Prof Yulin Chen

Abstract:

Impurity substitution is effective for studying the intrinsic properties of a quantum material. When the target element has multiple Wyckoff positions, it is challenging but essential to know the exact position and occupancy order of the impurity atoms. Via comprehensive experimental and theoretical investigations, we establish the Ni substitution road map in van der Waals ferromagnet Fe3GaTe2. The results unambiguously reveal that in (Fe1–xNix)3GaTe2, Ni atoms initially form interlayer gap Ni3 sites when x < 0.1 and then gradually occupy Fe2 sites. When x > 0.75, they start to substitute for Fe1 sites and eventually realize full occupation. Accordingly, TC and saturation moments both show nonlinear decreases tied to the different roles of Ni3, Fe1, and Fe2 sites in the spin Hamiltonian. The results not only yield fruitful insights into the roles of different Fe sites in Fe3GaTe2 but also set a paradigm for the future study of impurity substitution on other quantum materials.

Abstract:

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.

Abstract:

Over the years, great efforts have been devoted in introducing a sizable and tunable band gap in graphene for its potential application in next‐generation electronic devices. The primary challenge in modulating this gap has been the absence of a direct method for observing changes of the band gap in momentum space. In this study, advanced spatial‐ and angle‐resolved photoemission spectroscopy technique is employed to directly visualize the gap formation in bilayer graphene, modulated by both displacement fields and moiré potentials. The application of displacement field via in situ electrostatic gating introduces a sizable and tunable electronic bandgap, proportional to the field strength up to 100 meV. Meanwhile, the moiré potential, induced by aligning the underlying hexagonal boron nitride substrate, extends the bandgap by ≈20 meV. Theoretical calculations effectively capture the experimental observations. This investigation provides a quantitative understanding of how these two mechanisms collaboratively modulate the band gap in bilayer graphene, offering valuable guidance for the design of graphene‐based electronic devices.