Observation of Ultrastrong Coupling between Substrate and the Magnetic Topological Insulator MnBi 2 Te 4 .

The intrinsic magnetic topological insulator MnBi 2 Te 4 has attracted significant interest recently as a promising platform for exploring exotic quantum phenomena. Here we report that, when atomically thin MnBi 2 Te 4 is deposited on a substrate such as silicon oxide or gold, there is a very strong mechanical coupling between the atomic layer and the supporting substrate. This is manifested as an intense low-frequency breathing Raman mode that is present even for monolayer MnBi 2 Te 4 . Interestingly, this coupling turns out to be stronger than the interlayer coupling between the MnBi 2 Te 4 atomic layers. We further found that these low-energy breathing modes are highly sensitive to sample degradation, and they become drastically weaker upon ambient air exposure. This is in contrast to the higher energy optical phonon modes which are much more robust, suggesting that the low-energy Raman modes found here can be an effective indicator of sample quality.