<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"> <mml:mrow> <mml:mi>d</mml:mi> </mml:mrow> </mml:math> -Wave Flat Fermi Surface in Altermagnets Enables Maximum Charge-to-Spin Conversion

Altermagnets combine antiferromagnetic order with ferromagnetlike 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 KV_{2}Se_{2}O, which exhibits a CSE of ∼78% at the charge neutrality point-nearly double that of RuO_{2}, setting a new record for T-odd CSE. Under electron doping, this efficiency further increases to ∼98%, approaching the theoretical limit. Our Letter 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.