  Spin-orbit interaction and structure inversion asymmetry in combination with
magnetic ordering is a promising route to novel materials with highly mobile
spin-polarized carriers at the surface. Spin-resolved measurements of the
photoemission current from the Si-terminated surface of the antiferromagnet
TbRh2Si2 and their analysis within an ab initio one-step theory unveil an
unusual triple winding of the electron spin along the fourfold-symmetric
constant energy contours of the surface states. A two-band k.p model is
presented that yields the triple winding as a cubic Rashba effect. The curious
in-plane spin-momentum locking is remarkably robust and remains intact across a
paramagnetic-antiferromagnetic transition in spite of spin-orbit interaction on
Rh atoms being considerably weaker than the out-of-plane exchange field due to
the Tb 4f moments.
