Hidden altermagnetism

Hidden spin polarization (HSP) with zero net spin polarization in total but non-zero local spin polarization has been proposed in certain nonmagnetic centrosymmetric compounds, where the individual sectors forming the inversion partners are all inversion asymmetry. Here, we extend this idea to antiferromagnetic materials with $PT$ symmetry (the joint symmetry of space inversion symmetry ($P$) and time-reversal symmetry ($T$)), producing zero net spin polarization in total, but either of the two inversion-partner sectors possesses altermagnetism, giving rise to non-zero local spin polarization in the real space, dubbed "hidden altermagnetism". By first-principle calculations, we predict that $PT$-symmetric bilayer $\mathrm{Cr_2SO}$ can serve as a possible candidate showing altermagnetic HSP. By applying an external electric field to break the global $P$ symmetry, the hidden altermagnetism can be separated and observed experimentally. Our works extend the hidden physics, and will also advance the theoretical and experimental search for new type of spin-polarized materials.