Layer Axial Phonons and Dipolar Thermal Response in Centrosymmetric Thin Films

In nonmagnetic centrosymmetric materials, $\mathcal{PT}$ symmetry enforces vanishing total phonon angular momentum (phonon AM) at every wave vector, making them appear unsuitable as hosts of axial phonons. Here, using first-principles calculations for BaAgAs and representative van der Waal's layered materials, we show that centrosymmetric slabs can still host large hidden layer-resolved phonon AM with both chiral and cycloidal modes. This hidden phonon AM texture produces layer resolved phonon thermal Edelstein-like responses and a real-space phonon AM dipole response, both tunable by strain. Our results establish centrosymmetric layered materials as an exciting platform for hidden axial phonon and multipolar phonon thermal response.