Negative Differential Resistance and Ultrahigh TMR in Altermagnetic Tunnel Junctions.

Altermagnets can replace ferromagnets in tunnel junctions, yielding a large tunneling magnetoresistance, ultrafast switching, and low-power functionality. While most studies explore the linear-response regime, interesting features emerge at finite bias, with their peculiar electronic structure giving rise to complex nonlinear behavior. Using density functional theory and nonequilibrium Green's functions, we predict a pronounced low-bias negative differential resistance in an altermagnetic tunnel junction, incorporating orbital-ordered KV2Se2O. This is ascribed to KV2Se2O's altermagnetic quasi-2D Fermi surface. Upon application of a finite-bias voltage, the current in the parallel configuration first increases sharply before decreasing and becoming almost completely suppressed at around 0.14 V, whereas the antiparallel configuration displays monotonic current-voltage characteristics. This behavior, together with the negative differential resistance, produces a large tunneling magnetoresistance with sign inversion at 0.13 V. Our results validate altermagnetic tunnel junctions as a platform for applications requiring low-bias strongly nonlinear responses.