Topological insulator with antiferromagnetic order can serve as an ideal platform to realize axion electrodynamics. In this paper, we report a systematic study of the axion topological insulator candidate ${\mathrm{EuIn}}_2{\mathrm{As}}_2$. A linear energy dispersion across the Fermi level reveals a hole-type Fermi pocket. The orientation of the magnetic moment for ground state is determined within the $ab$-plane by anisotropic magnetic behavior. Besides long-range antiferromagnetic order, magnetization and magnetotransport measurements indicate existence of the ferromagnetic orders and ferromagnetic correlation, suggesting the formation of the magnetic polarons. These ferromagnetic clusters can persist above the antiferromagnetic transition leading to unconventional transport properties. Our results suggest multiple magnetic orders and states in ${\mathrm{EuIn}}_2{\mathrm{As}}_2$, which is vital to understanding its topological nature.

Topological magnets are a new family of quantum materials providing great potential to realize emergent phenomena, such as the quantum anomalous Hall effect and the axion-insulator state. Here, we present our discovery that the stoichiometric ferromagnet ${\mathrm{MnBi}}_8{\mathrm{Te}}_{13}$ with natural heterostructure ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/({\mathrm{Bi}}_2{\mathrm{Te}}_3{)}_3$ is an unprecedented “half-magnetic topological insulator,” with the magnetization existing at the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ surface but not at the opposite surface terminated by triple ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ layers. Our angle-resolved photoemission spectroscopy measurements unveil a massive Dirac gap at the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ surface and a gapless Dirac cone on the other side. Remarkably, the Dirac gap (about 28 meV) at the ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ surface decreases monotonically with increasing temperature and closes right at the Curie temperature, thereby representing the first smoking-gun spectroscopic evidence of a magnetization-induced topological surface gap among all known magnetic topological materials. We further demonstrate theoretically that the half-magnetic topological insulator is desirable to realize the surface anomalous Hall effect, which serves as direct proof of the general concept of axion electrodynamics in condensed matter systems.

Beyond the absence of long-range magnetic orders, the most prominent feature of the elusive quantum spin liquid (QSL) state is the existence of fractionalized spin excitations, i.e., spinons. When the system orders, the spin-wave excitation appears as the bound state of the spinon-antispinon pair. Although scarcely reported, a direct comparison between similar compounds illustrates the evolution from spinon to magnon. Here, we perform the Raman scattering on single crystals of two quantum kagome antiferromagnets, of which one is the kagome QSL candidate Cu₃Zn(OH)₆FBr, and another is an antiferromagnetically ordered compound EuCu₃(OH)₆Cl₃. In Cu₃Zn(OH)₆FBr, we identify a unique one spinon-antispinon pair component in the E_2g magnetic Raman continuum, providing strong evidence for deconfined spinon excitations. In contrast, a sharp magnon peak emerges from the one-pair spinon continuum in the E_g magnetic Raman response once EuCu₃(OH)₆Cl₃ undergoes the antiferromagnetic order transition. From the comparative Raman studies, we can regard the magnon mode as the spinon-antispinon bound state, and the spinon confinement drives the magnetic ordering.

Signatures of both the quantum anomalous Hall effect and axion electrodynamics have been recently observed to exist in thin films of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, a stoichiometric antiferromagnetic topological insulator. Direct evidence of the bulk topological magnetoelectric response in an axion insulator requires an energy gap at its topological surface state (TSS). However, independent spectroscopic experiments revealed that such a surface gap is much smaller than previously thought. Here we utilize angle resolved photoemission spectroscopy and density functional theory calculations to demonstrate that a sizable TSS gap unexpectedly exists in Sb-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ where the bulk system remains topologically nontrivial. This gap is found to be insensitive to the bulk antiferromagnetic-paramagnetic transition, while it enlarges along with increasing Sb concentration, enabling simultaneous tunability of the Fermi level and the TSS gap size (up to $>100$ meV). Our work shows that Sb dopants in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ can not only control the Fermi level but also induce a tunable surface gap, providing a potential platform to observe the key features of the high-temperature axion-insulator phase.

The spin polarization in nonmagnetic materials is conventionally attributed to the outcome of spin-orbit coupling when the global inversion symmetry is broken. The recently discovered hidden spin polarization indicates that a specific atomic site asymmetry could also induce measurable spin polarization, leading to a paradigm shift in research on centrosymmetric crystals for potential spintronic applications. Here, combining spin- and angle-resolved photoemission spectroscopy and theoretical calculations, we report distinct spin-momentum-layer locking phenomena in a centrosymmetric, layered material, BiOI. The measured spin is highly polarized along the Brillouin zone boundary, while the same effect almost vanishes around the zone center due to its nonsymmorphic crystal structure. Our work demonstrates the existence of momentum-dependent hidden spin polarization and uncovers the microscopic mechanism of spin, momentum, and layer locking to each other, thus shedding light on the design metrics for future spintronic materials.