Tailoring Altermagnetic Spin Splitting via Strain-Induced Symmetry Reconstruction in CrSb Thin Films

Altermagnets represent a novel class of quantum magnets that emerge from specific crystal symmetry operations, particularly rotations or mirror reflections, exhibiting unique momentum-dependent spin-split band structures within the antiferromagnetic regime. The spin textures of altermagnets are fundamentally governed by spin group symmetry, offering unique opportunities for magnetic property control. However, experimentally tuning their spin-splitting via symmetry engineering remains a key challenge. Here, interfacial strain in 10 nm CrSb thin films by molecular beam epitaxy (MBE) is engineered to systematically modify its crystal symmetry. Remarkably, high-resolution spin- and angle-resolved photoemission spectroscopy (spin-ARPES) measurements revealed a transition to a spin-degenerate ground state throughout the entire Brillouin zone, quantitatively demonstrating strain control over altermagnetic spin splitting. Although bulk altermagnetic theory anticipates spin-splitting, thin films exhibit spin degeneracy instead, which this first-principles calculations trace to interfacial strain-mediated surface magnetic state stabilization. In CrSb thin films, strain-induced magnetic reconstruction preserves PT symmetry (spatial inversion followed by time reversal), thereby enforces spin degeneracy across the Brillouin zone, as evidenced by the quantitative agreement between calculated spin/electronic structures and spin-ARPES measurements. This strategy establishes a new paradigm for tailoring altermagnetic spin splitting through engineered symmetry, addressing a critical gap in spectroscopic investigations of altermagnetic systems.