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Angle-resolved photoemission spectroscopy (ARPES) — an experimental technique based on the photoelectric effect — is arguably the most powerful method for probing the electronic structure of solids [1]. In the first part of my talk, I will introduce two advanced ARPES apparatuses that I have been working with: ‘Dreamline’ ARPES at Shanghai and the high-harmonic-generation (HHG) -based time-resolved ARPES (tr-ARPES) at MIT. The ‘Dreamline’ ARPES is characterized by a wide photon energy range ranging from surface-sensitive vacuum ultraviolet light to bulk-sensitive soft X-rays. In the past few years, it has played a crucial role in the study of the non-trivial bulk and surface electronic structures of topological materials [2]. Prominent examples include the experimental discovery of Weyl semimetals [3, 4] and unconventional fermions [5]. On the other hand, tr-ARPES with discretely tunable HHG light makes it possible to probe the transient electronic structure over a wide energy and momentum range. Particularly, the Gedik group has recently implemented a high-repetition-rate HHG-based tr-ARPES system, enabling the study of light-matter interaction in a wide range of quantum materials.
Hysteresis underlies a large number of phase transitions in solids, giving rise to exotic metastable states that are otherwise inaccessible. In the second part of my talk, I will present our recent finding of an unconventional hysteretic transition in a quasi-2D material, EuTe4 [6]. By combining transport, photoemission, diffraction, and x-ray absorption measurements, we observed that the hysteresis loop has a temperature width of more than 400 K, setting a record among crystalline solids. The transition has an origin distinct from known mechanisms, lying entirely within the incommensurate charge-density-wave (CDW) phase of EuTe4 with no change in the CDW modulation periodicity. We interpret the hysteresis as an unusual switching of the relative CDW phases in different layers, a phenomenon unique to quasi-2D compounds that is not present in either purely 2D or strongly-coupled 3D systems. Our findings raise new possibilities of hysteretic behavior in the solid-state, offering an important dimension for how an order parameter evolves in a symmetry-broken phase.
1. B. Q. Lv, T. Qian, H. Ding. Nat. Rev. Phys. 1, 609 (2019)
2. B. Q. Lv, T. Qian. H. Ding. Rev. Mod. Phys, 93, 025002 (2021)
3. B. Q. Lv*, N. Xu*, H. M. Weng*, et al. Nat. Phys. 11, 724 (2015)
4. B. Q. Lv*, H. M. Weng*, et al. Phys. Rev. X 5, 031013 (2015)
5. B. Q. Lv*, Z.-L. Feng*, Q.-N. Xu*, et al. Nature 546, 627–631 (2017)
6. B. Q. Lv*, A. Zong*, et al. Phys. Rev. Lett. (2022)
Dr. Lv obtained his Ph.D. degree in 2018 from the Institute of Physics, Chinese Academy of Sciences under the supervision of Prof. Hong Ding and Prof. Tian Qian. He is currently a postdoc researcher at the Massachusetts Institute of Technology working with Prof. Nuh Gedik. Dr. Lv’s research interests lie in the understanding and manipulation of novel broken symmetry and topological quantum states in condensed matter physics.
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