Altermagnetism, a recently identified magnetic phase that combines vanishing net magnetization with momentum-dependent spin splitting, challenges the conventional dichotomy between ferromagnets and antiferromagnets. While several candidate materials have been proposed, direct experimental evidence linking crystal symmetry, electronic structure and d-wave spin polarization remains scarce. Here we report the visualization of a metallic d-wave altermagnet in KV2Se2O. Through spin-selective scanning tunneling microscopy powered by a topological insulator tip, we uncover symmetry-protected momentum-dependent spin splitting that follows a characteristic d-wave form factor. Our results establish KV2Se2O as a tunable platform to study the interplay between spin-valley locking, Fermi-surface instability and unconventional magnetism, and open a pathway toward symmetry-engineered spintronics without net magnetization.
@article{arxiv.2603.21969,
title = {Visualizing spin-polarization of an altermagnet KV$_2$Se$_2$O via spin-selective tunneling},
author = {Guofei Yang and Chuang Li and Chengwei Wang and Xudong Zhao and Yifan Wan and Hengrui Gui and Guoqing Zeng and Saizheng Cao and Chuqiao Hu and Dong Chen and Yu Liu and Yu Song and Fei Liu and Lun-Hui Hu and Lin Jiao and Huiqiu Yuan},
journal= {arXiv preprint arXiv:2603.21969},
year = {2026}
}