Topological altermagnetic Josephson junctions

Planar Josephson junctions are pivotal for engineering topological superconductivity, yet are severely hindered by orbital effects induced by in-plane magnetic fields. In this work, we introduce the generic topological altermagnetic Josephson junctions (TAJJs) by leveraging the intrinsic spin-polarized band splitting and zero net magnetization attributes of altermagnets. Our proposed TAJJs effectively mitigate the detrimental orbital effects while robustly hosting Majorana end modes (MEMs) at both ends of the junction. Specifically, we demonstrate that MEMs emerge in $d_{x^2-y^2}$-wave TAJJs but vanish in the $d_{xy}$-wave configuration, thereby establishing the crystallographic orientation angle $\theta$ of the altermagnet as a novel control parameter of topology. The distinct spin-polarization of the MEMs provides an unambiguous experimental signature for the spin-resolved measurement. Furthermore, by harnessing the synergy between the $d_{x^2-y^2}$-wave altermagnet and its superconducting counterpart, our proposal extends to high-$T_c$ platforms naturally. Overall, this work establishes altermagnets as a versatile paradigm for realizing topological superconductivity, bridging conceptual innovations with scalable quantum architectures devoid of orbital effects and stray fields.