"Emergent Pseudo-Gauge Field and Gravitational Field in Dirac Materials"

Presented by Dr. Chaoxing Liu, Professor, from Pennsylvania State University

Electrons in solids are usually described by non-relativistic Schrödinger equation since electron velocity is much slower than the speed of light. However, the relativistic Dirac equation can emerge as a low energy effective theory for electrons in certain solid materials. These systems are dubbed “Dirac materials” and provide a tunable platform to test quantum relativistic phenomena in table-top experiments. More interestingly, different types of perturbations in these Dirac materials, such as lattice vibration, strain, magnon and material inhomogeneity, can be coupled to relativistic electrons in a similar form as the gauge field or gravitational field. We thus dubbed these types of perturbations as the emergent background “pseudo-gauge field” or “gravitational field” in Dirac materials. In this talk, I will focus on electron-strain coupling in Dirac materials by treating the strain (or equivalently acoustic phonons) as emergent pseudo-gauge field or gravitational field. This viewpoint allows us to theoretically predict rich physical phenomena in Dirac materials, including (1) piezo-electric and piezo-magnetic response in 2D or 3D Dirac materials [Nat. Commun. 11, 2290 (2020); PRB 104, 174406 (2021)]; (2) phonon helicity, namely the “helical texture” of phonon angular momentum, induced by electron Berry curvature in 2D massive Dirac materials [PRL 127, 125901 (2021)] or Nieh-Yan anomaly of Weyl fermions in 3D chiral crystals [PRB 106, 115102 (2022)], (3) and localized phonon modes at the surface/interface of 3D Dirac materials, induced by the axion electrodynamical response or Nieh-Yan gravitational response of 3D massive Dirac fermions [PRB 110, L161301 (2024)].