Research

Novel Quasiparticles in Layered Quantum Materials

Our understanding of complex quantum materials is established on the notion of quasiparticles, which includes fermions, bosons, and anyons, that are the building blocks of future quantum hardware. In order to uncover, understand and ultimately control novel quasiparticles, I utilize several experimental approaches and focus on layered quantum materials.

Approach 1: Optical and Magneto-optical spectroscopy

In topological semimetals, Dirac-like band crossings appear in momentum space and is protected by discrete symmetries. Topological semimetals has attracted great interest since they host quasiparticles that may not exist in vacuum but do occur in solid-state systems. For example, the nodal-line semimetals extend the point-degeneracies in Dirac semimetals into lines/rings of degeneracies in momentum space. These unusual band structure and topology can be captured by powerful power-law analysis of response functions, e.g. optical spectroscopy and Landau level spectroscopy [1, 2]. One recent example is the realization of semi-Dirac fermions in crossing nodal-lines [3].

Semi-Dirac fermions represent a peculiar class of fermions that are massless in one direction and massive in the perpendicular direction in two-dimensions. The proximity of semi-Dirac fermions to various topological phases (e.g. a Chern insulating state) has drawn intense theoretical efforts since their prediction [4, 5], yet the signatures of semi-Dirac points near the Fermi level remain elusive.

References

Y. Shao et al., Proc. Natl. Acad. Sci. U. S. A. 116, 1168 (2019).
Y. Shao et al., Nat. Phys. 16, 636 (2020).
Y. Shao et al., Phys. Rev. X 14, 041057 (2024).
P. Dietl, F. Piéchon, and G. Montambaux, Phys. Rev. Lett. 100, 236405 (2008).
V. Pardo, and W. E. Pickett, Phys. Rev. Lett. 102, 166803 (2009).

Approach 2: Near-field optical imaging

The scattering-type Scanning Near-field Optical Microscope (SNOM) have been gaining momentum for the exploration of polaritons, the hybridized quasiparticle of light and matter [1]. In particular, hyperbolic polaritons, with the characteristic “ray-like” propagation, has been explored in polar insulators such as hBN [2] and are limited to the mid-infrared range.

Common metals bounce incident light off in the infrared range. Anisotropic metals do support propagating modes in the bulk provided that the dielectric response is metallic in-plane and dielectric-like out of plane. Predictions of hyperbolic metals were extensive yet experimental verifications of waveguiding have been elusive. This is because the inherently strong electronic losses in typical metals arrest the propagation of infrared modes. In our recent experiments [3], we discovered that a special class of metals, so-called nodal metals hosting the two-dimensional electronic structure with van Hove singularities, support waveguiding of infrared energy in their bulk. This novel plasmonic effect is observed over an extended frequency range in the technically important near-infrared range, offering an appealing platform to investigate quantum effects originating from the interplay of topology, reduced dimensionality and electronic correlations encoded in unconventional quasiparticles.

References

D. N. Basov, M. M. Fogler, and F. J. G. de Abajo, Science 354, aag1992 (2016).
S. Dai et al., Science 343, 1125 (2014).
Y. Shao et al., Sci. Adv. 8, eadd6169 (2022).

Approach 3: Multi-modal nanoscopy

Nonlinear optics in topological semimetals is a burgeoning field [1,2] of research with expanding list of new materials but limited choice of probes.

We devised, modeled and implemented an entirely new approach for investigating nonlinear optical responses at the nano-scale with a metallic tip [3]. Conventional far-field nonlinear optical microscopy has diffraction-limited spatial resolution and probes the in-plane responses only. Our tip-based approach circumvents the diffraction limit and provides strong field-enhancement for both in-plane and out-of-plane fields. We therefore gain access to complete nonlinear conductivity tensors including components not attainable in conventional optical experiments. One immediate application is the separation of surface state and bulk nonlinear responses in Weyl semimetals based on their distinct real-space photocurrent patterns (bottom panels). Multimodal nanoscopy, combining nano-optics, nano-photocurrent, MFM, and KPFM will create a synergistic platform for implementing new device paradigms and discovering innovative functionalities of emerging quantum materials.

References

J. Ahn, G.-Y. Guo, and Naoto Nagaosa, PRX 10, 041041 (2020)
Q. Ma, A. G. Grushin, and K. S. Burch, Nat. Mater. 20,1601 (2021).
Y. Shao et al., PNAS 118, e2116366118 (2021).

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