A3: Atomic-scale visualization of magnetic-field-tuned states in topological structures
Hidenori Takagi, Dennis Huang
The goal of the project is to combine scanning tunneling microscopy at cryogenic temperatures with the full three-dimensional tunability of a vector magnet to control topological band structures and realize exotic spatially varying states and boundary modes. We will investigate how Dirac fermions, flat bands, surface states and edge states in kagome materials respond to a magnetic field. We will also investigate how a parallel magnetic field may induce inhomogeneous and/or topological superconductivity in transition metal dichalcogenides.
Publications
2026 |
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Zhang, Y. -S.; Yang, Z.; Xiao, C.; Isobe, M.; Minola, M.; Takagi, H.; Huang, D. Substrate tuning of the structural and electronic transition in thin flakes of the excitonic insulator candidate Ta2NiSe5 Journal Article Commun. Phys. 9, 309 (2026), datadoi: 10.5281/zenodo.22107443. @article{zhangSubstrateTuningStructural2026,Ta2NiSe5 continues to draw interest for its Tc = 326 K phase transition, whose dual electronic and structural nature reflects a complex interplay of electron-hole (excitonic) and electron-lattice interactions. The majority of studies that have attempted to decipher the relative importance of these interactions, particularly through charge tuning, have been focused on bulk samples. Here, we utilize an all-dry exfoliation and transfer protocol to isolate ultrathin flakes of Ta2NiSe5 on insulating Al2O3 and conducting Au. Using polarized Raman spectroscopy, we uncover the following substrate dependence: Four layers of Ta2NiSe5 on Al2O3 show a sharp structural and electronic transition that is lowered by roughly 40 K from the bulk Tc. Meanwhile, four layers of Ta2NiSe5 on Au undergo a structural and electronic transition that is much more gradual with respect to temperature and finishes roughly 150 K below the bulk Tc. The pronounced broadening points to an atomic-scale interface effect, wherein electrostatic screening and charge transfer from Au produces a Tc gradient perpendicular to the layers of the flake. We discuss the role of excitonic physics and suggest the possibility for interface engineering to pattern nanoscale junctions in Ta2NiSe5. |  |
Zhang, Y. -S.; Isobe, M.; Takagi, H.; Huang, D. Electronic tuning of the soft-phonon transport anomaly in Ta2Ni(SxSe1-x)5 Journal Article Phys. Rev. Lett. 137, 106503 (2026), datadoi: 10.5281/zenodo.21493304. @article{l5xq-2yrt,Ta2NiSe5 continues to be investigated for its phase transition at 𝑇c=326 K, where it develops both an electronic gap and a distortion of its Ta/Ni chains. One intriguing feature at 𝑇c seen in thermal transport is the giant anisotropic scattering of phonons moving perpendicular to the chains, which is apparently associated with the softening of a transverse acoustic phonon, but whose microscopic origin and significance demand clarification. By tuning the normal-state band overlap or gap with S substitution, we uncover a close connection between this soft-phonon transport anomaly and underlying electronic instabilities: when Ta2Ni(S𝑥Se1−𝑥)5 approaches a band insulator at high 𝑥, and signatures of the electronic transition are suppressed, the soft-phonon transport anomaly concomitantly vanishes. Our results establish the following picture for the Ta2Ni(S𝑥Se1−𝑥)5 family: near the S end, a sole lattice instability gives rise to a weak structural transition with 𝑇c approaching 130 K. Near the Se end, additional electronic instabilities boost 𝑇c up to 326 K and amplify experimental signatures of the transition. The strong interaction between electrons, holes, and the lattice is manifested as a soft-phonon transport anomaly accompanied by electronic fluctuations, which include excitonic and hybridization-gap fluctuations. | |
