Kitaev quantum spin liquid driven by surface acoustic waves

Driving materials with external fields provides a pathway to realize novel quantum phases and responses. Recent theoretical and experimental studies showed that by using surface acoustic waves (SAWs), which are elastic waves that propagate on the surface of a piezoelectric substrate generating lattice deformation, new response phenomena intrinsic to SAWs are obtained in hexagonal materials. This new response phenomenon comes from the fact that if the target materials have a honeycomb structure, SAWs work as a pseudo-gauge field that couples to the system with an opposite sign at two different valley points without electric charge. Motivated by these works, we consider a Kitaev quantum spin liquid (Kitaev QSL) driven by surface acoustic waves. Since this system exhibits mixing of discrete translational symmetries in both the space and time directions, we apply the generalized Floquet-Bloch theorem for oblique space-time crystals which has been recently proposed. In the chiral spin liquid phase, the Majorana band is gapless. However, driven by SAWs, we find that the Majorana band becomes gapped and that topologically nontrivial edge modes in both the momentum and energy directions appear in the quasi-energy spectrum. This result shows that SAWs provide a new quantum phase of matter in spite of being chargeless and spinless systems.