Static and ultrafast engineering of low-dimensional quantum materials

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 such as Dirac, Majorana, and Weyl fermions. 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 characterizing new quantum phases and manipulating electronic properties under equilibrium and non-equilibrium.
First, I will discuss the engineering of topological insulator phases in quasi-1D bismuth halides utilizing their stacking sequences. Topological insulators (TIs) are classified into different phases based on their topological invariants. Among them, strong TI phases have been 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 show that quasi-1D bismuth halides offer the ideal playground to investigate and manipulate these topological phases since their characteristic boundary states emerge in the naturally cleaved surfaces. 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. To this end, I designed and built a white light-based broadband pump-probe reflection spectroscopy setup to detect transient changes in electronic structures. I applied this technique to the layered antiferromagnet MnPS3 and observed coherent phonon oscillation after Floquet driving. The relationship between the coherently excited phonons and Floquet band 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.