Que, X.; He, Q.; Zhou, L.; Prodan, L.; Tsurkan, V.; Kézsmárki, I.; Takagi, H.; Huang, D. Tunable flat band on the surface of a rhombohedral kagome ferromagnet Unpublished (2026), arXiv.2609.19857. @unpublished{que2026tunableflatbandsurface,A central goal in the exploration of kagome-based materials is the realization of a flat band that has meV bandwidth and lies close to the Fermi energy. The prevailing assumption is that band flattening originates from destructive hopping processes on the kagome lattice. We perform scanning tunneling microscopy (STM) on the layered kagome ferromagnet Fe3Sn2 and show that it indeed hosts a flat band near the Fermi energy, which is manifested as a sharp peak in the differential tunneling conductance. First-principles slab calculations reveal, however, that this band is flattened not by the destructive interference of intralayer hopping, but by interlayer hopping between rhombohedral-stacked kagome planes in Fe3Sn2, and is confined to the surface layer. This surface band, forming in the vicinity of the Brillouin zone corners K¯ and K′¯, exhibits rich magnetic-field dependence, including fine structure due to valley-symmetry breaking by rotated Fe moments, as well as a persistent diamagnetic shift associated with orbital magnetic moments, all reproduced by our calculations. Our results, highlighting the crucial role of layer stacking on the band structure of kagome magnets, demonstrate experimentally an alternative mechanism of generating magnetically tunable flat bands in atomically thin volumes of topological magnets. | |
2025 |
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Que, X.; He, Q.; Zhou, L.; Lei, S.; Schoop, L.; Huang, D.; Takagi, H. Visualizing the internal structure of the charge-density-wave state in CeSbTe Journal Article Nat. Commun. 16, 3053 (2025), datadoi: 10.6084/m9.figshare.27255576. @article{queVisualizingInternalStructure2025,The collective reorganization of electrons into a charge density wave has long served as a textbook example of an ordered phase in condensed matter physics. Two-dimensional square lattices with p electrons are well-suited to the realization of charge density waves, due to the anisotropy of the p orbitals and the resulting one dimensionality of the electronic structure. In spite of a long history of study of charge density waves in square-lattice systems, few reports have recognized the significance of a hidden orbital degree of freedom. The degeneracy of px and py electrons may give rise to orbital patterns in real space that endow the charge density wave with additional broken symmetries or unusual order parameters. Here, we use scanning tunneling microscopy to visualize the internal structure of the charge-density-wave state of CeSbTe, which contains Sb square lattices with 5p electrons. We image atomic-sized, anisotropic lobes of charge density with periodically modulating anisotropy, which we interpret in terms of a superposition of px and py bond density waves. Our results support the fact that delocalized p orbitals can reorganize into emergent electronic states of matter. | |
2024 |
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Matsumoto, Y.; Schnierer, S.; Bruin, J. A. N.; Nuss, J.; Reiss, P.; Jackeli, G.; Kitagawa, K.; Takagi, H. A quantum critical Bose gas of magnons in the quasi-two-dimensional antiferromagnet YbCl3 under magnetic fields Journal Article Nat. Phys. 20, 1131–1138 (2024), datadoi: 10.1038/s41567-024-02498-w. @article{matsumotoQuantumCriticalBose2024,Bose–Einstein condensation (BEC) is a quantum phenomenon in which a macroscopic number of bosons occupy the lowest energy state and acquire coherence at low temperatures. In three-dimensional antiferromagnets, a magnetic-field-induced transition has been successfully described as a magnon BEC. For a strictly two-dimensional (2D) system, it is known that BEC cannot take place due to the presence of a finite density of states at zero energy. However, in a realistic quasi-2D magnet consisting of stacked magnetic layers, a small but finite interlayer coupling stabilizes marginal BEC but such that 2D physics is still expected to dominate. This 2D-limit BEC behaviour has been reported in a few materials but only at very high magnetic fields that are difficult to access. The honeycomb S = 1/2 Heisenberg antiferromagnet YbCl3 exhibits a transition to a fully polarized state at a relatively low in-plane magnetic field. Here, we demonstrate the formation of a quantum critical 2D Bose gas at the transition field, which, with lowering the field, experiences a BEC marginally stabilized by an extremely small interlayer coupling. Our observations establish YbCl3, previously a Kitaev quantum spin liquid material, as a realization of a quantum critical BEC in the 2D limit. | |