The interplay between nontrivial band topology and layered antiferromagnetism in MnBi₂Te₄ has opened a new avenue for exploring topological phases of matter. The quantum anomalous Hall effect and axion insulator state have been observed in odd and even number layers of MnBi₂Te₄, and the quantum metric nonlinear Hall effect has been shown to exist in this topological antiferromagnet. The rich and complex antiferromagnetic spin dynamics in MnBi₂Te₄ is expected to generate new quantum anomalous Hall phenomena that are absent in conventional ferromagnetic topological insulators, but experimental observations are still unknown. Here we fabricate a device of 7-septuple-layer MnBi₂Te₄ covered with an AlO_x capping layer, which enables the investigation of antiferromagnetic quantum anomalous Hall effect over wide parameter spaces. By tuning the gate voltage and perpendicular magnetic field, we uncover a cascade of quantum phase transitions that can be attributed to the influence of complex spin configurations on edge state transport. Furthermore, we find that an in-plane magnetic field enhances both the coercive field and the exchange gap of the surface state, in contrast to that in the ferromagnetic quantum anomalous Hall state. Combined with numerical simulations, we propose that these peculiar features arise from the spin flip and flop transitions that are inherent to a van der Waals antiferromagnet. The versatile tunability of the quantum anomalous Hall effect in MnBi₂Te₄ paves the way for potential applications in topological antiferromagnetic spintronics.

Real-space, three-dimensional imaging of atomic structures in materials science is a critical yet challenging task. Although scanning transmission electron microscopy has achieved sub-angstrom lateral resolution through techniques like electron ptychography, depth resolution remains limited to only 2 to 3 nanometers using single-projection setups. Attaining better depth resolution often requires large sample tilt angles and numerous projections, as demonstrated in atomic electron tomography. Here, we introduce an extension of multislice electron ptychography, which couples only a few small-angle projections to improve depth resolution by more than threefold, reaching the sub-nanometer scale and potentially approaching the atomic level. This technique maintains high resolving power for both light and heavy atoms, significantly enhancing the detection of individual dopants. We experimentally demonstrate three-dimensional visualization of dilute praseodymium dopants in a brownmillerite oxide, Ca₂Co₂O₅, along with the accompanying lattice distortions. This approach can be implemented on widely available transmission electron microscopes equipped with hybrid pixel detectors, with data processing achievable using high-performance computing systems.

The recent discovery of superconductivity in La₃Ni₂O_7−δ under high pressure with a transition temperature around 80 K has sparked extensive experimental and theoretical efforts. Several key questions regarding the pairing mechanism remain to be answered, such as the most relevant atomic orbitals and the role of atomic deficiencies. Here we develop a new, energy-filtered, multislice electron ptychography technique, assisted by electron energy-loss spectroscopy, to address these critical issues. Oxygen vacancies are directly visualized and are found to primarily occupy the inner apical sites, which have been proposed to be crucial to superconductivity. We precisely determine the nanoscale stoichiometry and its correlation to the oxygen K-edge spectra, which reveals a significant inhomogeneity in the oxygen content and electronic structure within the sample. The spectroscopic results also reveal that stoichiometric La₃Ni₂O₇ has strong charge-transfer characteristics, with holes that are self-doped from Ni sites into O sites. The ligand holes mainly reside on the inner apical O and the planar O, whereas the density on the outer apical O is negligible. As the concentration of O vacancies increases, ligand holes on both sites are simultaneously annihilated. These observations will assist in further development and understanding of superconducting nickelate materials. Our imaging technique for quantifying atomic deficiencies can also be widely applied in materials science and condensed-matter physics.

In conventional metallic superconductors such as aluminium, the large number of weakly bounded Cooper pairs become phase-coherent as soon as they start to form. The cuprate high-temperature superconductors belong to a different category because, being doped Mott insulators, they are known to have low superfluid density and are therefore susceptible to phase fluctuations. It has been proposed that pairing and phase coherence may occur separately in cuprates, and that the measured critical temperature corresponds to the phase-coherence temperature controlled by the superfluid density. Here we examine the evolution of pairing and phase-ordering in underdoped cuprates, and find that a chequerboard plaquette pattern of charge order plays a crucial role, such that the global phase coherence is established once its spatial occupation exceeds a threshold. We make these observations via scanning tunnelling microscopy on underdoped Bi₂La_xSr_2−xCuO_6+δ. We observe a smooth crossover from the Mott insulator to superconductor on small islands that have chequerboard order. Each chequerboard plaquette contains approximately two holes and exhibits a stripy internal structure that has a strong influence on the superconducting spectroscopic features. The local spectra remain qualitatively the same across the insulator-to-superconductor transition, and the quasiparticle interference becomes long-ranged.

Layered superconductors exhibit strong anisotropic responses to magnetic fields in out-of-plane and in-plane orientations, due to their distinct vortex structures and upper critical field values. Here, we utilize the planar tunnel junction technique to perform continuous magnetic field-dependent dI/dV spectroscopy measurements on 2H-NbSe₂ under different field orientations. We observe characteristic kink features for weak in-plane magnetic fields, but the overall behaviors are quite similar for different field orientations despite the distinct vortex generation processes and widely different upper critical field values. Especially, the generic square root dependence of the Fermi level density of state on magnetic field indicates that the Doppler shift plays a central role in the low energy excitations of 2H-NbSe₂ in the presence of magnetic field.

A promising approach to the next generation of low-power, functional, and energy-efficient electronics relies on novel materials with coupled magnetic and electric degrees of freedom. In particular, stripy antiferromagnets often exhibit broken crystal and magnetic symmetries, which may bring about the magnetoelectric (ME) effect and enable the manipulation of intriguing properties and functionalities by electrical means. The demand for expanding the boundaries of data storage and processing technologies has led to the development of spintronics toward two-dimensional (2D) platforms. This work reports the ME effect in the 2D stripy antiferromagnetic insulator CrOCl down to a single layer. By measuring the tunneling resistance of CrOCl on the parameter space of temperature, magnetic field, and applied voltage, we verified the ME coupling down to the 2D limit and probed its mechanism. Utilizing the multi-stable states and ME coupling at magnetic phase transitions, we realize multi-state data storage in the tunneling devices. Our work not only advances the fundamental understanding of spin-charge coupling, but also demonstrates the great potential of 2D antiferromagnetic materials to deliver devices and circuits beyond the traditional binary operations.

We demonstrate a method for fabricating a high-quality AlO_x-based planar tunnel junction using atomic layer deposition, integrated with the exfoliation and transfer techniques for van der Waals (vdW) materials. The tunneling spectroscopy results on exfoliated Bi₂Sr₂CaCu₂O_8+δ and 2H-NbSe₂ vdW superconductors are highly consistent with that obtained by ultrahigh vacuum scanning tunneling spectroscopy on atomically clean surfaces. The planar tunneling devices enable high-precision spectroscopy over a wide range of temperatures and magnetic fields and reveal novel features and stark contrast between high-T_C cuprates and conventional superconductors. This method represents a universally applicable technique for probing the electronic structure of various two-dimensional vdW materials.