Bachelor of Science, Tsinghua University, Mathematics and Physics (2012)
Master of Engineering, University of Tokyo, Applied Physics (2014)
Doctor of Philosophy, Massachusetts Institute of Technology, Physics (2020)
Ian Fisher, Postdoctoral Faculty Sponsor
Evidence of two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn.
2021; 12 (1): 5345
The kagome lattice has long been regarded as a theoretical framework that connects lattice geometry to unusual singularities in electronic structure. Transition metal kagome compounds have been recently identified as a promising material platform to investigate the long-sought electronic flat band. Here we report the signature of a two-dimensional flat band at the surface of antiferromagnetic kagome metal FeSn by means of planar tunneling spectroscopy. Employing a Schottky heterointerface of FeSn and an n-type semiconductor Nb-doped SrTiO3, we observe an anomalous enhancement in tunneling conductance within a finite energy range of FeSn. Our first-principles calculations show this is consistent with a spin-polarized flat band localized at the ferromagnetic kagome layer at the Schottky interface. The spectroscopic capability to characterize the electronic structure of a kagome compound at a thin film heterointerface will provide a unique opportunity to probe flat band induced phenomena in an energy-resolved fashion with simultaneous electrical tuning of its properties. Furthermore, the exotic surface state discussed herein is expected to manifest as peculiar spin-orbit torque signals in heterostructure-based spintronic devices.
View details for DOI 10.1038/s41467-021-25705-1
View details for PubMedID 34526494
Topological flat bands in frustrated kagome lattice CoSn
2020; 11 (1): 4004
Electronic flat bands in momentum space, arising from strong localization of electrons in real space, are an ideal stage to realize strongly-correlated phenomena. Theoretically, the flat bands can naturally arise in certain geometrically frustrated lattices, often with nontrivial topology if combined with spin-orbit coupling. Here, we report the observation of topological flat bands in frustrated kagome metal CoSn, using angle-resolved photoemission spectroscopy and band structure calculations. Throughout the entire Brillouin zone, the bandwidth of the flat band is suppressed by an order of magnitude compared to the Dirac bands originating from the same orbitals. The frustration-driven nature of the flat band is directly confirmed by the chiral d-orbital texture of the corresponding real-space Wannier functions. Spin-orbit coupling opens a large gap of 80 meV at the quadratic touching point between the Dirac and flat bands, endowing a nonzero Z2 invariant to the flat band. These findings demonstrate that kagome-derived flat bands are a promising platform for novel emergent phases of matter at the confluence of strong correlation and topology.
View details for DOI 10.1038/s41467-020-17465-1
View details for Web of Science ID 000561071600009
View details for PubMedID 32778669
View details for PubMedCentralID PMC7417556
- Second harmonic generation as a probe of broken mirror symmetry PHYSICAL REVIEW B 2020; 101 (24)
Dirac fermions and flat bands in the ideal kagome metal FeSn
2020; 19 (2): 163-+
A kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice-Dirac fermions and flat bands-have not been simultaneously observed. Here, we use angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, which has spatially decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that the coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in a kagome lattice is a frontier of great promise at the confluence of topology, magnetism and strongly correlated physics.
View details for DOI 10.1038/s41563-019-0531-0
View details for Web of Science ID 000511169400011
View details for PubMedID 31819211
- Creating Weyl nodes and controlling their energy by magnetization rotation PHYSICAL REVIEW RESEARCH 2019; 1 (3)
de Haas-van Alphen effect of correlated Dirac states in kagome metal Fe3Sn2
2019; 10: 4870
Primarily considered a medium of geometric frustration, there has been a growing recognition of the kagome network as a harbor of lattice-borne topological electronic phases. In this study we report the observation of magnetoquantum de Haas-van Alphen oscillations of the ferromagnetic kagome lattice metal Fe3Sn2. We observe a pair of quasi-two-dimensional Fermi surfaces arising from bulk massive Dirac states and show that these band areas and effective masses are systematically modulated by the rotation of the ferromagnetic moment. Combined with measurements of Berry curvature induced Hall conductivity, our observations suggest that the ferromagnetic Dirac fermions in Fe3Sn2 are subject to intrinsic spin-orbit coupling in the d electron sector which is likely of Kane-Mele type. Our results provide insights for spintronic manipulation of magnetic topological electronic states and pathways to realizing further highly correlated topological materials from the lattice perspective.
