  Recently, a novel magnetic phase called altermagnetism has been proposed,
ushering in a third distinct magnetic phase beyond ferromagnetism and
antiferromagnetism. It is expected that this groundbreaking phase exhibits
unique physical properties such as C-paired spin-valley locking, anomalous Hall
effect, nontrivial Berry phase, and giant magnetoresistance, etc. Among all the
predicted candidates, several room temperature altermagnets are suggested to
host significant potential applications in the near future. Nevertheless,
direct evidence about the spin pattern of the room temperature altermagnet is
still unrevealed. Previous studies found that RuO2 is identified as the most
promising candidate for room temperature d-wave altermagnetism, exhibiting a
substantial spin splitting of up to 1.4 eV. In this study, utilizing
angle-resolved photoemission spectroscopy (ARPES), we report experimental
observation of the spin splitting in RuO2. Furthermore, employing spin-ARPES,
we directly observed the d-wave spin pattern. Our results unequivocally show
that RuO2 is a perfect d-wave altermagnet with great potential for upcoming
spintronic applications.
