Zhengtai Liu

The discovery of high-temperature superconductivity in bilayer and trilayer nickelates has spurred intense interest in the Ruddlesden-Popper nickelates, yet the fundamental properties of the NiO 2 layer remain obscured by interlayer coupling. It is therefore imperative to investigate the electronic properties of their single-layer counterpart—though not superconducting—to isolate the intrinsic physics of the Ni-O layer. In this work, we present a systematic study of the single-layer nickelate La 1.4 Sr 0.6 NiO 4 using high-resolution angle-resolved photoemission spectroscopy and theoretical calculations. We reveal strong electron correlation effects manifested by high-energy kinks in band dispersions and a pronounced orbital-dependent band renormalization. Interestingly, we observe a quasi-one-dimensional electronic structure characterized by straight Fermi surface sheets along the diagonal momentum directions. Such a squarelike Fermi surface topology facilitates non-Fermi-liquid behavior consistent with the Luttinger liquid model, as evidenced by the power-law spectral function, robust temperature scaling, and spectroscopic features reminiscent of spin-charge separation. Our results therefore not only point toward an exotic Luttinger liquid phase emerging from the unexpected dimensional reduction in an intrinsically quasi-two-dimensional nickelate but also provide a new perspective for understanding the intriguing physics in multilayer nickelates.

Bismuth oxyselenide (Bi ₂ O ₂ Se) has emerged as a leading semiconductor candidate for next‐generation electronics and quantum phenomena research. However, its strong ionic interlayer bonding of Se layers fundamentally restricts the full potential of this otherwise promising quasi‐two‐dimensional (2D) material. In this work, we demonstrate chlorine substitution as an effective strategy to introduce van der Waals gaps and modify the electronic structure of the system. Through comprehensive transport and angle‐resolved photoemission spectroscopy (ARPES) measurements on Bi ₆ O ₆ Se ₂ Cl ₂ single crystal, an optimal Cl‐doping compound, we show that the key electronic features, including indirect bandgap, high electron mobility, and carrier effective mass, are well preserved, while the 2D character of the system is sufficiently enhanced. First‐principles calculations attribute this selective tunability to the unique spatial separation between conductive Bi‐O layers and dopant Se/Cl layers, which enables independent optimization of interlayer coupling and intralayer transport. Remarkably, our theoretical calculations combined with experimental observations project a broad bandgap engineering across a wide range (0.8 ∼ 3.5 eV) within the Bi ₂ O ₂ Se system. Our findings not only expand the application potential of Bi ₂ O ₂ Se‐based materials in optoelectronics, nanoelectronics, and fundamental research but also highlight spatial decoupling as a viable strategy for modifying the detailed electronic structure of quasi‐2D quantum systems.

Trilayer nickelates provide a tunable platform for investigating the interplay between structural geometry, electron correlation, and unconventional superconductivity. By directly comparing Pr${}_{4}$Ni${}_{3}$O${}_{10}$ and La${}_{4}$Ni${}_{3}$O${}_{10}$, this work uncovers an orbital differentiation enhanced by structural modification in Pr${}_{4}$Ni${}_{3}$O${}_{10}$, where orbitals become selectively incoherent and significantly renormalized, while orbitals remain coherent. This dichotomy leads to a marked suppression of interorbital hybridization in Pr${}_{4}$Ni${}_{3}$O${}_{10}$. This work suggests that the interlayer bonding angle serves as an active tuning parameter for the electronic properties, bridging multiorbital correlated physics and superconductivity in nickelates.

The van der Waals room-temperature ferromagnet Fe_{5}GeTe_{2} has garnered considerable research interest due to its intriguing magnetic and transport properties, as well as its potential applications, yet their microscopic origin remains unclear. Using angle-resolved photoemission spectroscopy and density functional theory calculations, we resolve an electronic structure corresponding to the up-up-up-type configuration. Pronounced Fermi surface reconstruction and a 25 meV dispersion kink referring to the strong electron-magnon coupling have been observed near T_{1}∼105 K, coincident with anomalies in resistivity and magnetic susceptibility. Our calculations identify the emergence of Fe(1) sublattice spin order as the driving force behind both phenomena. These findings reveal the decisive role of sublattice-dependent magnetism in governing the coupled electronic and magnetic properties of Fe_{5}GeTe_{2}, establishing sublattice engineering as a pathway for tuning correlated behavior in two-dimensional magnets.

The interplay between local and itinerant electrons underpins many correlated and topological quantum states. Kagome lattices provide an ideal platform by hosting both flat (localized states) and dispersive bands (itinerant states), yet direct spectroscopic evidence of their dynamical coupling has remained elusive. Here we report the long-sought flat band resonance in the quasi-two-dimensional kagome bilayer material CsCr₆Sb₆. Using angle-resolved photoemission spectroscopy, transport measurements, and combined density functional theory and dynamical mean-field theory, we identify coexisting flat band doublets and dispersive bands near the Fermi energy. Upon cooling, the flat and dispersive bands exhibit a pronounced enhancement of spectral weight and hybridization, directly evidencing flat band resonance. Crucially, this emergence coincides with the onset of short-range antiferromagnetic correlations, contrasting sharply with conventional Kondo lattice behavior. Our findings demonstrate not only the long-sought flat band resonance in kagome materials, but also its unconventional correlation with magnetism.

Altermagnetism arises from composite real-space and spin-space symmetries, combining zero net magnetization with pronounced momentum-dependent spin splitting. This review highlights the pivotal role of angle-resolved photoemission spectroscopy (ARPES), along with its spin-resolved (SARPES) and circular-dichroism (CD-ARPES) variants, in directly visualizing nonrelativistic band splitting and spin textures in altermagnets. Within the spin-group framework, we distinguish ferromagnetic, antiferromagnetic, and altermagnetic orders and elucidate the symmetry origin of spin polarization. We then systematically review representative systems: the debated d-wave prototype RuO2, layered d-wave altermagnets KV2Se2O and Rb1-delta V2Te2O, and a series of g-wave compounds including MnTe (domain-tunable) and CrSb (topological), together with the noncoplanar antiferromagnet MnTe2 and other emerging candidate platforms. Overall, ARPES has become a key probe for resolving symmetry-driven spin splitting. Future advances in micro/nano-beam and in-situ spectroscopies, combined with strain and domain engineering, heterostructure design, and exploration of broader unconventional magnetic states, are expected to drive the joint evolution of altermagnetism and photoemission spectroscopy, paving the way for spintronic and correlated quantum research.

Altermagnetism arises from composite real-space and spin-space symmetries, combining zero net magnetization with pronounced momentum-dependent spin splitti