View details for DOI 10.1038/s41467-019-12822-1
View details for Web of Science ID 000492835100008
View details for PubMedID 31653866
View details for PubMedCentralID PMC6814717
- Molecular beam epitaxy growth of antiferromagnetic Kagome metal FeSn APPLIED PHYSICS LETTERS 2019; 115 (7)
Ultrafast manipulation of mirror domain walls in a charge density wave
2018; 4 (10): eaau5501
Domain walls (DWs) are singularities in an ordered medium that often host exotic phenomena such as charge ordering, insulator-metal transition, or superconductivity. The ability to locally write and erase DWs is highly desirable, as it allows one to design material functionality by patterning DWs in specific configurations. We demonstrate such capability at room temperature in a charge density wave (CDW), a macroscopic condensate of electrons and phonons, in ultrathin 1T-TaS2. A single femtosecond light pulse is shown to locally inject or remove mirror DWs in the CDW condensate, with probabilities tunable by pulse energy and temperature. Using time-resolved electron diffraction, we are able to simultaneously track anti-synchronized CDW amplitude oscillations from both the lattice and the condensate, where photoinjected DWs lead to a red-shifted frequency. Our demonstration of reversible DW manipulation may pave new ways for engineering correlated material systems with light.
View details for DOI 10.1126/sciadv.aau5501
View details for Web of Science ID 000449221200072
View details for PubMedID 30345365
View details for PubMedCentralID PMC6195337
Massive Dirac fermions in a ferromagnetic kagome metal
2018; 555 (7698): 638-+
The kagome lattice is a two-dimensional network of corner-sharing triangles that is known to host exotic quantum magnetic states. Theoretical work has predicted that kagome lattices may also host Dirac electronic states that could lead to topological and Chern insulating phases, but these states have so far not been detected in experiments. Here we study the d-electron kagome metal Fe3Sn2, which is designed to support bulk massive Dirac fermions in the presence of ferromagnetic order. We observe a temperature-independent intrinsic anomalous Hall conductivity that persists above room temperature, which is suggestive of prominent Berry curvature from the time-reversal-symmetry-breaking electronic bands of the kagome plane. Using angle-resolved photoemission spectroscopy, we observe a pair of quasi-two-dimensional Dirac cones near the Fermi level with a mass gap of 30 millielectronvolts, which correspond to massive Dirac fermions that generate Berry-curvature-induced Hall conductivity. We show that this behaviour is a consequence of the underlying symmetry properties of the bilayer kagome lattice in the ferromagnetic state and the atomic spin-orbit coupling. This work provides evidence for a ferromagnetic kagome metal and an example of emergent topological electronic properties in a correlated electron system. Our results provide insight into the recent discoveries of exotic electronic behaviour in kagome-lattice antiferromagnets and may enable lattice-model realizations of fractional topological quantum states.
View details for DOI 10.1038/nature25987
View details for Web of Science ID 000428617600045
View details for PubMedID 29555992
- Extreme magnetoresistance in magnetic rare-earth monopnictides PHYSICAL REVIEW B 2018; 97 (8)
- Electronic transport on the Shastry-Sutherland lattice in Ising-type rare-earth tetraborides PHYSICAL REVIEW B 2017; 95 (17)
- Thermoelectric probe for Fermi surface topology in the three-dimensional Rashba semiconductor BiTeI PHYSICAL REVIEW B 2015; 92 (11)
- Transport signatures of Fermi surface topology change in BiTeI PHYSICAL REVIEW B 2015; 91 (20)