Gate engineering Fabry-Pérot resonance in altermagnetic junctions.

Electrically generating and controlling spin-polarized transport is a central objective in spintronics. Altermagnets, which exhibit compensated magnetic order and symmetry-protected spin splitting, offer a promising route toward this goal without requiring net magnetization, magnetic fields, or spin-orbit coupling. Here, we investigate coherent spin transport in a two-dimensional d-wave altermagnetic junction connected to normal metal leads. The anisotropic exchange field induces spin-dependent effective masses, resulting in distinct Fabry-Pérot resonance conditions for spin-up and spin-down electrons. When the junction length matches half-integer multiples of the spin-dependent wavelength, one spin channel is resonantly transmitted while the other is suppressed, yielding fully spin-polarized transport. Employing the quantum scattering formalism, we show that the spin polarization of the transmitted current can be controlled by tuning (i) the gate potential in the altermagnet, (ii) the interfacial barrier strength, and (iii) the orientation of the altermagnetic field. These control parameters enable electrically tunable spin filtering and provide a diagnostic to distinguish between distinct d-wave altermagnetic symmetries. In particular, we show that the [Formula: see text]-wave altermagnet supports robust gate-controlled spin-polarized current even in the high-barrier tunneling regime, a behavior absent in its [Formula: see text]-wave counterpart. Our results establish a field-free, gate-controlled mechanism for spintronic functionality rooted in the crystalline anisotropy of altermagnetic materials.