Altermagnets combine antiferromagnetic order with ferromagnet-like spin splitting, a duality that unlocks ultrafast spin-dependent responses. This unique property creates unprecedented opportunities for spin-current generation, overcoming the intrinsic limitations of conventional spin-transfer and spin-orbit torque approaches in magnetic memory technologies. Here, we establish a fundamental relationship between Fermi surface geometry and time-reversal-odd (T-odd) spin currents in altermagnets through combined model analysis and first-principles calculations. We demonstrate that a d-wave altermagnet with a flat Fermi surface can achieve a theoretical upper limit of charge-to-spin conversion efficiency (CSE) of 100%. This mechanism is realized in the newly discovered room-temperature altermagnetic metal KV2O2Se, which exhibits a CSE of ∼78% at the charge neutrality point, nearly double that of RuO2, setting a new record for T-odd CSE. Under electron doping, this efficiency further increases to ∼98%, approaching the theoretical limit. Our work advances the fundamental understanding of T-odd spin currents via Fermi surface geometry engineering and provides key insights for developing next-generation altermagnet-based memory devices.
@article{arxiv.2506.07703,
title = {$d$-Wave Flat Fermi Surface in Altermagnets Enables Maximum Charge-to-Spin Conversion},
author = {Junwen Lai and Tianye Yu and Peitao Liu and Long Liu and Guozhong Xing and Xing-Qiu Chen and Yan Sun},
journal= {arXiv preprint arXiv:2506.07703},
year = {2025}
}