Jieyi 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.

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.

In low-dimensional superconductors, the interplay between quantum confinement and interfacial hybridization effects can reshape Cooper-pair wavefunctions and give rise to unconventional superconducting states. Here we use plasma-free confinement epitaxy assisted by a carbon buffer layer to synthesize a gallium trilayer sandwiched between graphene and a 6H-SiC(0001) substrate. Within this confined gallium layer, we demonstrate interfacial Ising-type superconductivity driven by atomic orbital hybridization. Electrical transport measurements reveal that the in-plane upper critical magnetic field reaches ~21.98 T at T = 400 mK, approximately 3.38 times the Pauli paramagnetic limit. Angle-resolved photoemission spectroscopy measurements, combined with theoretical calculations, confirm the presence of split Fermi surfaces with Ising-type spin textures at the K and K′ valleys of the confined gallium layer, originating from strong hybridization with the SiC substrate. This work establishes a strategy for realizing unconventional pairing wavefunctions through the synergistic combination of quantum confinement and interfacial hybridization effects.

This study investigates the intrinsic magnetism and field‐driven spin alignment in NiI ₂ using X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism (XMCD). NiI ₂ , a van der Waals material, exhibits helimagnetic and type‐II multiferroic behavior. This study reveals robust XMCD signals across paramagnetic, antiferromagnetic, and helimagnetic phases under applied out‐of‐plane fields up to 6 T, while no net moment emerges at zero field. Atomic multiplet calculations confirm a covalent Ni 3 d ground state with a significantly reduced spin moment. The results establish the intrinsic nature of NiI ₂ 's magnetism and clarify its field‐driven spin alignment mechanism. This comprehensive spectroscopic characterization lays the foundation for future applications of NiI ₂ in advanced spintronic and multiferroic devices, despite challenges posed by its low transition temperature in the monolayer limit. Future research should focus on enhancing its critical temperature through doping, strain engineering, or heterostructure fabrication.

Berry curvature and spin texture are representative tuning parameters that govern spin-orbit coupling–related physics and are also the foundation for future device applications. Here, we investigate the impact of the Sb-to-Bi ratio on shaping the electronic band structure and its correlated first- and second-harmonic magneto-transport signals in the intrinsic magnetic topological insulator Mn(Bi _{1− x} Sb _x ) ₂ Te ₄ . First-principles calculations reveal that the introduction of Sb not only triggers a topological phase transition but also changes the integral of the Berry curvature at the shifted Fermi level, which leads to the reversal of the anomalous Hall resistance polarity for Sb fractions x > 0.67. Moreover, it also induces the opposite spin splitting of the valence bands compared to the Sb-free host, and the resulting clockwise/counterclockwise spin chirality gives rise to a tunable unidirectional second-harmonic anomalous Hall response. Our findings pave the way for constructing chiral spin-orbitronic devices through band structure engineering.

The resurgence of interest in Kondo insulators has been driven by two major mysteries: the presence of metallic surface states and the observation of quantum oscillations. To further explore these mysteries, it is crucial to investigate another similar system beyond the two existing ones, SmB ₆ and YbB ₁₂ . Here, we address this by reporting on a Kondo insulator, U ₃ Bi ₄ Ni ₃ . Our transport measurements reveal that a surface state emerges below 250 kelvin and dominates transport properties below 150 kelvin, which is well above the temperature scale of SmB ₆ and YbB ₁₂ . At low temperatures, the surface conductivity is about one order of magnitude higher than the bulk. The robustness of the surface state indicates that it is inherently protected. The similarities and differences between U ₃ Bi ₄ Ni ₃ and the other two Kondo insulators will provide valuable insights into the nature of metallic surface states in Kondo insulators and their interplay with strong electron correlations.

Using x-ray photoelectron diffraction (XPD) and angle-resolved photoemission spectroscopy, we study photoemission intensity changes related to changes in the geometric and electronic structure in the kagome metal CsV_{3}Sb_{5} upon transition to an unconventional charge density wave (CDW) state. The XPD patterns reveal the presence of a chiral atomic structure in the CDW phase. Furthermore, using circularly polarized x-rays, we have found a pronounced nontrivial circular dichroism in the angular distribution of the valence band photoemission in the CDW phase, indicating a chirality of the electronic structure. This observation is consistent with the proposed orbital loop current order. In view of a negligible spontaneous Kerr signal in recent magneto-optical studies, the results suggest an antiferromagnetic coupling of the orbital magnetic moments along the c axis. While the inherent structural chirality may also induce circular dichroism, the observed asymmetry values seem to be too large in the case of the weak structural distortions caused by the CDW.

The physical properties of magnetic topological materials are strongly influenced by their nontrivial band topology coupled with the magnetic structure. Co₃Sn₂S₂ is a ferromagnetic kagome Weyl semimetal displaying giant intrinsic anomalous Hall effect which can be further tuned via elemental doping, such as Ni substitution for Co. Despite significant interest, the exact valency of Co and the magnetic order of the Ni dopants remained unclear. Here, we report a study of Ni-doped Co₃Sn₂S₂ single crystals using synchrotron-based X-ray magnetic circular dichroism (XMCD), X-ray photoelectron emission microscopy (XPEEM), and hard/soft X-ray photoemission spectroscopy (XPS) techniques. We confirm the presence of spin-dominated magnetism from Co in the host material, and also the establishment of ferromagnetic order from the Ni dopant. The oxygen-free photoemission spectrum of the Co 2p core levels in the crystal well resembles that of a metallic Co film, indicating a Co⁰⁺ valency. Surprisingly, we find the electron filling in the Co 3d state can reach 8.7-9.0 electrons in these single crystals. Our results highlight the importance of element-specific X-ray spectroscopy in understanding the electronic and magnetic properties that are fundamental to a heavily studied Weyl semimetal, which could aid in developing future spintronic applications based on magnetic topological materials.

In (Bi _{1− x} Sb _x ) ₂ Te ₃ intercalated MnBi ₂ Te ₄ multilayers, structural engineering is employed to modulate both the anomalous Hall response and magnetism.