Floquet control of topological phases and Hall effects in Z2 nodal line semimetals

Dynamic control of topological properties in materials is central to modern condensed matter physics, and Floquet engineering, utilizing periodic light fields, provides a promising avenue. Here, we use Floquet theory to theoretically study the topological response of a Z2 nodal line semimetal (NLSM) when driven by circularly polarized light (CPL). We demonstrate that the direction of CPL irradiation critically dictates the resulting topological phase transitions. Specifically, when light is incident perpendicular to the nodal line plane, increasing the light amplitude induces two successive topological phase transitions: first, from the Z2 NLSM to a vortex NLSM, a rare and intriguing topological state; and second, a transition from the vortex NLSM to a semimetal with a pair of Weyl points (WPs). In stark contrast, irradiation along other directions directly transforms the Z2 nodal line into a pair of WPs. We further investigate the transport properties of the Floquet Z2 NLSM, focusing on the anomalous and planar Hall effects. The anomalous Hall effect exhibits a direction-dependent amplitude variation, deviating from conventional two-band NLSM behavior. Importantly, we reveal a significant and tunable planar Hall effect, a phenomenon largely unexplored in Floquet topological materials, which is highly sensitive to both light amplitude and direction. Our findings not only present a novel route to realize the vortex NLSM, but also establish an efficient way to control Hall transport phenomena in Z2 NLSMs.