  Beyond conventional ferromagnetism and antiferromagnetism, altermagnetism is
a recently discovered unconventional magnetic phase characterized by
time-reversal symmetry breaking and spin-split band structures in materials
with zero net magnetization. This distinct magnetic phase not only enriches the
understanding of fundamental physical concepts but also has profound impacts on
condense-matter physics research and practical device applications.
Spin-polarized band structures have been recently observed in semiconductors
MnTe and MnTe2 with vanishing net magnetization, confirming the existence of
this unconventional magnetic order. Metallic altermagnets have unique
advantages for exploring novel physical phenomena related to low-energy
quasiparticle excitations and for applications in spintronics as electrical
conductivity in metals allows the direct manipulation of spin current through
electric field. Here, through comprehensive characterization and analysis of
the magnetic and electronic structures of KV2Se2O, we have unambiguously
demonstrated a metallic room-temperature altermaget with d-wave spin-momentum
locking. The highly anisotropic spin-polarized Fermi surfaces and the
spin-density-wave order emerging in the altermagnetic phase make it an
extraordinary platform for designing high-performance spintronic devices and
studying many-body effects coupled with the unconventional magnetism.
