Probing rigid-layer modes in a van der Waals mineral by terahertz time-domain spectroscopy

Since the discovery of graphene, great attention has been attracted to naturally occurring van der Waals materials [1], which can be exfoliated down to a few layers, thus being a low-cost source for two-dimensional materials. As interesting quantum phenomena can emerge from low-dimensionality, these systems have become the promising building blocks for novel electronic, optoelectronic, and mechanical nanodevices. Among the candidate materials one can highlight the mineral clinochlore [Mg5Al(AlSi3)O10(OH)8], one of the most abundant phyllosilicates in nature [2]. In this work [3], we have studied low-frequency rigid-layer modes as a path to shed light on the strength of the inter-layer coupling. We have employed our terahertz time-domain spectroscopy (THz-TDS) experimental setup [4] to analyze bulk samples of clinochlore. This powerful technique is known to enable direct determination of the complex refractive index of materials. From our measurements, we have found a very distinctive sharp resonance centered in 1.13 THz, which exhibited a small temperature dependence. Further polarization dependent measurements have also demonstrated a birefringent behavior of clinochlore, which was observed through the detection of Faraday rotation and ellipticity on and off resonance. Moreover, THz-TDS measurements were also conducted on the related mineral talc, helping us to discuss the possible origins of the resonance in clinochlore. Finally, density function theory simulations were made considering the existence of iron impurities on the mineral and its effect on the frequency of the rigid-layer modes and refractive index.

[1] R. Frisenda et al., npj 2D Materials and Applications 4, 38 (2020).

[2] R. de Oliveira et al., Applied Surface Science 599, 153959 (2022).

[3] N. M. Kawahala et al., arxiv: 2409.18933 (2024).

[4] N. M. Kawahala et al., Coatings 13, 1855 (2023).