Symmetry-protected nodal planes and accidental nodal surfaces in mixed odd-even wave spin-momentum locking of relativistic altermagnets

Non-relativistic spin--momentum locking in altermagnets exhibits an even number of nodal planes. In the relativistic limit, the number of nodal planes can be lowered by symmetry reduction due to the N\'eel vector and spin--orbit coupling in noncentrosymmetric systems. Therefore, an analysis of the evolution of the nodal planes in relativistic altermagnets is required. While $g$-wave spin--momentum locking is straightforward to realize in non-relativistic altermagnets, this $g$-wave does not necessarily survive in the relativistic case. In this work, we investigate the relativistic spin--momentum locking of the centrosymmetric CrSb and the noncentrosymmetric wurtzite MnTe. As a first result, we show that in both systems the dominant spin component retains its $g$-wave character in the relativistic regime only when the N\'eel vector is oriented along the $z$-axis, while the subdominant components exhibit $d$-wave symmetry in CrSb and $p$-wave symmetry in ferroelectric wurtzite MnTe. More generally, the $g$-wave character is preserved in the relativistic limit only when both the N\'eel vector and the electric field associated with inversion-symmetry breaking are oriented along the $z$-axis. As a second result, we show that relativistic spin--momentum locking of ferroelectric altermagnets can exhibit $p$-wave magnetism with one symmetry-protected nodal plane and an accidental nodal surface not protected by symmetry, or can have two accidental nodal surfaces. With the N\'eel vector aligned along the $x$-axis, selected bands of ferroelectric altermagnet wurtzite MnTe exhibit $p$-wave magnetism. Our results establish that altermagnets can host distinct spin components that realize a mixture of angular-momentum wave symmetries in momentum space in the relativistic limit.