheng Xie, together with their collaborator Sadamichi Maekawa from Japan RIKEN Center for Emergent Matter Science were invited to write a review article in Nature Materials to introduce the emergent research direction of using spin current as a probe of quantum materials.

Spin current, one of the most important concepts in spintronics, historically referred to the flow of electrons carrying spin information, in particular since the discovery of giant magnetoresistance in the 1980s. Different from conventional charge current, spin current with low energy dissipation has great potential for further application and fundamental physical investigation. Recently, it has been found that spin current can also be mediated by spin-triplet supercurrent, superconducting quasiparticles, spinons, magnons, spin superfluidity, and so on.

Quantum materials are of central interest in the fields of condensed matter physics and materials science. They are a class of materials whose quantum properties stem from an intimate connection between reduced dimensionality, quantum confinement, quantum coherence, quantum fluctuations, topology of wavefunctions, relativistic spin–orbit interactions, fundamental symmetries, and so on. Examples of quantum materials include graphene, high-temperature superconductors (SCs), topological insulators, Weyl semimetals, quantum spin liquids, spin superfluidity and so on. To measure spin-dependent properties of quantum materials, it’s necessary to develop a powerful tool, which should be easily achieved and manipulated. Fortunately, experimental physical scientists, together with theoretical physical scientists, utilize spin current as a probe to quantum materials successfully. A series of methods have come true to activate and detect the spin current in quantum materials effectively. Abundant experimental and theoretical works have already shown the potential of spin current as a probe to quantum materials.

Recently, it is demonstrated that the spin current can be mediated by spin-triplet pairs and superconducting quasiparticles in superconductors (SCs), spinons in quantum spin liquids, magnons in magnetic insulators and spin superfluidity, as summarized in Table 1.

This review article highlights the key progress concerning the use of spin current as a novel probe for a series of quantum materials, including spin-triplet supercurrent and spin dynamics in superconductors, quantum spin liquids, magnetic phase transitions, hybrid quantum collective modes, such as magnon-polarons, magnon-polaritons, magnon Bose-Einstein condensate (BEC) as well as spin superfluidity in canted antiferromagnets and electron-hole excitons.

These important progresses have identified spin current as a novel probe of quantum materials. Looking forward, a probe based on spin current will have a bright future for further investigation of new quantum materials, for example quantum Hall structures and quantum spin Hall materials, quantum magnets and antiferromagnets, quantum chiral phonons in hexagonal lattice systems, spin-mechanical motion coupled quantum systems, spin dynamics of SCs and the superconducting gap at the FM/SC interfaces, pairing symmetry of spin-triplet SC candidates, new hybridized excitations of strongly coupled magnon and coupled spin systems, quantum and topological magnon materials, spinons in various quantum spin liquids, and the spin superfluid Josephson effect, as well as the other quantum states that can mediate spin current. The progress in this exciting field will motivate the development of spin-dependent imaging techniques to directly measure the spin current flow such as spin-polarized STM and the nitrogen vacancy center microscopy.

This review has been published online in Nature Materials on August 26th, 2019 (Nature Materials, DOI: https://doi.org/10.1038/s41563-019-0456-7). The work is financially supported by National Basic Research Programs of China, National Natural Science Foundation of China, and the Strategic Priority Research Program of Chinese Academy of Sciences.