  Spatial, momentum and energy separation of electronic spins in condensed
matter systems guides the development of novel devices where spin-polarized
current is generated and manipulated. Recent attention on a set of previously
overlooked symmetry operations in magnetic materials leads to the emergence of
a new type of spin splitting, enabling giant and momentum-dependent spin
polarization of energy bands on selected antiferromagnets. Despite the
ever-growing theoretical predictions, the direct spectroscopic proof of such
spin splitting is still lacking. Here, we provide solid spectroscopic and
computational evidence for the existence of such materials. In the noncoplanar
antiferromagnet MnTe$_2$, the in-plane components of spin are found to be
antisymmetric about the high-symmetry planes of the Brillouin zone, comprising
a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an
unconventional spin pattern, further found to diminish at the high-temperature
paramagnetic state, stems from the intrinsic AFM order instead of spin-orbit
coupling (SOC). Our finding demonstrates a new type of quadratic spin texture
induced by time-reversal breaking, placing AFM spintronics on a firm basis and
paving the way for studying exotic quantum phenomena in related materials.
