Observation of van der Waals phonons in the single-layer cuprate (Bi, Pb)2(Sr, La)2CuO6+δ

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 ultralow energy phonon mode along the Cu -O bond direction in the high-Tc cuprate (Bi,Pb)2(Sr,La)2CuO6+delta (Bi2201). The energy and full -width half -maximum (FWHM) of these modes are independent of temperature, while their 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 vibrational properties similar to graphene and transition -metal dichalcogenides, which can be exploited to engineer novel heterostructures.