A hybrid topological quantum state in an elemental solid
Nature, Nature volume 628, pages527–533 (2024), Princeton University
Projects and Talks
Untangling charge-order dependent bulk states from surface effects in a topological kagome metal ScV$_6$Sn$_6$
PRB, Phys. Rev. B 109, 075150, Princeton University
Kagome metals with charge density wave (CDW) order exhibit a broad spectrum of intriguing quantum phenomena. The recent discovery of the novel kagome CDW compound ScV$_6$Sn$_6$ has spurred significant interest. However, understanding the interplay between CDW and the bulk electronic structure has been obscured by a profusion of surface states and terminations in this quantum material. Here, we employ photoemission spectroscopy and potassium dosing to elucidate the complete bulk band structure of ScV$_6$Sn$_6$, revealing multiple van Hove singularities near the Fermi level. We surprisingly discover a robust spin-polarized topological Dirac surface resonance state at the M point within the twofold van Hove singularities. Assisted by first-principles calculations, the temperature dependence of the kz-resolved angle-resolved photoemission spectroscopy spectrum provides unequivocal evidence for the proposed $\sqrt{3} \times \sqrt{3} \times 3 $ charge order over other candidates. Our work not only enhances the understanding of the CDW-dependent bulk and surface states in ScV$_6$Sn$_6$, but also establishes an essential foundation for potential manipulation of the CDW order in kagome materials.
Topological ferromagnetic Kondo lattice
ARXIV, arXiv:2302.12113, Princeton University & Peking University
Magnetic topological semimetals have recently emerged as a topic of central interest in condensed matter physics, but have mostly been explored in weakly-correlated materials. Strong interacting systems, such as Kondo lattices, can host tunable unconventional topological states thanks to their rich phase diagram with multiple intertwined degrees of freedom. Here we unravel the existence of Kondo-induced Weyl-loop states in CeCo2As2. Using angle-resolved photoemission spectroscopy and first-principle calculation, we visualize f-orbital-dominated heavy nodal rings near the Fermi level in the magnetic Kondo lattice phase. Remarkably, by tuning the ratio of Ce/La, we observe a substantial enhancement of anomalous Hall conductivity in the coherent Kondo lattice regime. The value of the Hall conductivity quantitatively matches with the first-principle calculation that optimized with our ARPES results and can be attributed to the large Berry curvature (BC) density engendered by the topological nodal rings. Our results promote $CeCo_2As_2$ as a topological Kondo magnet candidate and underpin the realization of various topological responses in a highly sensitive ferromagnetic Kondo lattice setting.
Visualization of Tunable Weyl Line in A–A Stacking Kagome Magnets
Advanced Materials, Advanced Materials 35.3 (2023): 2205927., Princeton Uiversity
Kagome magnets provide a fascinating platform for a plethora of topological quantum phenomena, in which the delicate interplay between frustrated crystal structure, magnetization, and spin–orbit coupling (SOC) can engender highly tunable topological states. Here, utilizing angle-resolved photoemission spectroscopy, the Weyl lines are directly visualized with strong out-of-plane dispersion in the A–A stacked kagome magnet $GdMn_6Sn_6$. Remarkably, the Weyl lines exhibit a strong magnetization-direction-tunable SOC gap and binding energy tunability after substituting Gd with Tb and Li, respectively. These results not only illustrate the magnetization direction and valence counting as efficient tuning knobs for realizing and controlling distinct 3D topological phases, but also demonstrate $AMn_6Sn_6$ (A = rare earth, or Li, Mg, or Ca) as a versatile material family for exploring diverse emergent topological quantum responses.
Observation of a linked-loop quantum state in a topological magnet
Nature, Nature 604, 647–652 (2022), Princeton Uiversity
Quantum phases can be classified by topological invariants, which take on discrete values capturing global information about the quantum state. Over the past decades, these invariants have come to play a central role in describing matter, providing the foundation for understanding superfluids5, magnets, the quantum Hall effect, topological insulators, Weyl semimetals and other phenomena. Here we report an unusual linking-number (knot theory) invariant associated with loops of electronic band crossings in a mirror-symmetric ferromagnet. Using state-of-the-art spectroscopic methods, we directly observe three intertwined degeneracy loops in the material’s three-torus, T3, bulk Brillouin zone. We find that each loop links each other loop twice. Through systematic spectroscopic investigation of this linked-loop quantum state, we explicitly draw its link diagram and conclude, in analogy with knot theory, that it exhibits the linking number (2, 2, 2), providing a direct determination of the invariant structure from the experimental data. We further predict and observe, on the surface of our samples, Seifert boundary states protected by the bulk linked loops, suggestive of a remarkable Seifert bulk–boundary correspondence. Our observation of a quantum loop link motivates the application of knot theory to the exploration of magnetic and superconducting quantum matter.
Signatures of Weyl Fermion Annihilation in a Correlated Kagome Magnet
Physical Review Letters, Phys. Rev. Lett. 127, 256403, Princeton University
The manipulation of topological states in quantum matter is an essential pursuit of fundamental physics and next-generation quantum technology. Here we report the magnetic manipulation of Weyl fermions in the kagome spin-orbit semimetal $Co_3Sn_2S_2$, observed by high-resolution photoemission spectroscopy. We demonstrate the exchange collapse of spin-orbit-gapped ferromagnetic Weyl loops into paramagnetic Dirac loops under suppression of the magnetic order. We further observe that topological Fermi arcs disappear in the paramagnetic phase, suggesting the annihilation of exchange-split Weyl points. Our findings indicate that magnetic exchange collapse naturally drives Weyl fermion annihilation, opening new opportunities for engineering topology under correlated order parameters.