The discovery of quantum materials has led to significant advancements in condensed matter physics. Conventional fields such as magnetism and superconductivity were revolutionized by the concept of topology, which offers an ideal platform for realizing novel physical phenomena. To deepen our understanding of the quantum phenomena, it is crucial to search for various quantum phases and establish ways to control their properties on demand. In this presentation, I will introduce my studies on controlling low-dimensional quantum materials under equilibrium and non-equilibrium.

First, I will discuss the engineering of topological insulator phases in quasi-1D bismuth halides utilizing their stacking sequences. Among various kinds of topological insulators (TIs), strong TIs were immediately demonstrated after their theoretical prediction by observing the surface states through photoemission spectroscopy. In contrast, weak TIs and higher-order TIs remained elusive due to the experimental difficulties in detecting their boundary states. Here, I will show that quasi-1D bismuth halides offer the ideal playground to investigate and manipulate these topological phases. Utilizing a high-resolution laser and a synchrotron-based nano-beam, I have found evidence of weak and higher-order TI phases in bismuth halides depending on their stacking sequences [1-3].

​ Second, I will discuss the ultrafast manipulation of electronic states in a layered antiferromagnet. Here, I have explored the possibility of Floquet engineering, where an ultrafast intense electromagnetic field is applied to create or control novel quantum properties. I applied white light-based broadband transient reflection spectroscopy to the layered antiferromagnet MnPS₃ and observed coherent phonon generation after Floquet driving. The relationship between the coherently excited phonons and Floquet engineering will be discussed [4].
 

[1] R. Noguchi et al., Nature 566, 518–522 (2019).
[2] R. Noguchi et al., Nat. Mater. 20, 473 (2021).
[3] R. Noguchi et al., arXiv:2301.07158.
[4] R. Noguchi, D. Hsieh et al., in preparation.

Ryo Noguchi is a postdoctoral scholar at the California Institute of Technology. He received his PhD from the University of Tokyo in 2021 under the supervision of Prof. Takeshi Kondo. Before joining Prof. David Hsieh's group at Caltech, he worked as a postdoctoral researcher with Prof. Changyoung Kim at Seoul National University. Ryo is interested in uncovering and controlling novel quantum states using spectroscopic techniques. Currently, he is conducting research on optical manipulation of layered antiferromagnets, with a specific focus on Floquet effects.

Please contact phweb@ust.hk should you have questions about the talk.