Prof Yulin Chen

Abstract:

Correlated kagome materials exhibit a compelling interplay between lattice geometry, electron correlation, and topology. In particular, the flat bands near the Fermi level provide a fertile playground for novel many-body states. Here we investigate the electronic structure of CsCr3Sb5 using high-resolution angle-resolved photoemission spectroscopy and ab-initio calculations. Our results suggest that Cr 3d electrons are intrinsically incoherent, showing strong electron correlation amplified by Hund’s coupling. Notably, we identify incipient flat bands close to the Fermi level, which are expected to significantly influence the electronic properties of the system. Across the density-wave-like transition at 55 K, we observe a drastic enhancement of the electron scattering rate, which aligns with the semiconducting-like property at high temperatures. These findings establish CsCr3Sb5 as a strongly correlated Hund’s metal with incipient flat bands near the Fermi level, which provides an electronic basis for understanding its novel properties compared to the weakly correlated AV3Sb5.

Abstract:

The 2M-phase transition metal dichalcogenides have recently attracted intensive research interest due to their rich topological and superconducting phase diagrams. Apart from the topological surface states of 2M-WS2 near Γ that originated from the strong topological order, using angle-resolved photoemission spectroscopy, we discover additional Rashba-split states on the surfaces of both 2M-WS2 and 2M-WSe2, which extend in large momentum-energy regions. First-principles calculations well reproduce these states and attribute them to the weak topological orders. The calculations further indicate that the surface state connecting configurations are tunable under moderate pressure, suggesting that 2M-WS2 and WSe2 are promising platforms to study topological phase transition and explore topological superconductivity.

Abstract:

In two-dimensional van der Waals magnetic materials, the interplay between magnetism and electron correlation can give rise to new ground states and lead to novel transport and optical properties. A fundamental question in these materials is how the electron correlation manifests and interacts with the magnetic orders. In this study, we demonstrate that the recently discovered 2D antiferromagnetic material, CrSBr is a Mott insulator, through the combined use of resonant and temperature-dependent angle-resolved photoemission spectroscopy techniques, supplemented by dynamical mean-field theory analysis. Intriguingly, we found that as the system transitions from the antiferromagnetic to the paramagnetic phases, its Mott bands undergo a reconfiguration, and a coherent-incoherent crossover, driven by the dissolution of the magnetic order. Our findings reveal a distinctive evolution of band structure associated with magnetic phase transitions, shedding light on the investigation of the intricate interplay between correlation and magnetic orders in strongly correlated van der Waals magnetic materials.

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 (Fe_1-<i>x</i>Ni_<i>x</i>)₃GaTe₂, Ni atoms initially form interlayer gap Ni3 sites when <i>x</i> < 0.1 and then gradually occupy Fe2 sites. When <i>x</i> > 0.75, they start to substitute for Fe1 sites and eventually realize full occupation. Accordingly, <i>T</i>_C 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.