  Spintronics has emerged as a viable alternative to traditional electronics
based technologies in the past few decades. While on one hand, the discovery of
topological phases of matter with protected spin-polarized states has opened up
exciting prospects, recent revelation of intriguing non-relativistic spin
splitting in collinear antiferromagnetic materials with unique symmetries
facilitate a wide possibility of realizing both these features simultaneously.
In this work, we report the co-existence of these two intriguing properties
within a single material: GdAlSi. It crystallizes in a body-centered tetragonal
structure with a non-centrosymmetric space group $I4_{1}md$ ($109$). The
magnetization data indicates antiferromagnetic ordering with an ordering
temperature ($T_N$) 32 K. Ab-initio calculations reveal GdAlSi to be a
collinear antiferromagnetic Weyl semimetal with an unconventional,
momentum-dependent spin splitting, also referred to as altermagnet.
Angle-resolved photoemission spectroscopy measurements on GdAlSi single
crystals subsequently confirm the presence of Fermi arcs, a distinctive
hallmark of Weyl semimetals. Electric and magnetic multipole analysis provides
a deeper understanding of the symmetry-mediated, momentum-dependent spin
splitting, which has strictly non-relativistic origin. To the best of our
knowledge, such co-existence of unconventional antiferromagnetic order and
non-trivial topology is unprecedented and has never been observed before in a
single material, rendering GdAlSi a special and promising candidate material.
We propose a device harnessing these features, poised to enable practical and
efficient topotronic applications.
