Magnetic symmetry breaking driven "inverse magnetic breakdown" in a d-wave altermagnet KV$_{2}$Se$_{2}$O

Altermagnets, combining zero net magnetization with intrinsic spin splitting, demonstrate unique quantum phenomena crucial for spintronic applications. KV$_{2}$Se$_{2}$O is proven to be a d-wave altermagnet with phase transition from a checkerboard-type (C-type) antiferromagnetic (AFM) state to a spin density wave (SDW) state as the temperature decreases. After phase transition, an apparent paradox emerges where angle-resolved photoemission spectroscopy (ARPES) reveals negligible Fermi surface modifications, while physical property measurement system (PPMS) measurements uncover substantial changes in transport properties. Our study explores the microscopic mechanisms governing phase-dependent transport properties of KV$_{2}$Se$_{2}$O based on first-principles calculations. The spin canting driven by periodic spin modulation in the SDW phase reduces the magnetic symmetry of KV$_{2}$Se$_{2}$O. The resultant band degeneracy lifting and Fermi surface reconstruction induce the “inverse magnetic breakdown" phenomenon, which alters carrier trajectories, modifies carrier concentration, strengthens electron-hole compensation, and ultimately accounts for the contrasting magnetic-field-dependent Hall resistivity relative to the C-type AFM state. Our work proposes an innovative method for identifying the electronic structure evolution across a phase transition from transport signatures, providing a novel paradigm for altermagnet research.