Multiband-driven anisotropic vortex topology in iron-based superconductors yielding a strain-tunable x-vortex Majorana platform

We uncover a pronounced anisotropy in Majorana vortex topology arising from the interaction between vortex orientation and multiband topologies, exemplified by iron-based superconductors (FeSCs). This anisotropy manifests in two distinct vortex configurations: the z-vortex and x-vortex, oriented perpendicular and parallel to the Dirac axis (z-axis for FeSCs), respectively. The x-vortex exhibits a unique bifurcation, displaying two distinct topological phase diagrams. One is strikingly simple, comprising only trivial and topological superconducting phases, and remains resilient to multiband entanglement. The other mirrors the z-vortex's complex diagram, featuring alternating trivial, topological crystalline, and topological superconducting phases. The former is exclusive to the x-vortex and supports unpaired Majorana vortices across a wide parameter range, even in the presence of normal-state Dirac nodes. Notably, uniaxial strain can modulate these x-vortex phases, enabling the x-vortex to support both stable Majorana vortices and rich exotic physics in a controllable manner. Moreover, we propose that the x-vortex offers promising advantages for developing Majorana nanowire devices in FeSCs. Our findings introduce a novel paradigm in vortex topology within multiband superconducting systems, highlighting the x-vortex as a promising platform for exploring Majorana physics and advancing FeSC quantum devices.