d-wave altermagnets possess nearly orthogonal flat Fermi surfaces, which in an idealized limit enable complete spin-channel separation and a theoretical charge-to-spin conversion efficiency (CSE) of 100%. The recently discovered metallic altermagnet KV2Se2O exemplifies this class, yet realistic samples host residual elliptical Fermi pockets that enhance charge conductivity while suppressing spin conductivity, drastically reducing the CSE. Here we show that in-plane equibiaxial tensile strain systematically eliminates these parasitic pockets, restoring the flat-band geometry. Our first-principles calculations reveal that the CSE increases monotonically with strain, reaching a record value of approximately 96% at 4% strain. An effective tight-binding model fitted to the computed band structure accurately captures the evolution of the Fermi surface and confirms that the suppression of the pockets -- governed by reduced next-nearest-neighbor hoppings -- is the dominant mechanism for the strain-enhanced CSE. We further identify an unconventional out-of-plane spin current component that emerges under tilted electric fields and achieves a CSE of nearly 55% at optimal orientations, offering a promising pathway for field-free perpendicular magnetization switching. Our findings establish strain engineering as a practical route to approach the ultimate conversion limit in d-wave altermagnets and provide a design principle for high-efficiency spintronic devices.
@article{arxiv.2604.21779,
title = {Nearly Complete Charge--Spin Conversion via Strain-Eliminated Fermi Pockets in $d$-Wave Altermagnets},
author = {Wancheng Zhang and Zhenhua Zhang and Rui Xiong and Zhihong Lu},
journal= {arXiv preprint arXiv:2604.21779},
year = {2026}
}