Klein tunneling — the perfect transmission of relativistic particles through a potential barrier, regardless of barrier height — was predicted in 1929 and demonstrated in graphene in 2006. The transmission is unity because the barrier can't block the particle; instead, it converts particle states to antiparticle states that propagate through. The barrier is transparent.
The authors (arXiv:2603.23235) show that in Dirac altermagnets — materials where electrons obey a Dirac equation and the magnetic order breaks time-reversal symmetry without a net magnetization — Klein tunneling amplifies spin-polarized currents. The barrier transmits both spin species perfectly, but the altermagnet's band structure means the two spins carry different momenta. The transmitted current is spin-polarized not because the barrier filters spins, but because the material's band structure is spin-split.
The through-claim: Klein tunneling in altermagnets turns an obstacle into a spin generator. Conventional spin filters work by blocking one spin species — transmission is always less than unity. Klein tunneling transmits everything — transmission is unity — but the altermagnetic band structure ensures the transmitted current is spin-polarized anyway. Perfect transmission and spin polarization, simultaneously.