Observation of van der Waals phonons in the single-layer cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+δ}$
cond-mat.supr-con
/ Authors
Y. Y. Peng, I. Boukahil, K. Krongchon, Q. Xiao, A. A. Husain, Sangjun Lee, Q. Z. Li, A. Alatas, A. H. Said, H. T. Yan
and 7 more authors
Y. Ding, L. Zhao, X. J. Zhou, T. P. Devereaux, L. K. Wagner, C. D. Pemmaraju, P. Abbamonte
/ Abstract
Interlayer van der Waals (vdW) coupling is generic in two-dimensional materials such as graphene and transition metal dichalcogenides, which can induce very low-energy phonon modes. Using high-resolution inelastic hard x-ray scattering, we uncover the ultra-low energy phonon mode along the Cu-O bond direction in the high-$T_c$ cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+δ}$ (Bi2201). This mode is independent of temperature, while its intensity decreases with doping in accordance with an increasing c-axis lattice parameter. We compare the experimental results to first-principles density functional theory simulations and identify the observed mode as a van der Waals phonon, which arises from the shear motion of the adjacent Bi-O layers. This shows that Bi-based cuprate has similar vibrational properties as graphene and transition metal dichalcogenides, which can be exploited to engineer novel heterostructures.