  The fermionic Hubbard model (FHM)[1], despite its simple form, captures
essential features of strongly correlated electron physics. Ultracold fermions
in optical lattices[2, 3] provide a clean and well-controlled platform for
simulating FHM. Doping its antiferromagnetic ground state at half filling,
various exotic phases are expected to arise in the FHM simulator, including
stripe order[4], pseudogap[5], and d-wave superconductors[6], offering valuable
insights into high-temperature superconductivity[7{9]. Although notable
progress, such as the observation of antiferromagnetic correlations over
short[10] and extended distances[11], has been obtained, the antiferromagnetic
phase has yet to be realized due to the significant challenges of achieving low
temperatures in a large and uniform quantum simulator. Here, we report the
observation of the antiferromagnetic phase transition in a three-dimensional
fermionic Hubbard system comprising lithium-6 atoms in a uniform optical
lattice with approximately 800,000 sites. When the interaction strength,
temperature, and doping concentration are finely tuned to approach their
respective critical values, sharp increases in the spin structure factor (SSF)
are observed. These observations can be well described by a power-law
divergence, with a critical exponent of 1.396 from the Heisenberg universality
class[12]. At half filling and with optimal interaction strength, the measured
SSF reaches 123(8), signifying the establishment of an antiferromagnetic phase.
Our results set the stage for exploring the low-temperature phase diagram of
FHM.
