Title: Novel properties due to crystal symmetry in altermagnets and topological superconductors

Speaker: LIU Junwei，The Hong Kong University of Science and Technology

Time：2025-09-01 16:00

Venue: Physics Building W105

Abstract:I will talk about the novel properties due to crystal symmetry spin splitting antiferromagnets (AFMs) [Nat. Commun. 12, 2846 (2021)] (named as altermagnet in 2022) and in topological superconductor [Nature 633, 71 (2024)]. In spin-splitting AFMs, we propose the crystal-symmetry-paired spin-valley/momentum locking (CSVL/CSML), which is enabled by crystal symmetries intrinsically in AFMs (e.g., V₂Se₂O, V₂Te₂O, MnTe and RuO₂) [Nat. Commun. 12, 2846 (2021); Phys. Rev. X 15, 021083 (2025)]. CSML enables feasible controls of AFMs by manipulating the corresponding crystal symmetry. Typically, one can use a strain field to induce net valley polarization/magnetization and use an electric field to generate a noncollinear spin current even without spin-orbit coupling. All the predictions have been confirmed in experiments [Nat. Phys. (2025); Nat. Phys. (2025)]. These properties have helped us realize the electric readout and 180^o deterministic switching of the Néel order in our experimental work [Sci. Adv. 10, eadn0479 (2024); Nature 638, 645 (2025)].

In topological superconductors, magnetic mirror symmetry can protect multiple Majorana zero modes (MZMs) in a single vortex, which allows feasible controls of hybridization of MZMs simply using an external field. This has been realized in our recent collaborative experimental works with Prof Jin-Feng Jia’s group [Nature 633, 71 (2024); Quantum Frontiers 3, 20 (2024)]. Similar properties can also be realized in superconducting CSVL/CSML materials.

Bio: Prof. Junwei Liu obtained his PhD at Tsinghua University in 2014 and then did his postdoctoral research at MIT from 2014 to 2017. He then joined HKUST as an assistant professor and was promoted to an associate professor in 2023. He has a broad interest in condensed matter physics, quantum physics and materials science. His major contributions include (1) theoretical prediction of SnTe-type topological crystalline insulator, WTe₂-type and TaIrTe₄-type quantum spin Hall insulator; (2) discovery of world-first monolayer ferroelectricity in SnTe thin films; (3) proposal of self-learning Monte Carlo methods that can be thousands of times faster than conventional methods without loss of any accuracy; (4) design and realization of the world-first all-optical neural networks; and (5) proposal of crystal-symmetry-paired spin-valley locking (also named altermagnet) and realization of world-first electrical readout and 180° switching of the Néel order in spin-splitting antiferromagnetic materials. He has published more than 70 papers including 2 in Science, 3 in Nature, 2 in Nat. Phys., 3 in Nat. Mater., 7 in Nat. Commun., 3 in Phys. Rev. Lett., 1 in Optica, etc.