Chaoyu Chen

Recent theoretical and experimental efforts have revealed the existence of a new class of magnetic materials, altermagnets, characterized by distinct symmetries in spin and real space. Combining the properties of ferromagnets, such as band spin polarization and time-reversal symmetry breaking responses, and antiferromagnets, like vanishing stray field and terahertz magnon dynamics, altermagnets are attracting enormous attention in both magnetism and spintronics community. While numerous altermagnetic candidates and emerging physical properties have been studied in theory extensively and enthusiastically, the experimental investigation of altermagnetism is struggling against the lack of proper altermagnetic materials and the controversial results reported for specific candidates. In this review, we first give a general description on the concept and key properties of altermagnetism, outline its distinction from other compensated magnetic order from the aspects of time-reversal symmetry in the spin momentum locking texture and time-reversal symmetry breaking responses. The main body of this review concentrates on the progress in altermagnetic material study with special focus on their band structure and spin polarization features, as well as the controversies over several candidate materials. Going forward, we point out several directions which altermagnetic material investigation might follow and guidelines which could help to avoid the pitfalls in the experimental pursuit of ideal altermagnets.

Altermagnetism has recently emerged as a third fundamental branch of magnetism, distinct from conventional ferromagnetism and antiferromagnetism (AFM). Characterized by zero net magnetization yet exhibiting time-reversal symmetry breaking and non-relativistic spin splitting in momentum space, altermagnets offer a unique platform for next-generation spintronics. This review highlights the pivotal role of angle-resolved photoemission spectroscopy (ARPES) in experimentally validating this phase by directly visualizing its hallmark spin-split electronic bands. We critically examine recent ARPES investigations across a spectrum of representative candidate materials, ranging from the debated d-wave prototype RuO ₂ , where spin splitting remains elusive, to the robust observation of crystal-symmetry-paired spin-valley locking in layered systems such as KV ₂ Se ₂ O and Rb _{1– δ} V ₂ Te ₂ O. Furthermore, we discuss the direct detection of giant spin splitting in g-wave compounds like MnTe and high-temperature CrSb, alongside the interplay of altermagnetism with correlated orders in CoNb ₄ Se ₈ . On the other hand, we also discuss the observation of AFM-induced spin splitting in MnTe ₂ , which is not an altermagnet but belongs to an extended category of spin-split antiferromagnet. Finally, we provide an outlook on emerging opportunities, including the use of nano-ARPES for domain imaging, the search for new candidate materials, and the potential for realizing topological superconductivity in altermagnetic heterostructures.

The primary focus of spintronics is the investigation of novel spin splitting effects and related spin-polarized quantum materials, which have been extensively pursued for their potential applications. The structural inversion asymmetric Rashba splitting, bulk inversion asymmetric Dresselhaus splitting, and ferromagnetic spin polarization derived from Zeeman splitting constitute the foundation of traditional spintronics. From a symmetry perspective, ferromagnets achieve spin splitting through the breaking of time-reversal symmetry. However, in time-reversal symmetric and inversion symmetric materials with spin–orbit coupling, unexpected forms of spin-splitting can also arise by breaking local inversion symmetry, known as hidden spin-momentum locking (SML), bringing infinite vitality to fundamental research and future applications. This review first highlights notable advancements in spin-splitting within centrosymmetric systems, then examines the influence of hidden SM locking on superconducting and topological behaviors, concluding with a discussion on prospective opportunities in this emerging field. Given the rapid progress in non-relativistic spin splittings—particularly within altermagnetism—we develop appropriately scaled extensions to advance this emerging field. This review seeks to enhance our understanding of the ‘hidden effect’ in fundamental research while uncovering additional quantum phenomena that emerge from introducing extra degrees of freedom—an aspect that underscores the unique appeal of quantum materials capable of continuously demonstrating novel effects.

We have investigated the surface and bulk electronic structures of the superconducting type-II Dirac semimetal 1T-PdSeTe. The superconducting transition temperature T c = 3.2 K was almost twice as high as T c = 1.6 K in 1T- PdTe 2 . Scanning transmission electron microscopy measurements showed homogeneously mixed Se and Te atoms in the chalcogen layers, consistent with the CdI 2 -type crystal structure. Angle-resolved photoemission spectroscopy measurements and density functional theory calculations indicated the existence of the topological surface states, and the overall band structures were similar to those of 1T- PdTe 2 . These results suggest that the CdI 2 -type lattice symmetry dictates the band dispersion, regardless of atomic disorder in the chalcogen layers. As the electronic band dispersion and the local structures were persistent upon substitution, the enhancement of T c is likely associated with the chemical pressure. Our results provide insight into the effects of the solid solution on the surface and bulk electronic states as well as the superconducting transition temperature. Published by the American Physical Society 2025

To generate and manipulate spin‐polarized electronic states in solids are crucial for modern spintronics. The textbook routes employ quantum well states or Shockley/topological type surface states whose spin degeneracy is lifted by strong spin‐orbit coupling and inversion symmetry breaking at the surface/interface. The resultant spin polarization is usually truncated because of the intertwining between multiple orbitals. Here a unique type of surface states is realized, namely, dangling bond surface states in a family of ternary rare‐earth pnictide tellurides RePnTe (Re = La, Gd, Ce; Pn = Sb, Bi), with robust band structure and sizeable spin splitting. Spin and angle‐resolved photoemission spectroscopy measurements reveal high spin polarization and distinct spin‐momentum locking texture, which, according to the theoretical analysis, arise from local site asymmetry and surface‐purified spin‐orbital texture. The work extends the so‐called “hidden spin polarization” from the bulk to the surface, presenting an intriguing spin‐orbital‐momentum‐layer locking phenomenon, which may shed lights on potential spintronic applications.

The recently discovered kagome superconductors AV₃Sb₅ (A= K, Rb, Cs) provide a new platform to explore intertwined symmetry-breaking orders. However, great controversies exist to date, including the origin of charge density wave (CDW), the unconventional or conventional nature of superconductivity, and the presence or absence of time-reversal symmetry breaking. A thorough understanding of the fundamental electronic structure is crucial for addressing these disputes. In this review, we provide an extensive summary of the key structural and electronic properties of AV₃Sb₅ compounds and evaluate the current research on their unconventional electronic order, especially the superconductivity and CDW, with a particular focus on insights from angle-resolved photoemission spectroscopy studies. We expect this review to be timely due to the convergence of various experimentally observed phenomena related to the CDW and superconducting order parameters in AV₃Sb₅ compounds. Our goal is to guide future investigations aimed at uncovering the microscopic origins of these unconventional electronic properties in kagome superconductors.