Observation of van der Waals phonons in the single-layer cuprate (Bi,Pb)$_2$(Sr,La)$_2$CuO$_{6+\delta}$

TL;DR

This study reveals the presence of van der Waals phonons in a single-layer cuprate superconductor, showing similarities to 2D materials like graphene, and suggests potential for engineering novel heterostructures.

Contribution

It provides experimental evidence of van der Waals phonons in a cuprate, supported by first-principles calculations, highlighting a new vibrational property in high-$T_c$ superconductors.

Findings

Identification of an ultra-low energy van der Waals phonon mode

Mode is independent of temperature and decreases with doping

Comparison with theoretical simulations confirms shear motion origin

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$_{6+\delta}$ (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.