Active Nematics with Integer Defects in Active Colloids of Ellipsoidal Rollers

How propulsion direction relative to particle shape controls topological ordering remains unclear. Here, we study dense ellipsoidal Quincke rollers driven perpendicular to their long axes. It is found that under a high-frequency alternating current (AC) electric field, large-scale polar order emerges through the merging of local nematic domains. Unlike conventional active nematics dominated by half-integer defects, the transversely driven prefers integer defects: +1 vortices with radially aligned particle axes and -1 saddle defects with fourfold symmetry. These defects are confined in domain walls, along which oppositely charged pairs approach and annihilate. These results demonstrate that a polar velocity field can coexist with a perpendicular nematic body axis. This coupling produces unconventional defect textures and a distinct route to global ordering. Our findings offer a robust protocol that the angle between propulsion and polar axis as a practical parameter for controlling topological properties of active nematics.