Altermagnetism (AM), the recently discovered third class of collinear magnetic order, is characterized by a nonrelativistic momentum-dependent spin-split electronic structure with compensated zero net magnetization. It can arise from the conventional antiferromagnetism by introducing local anisotropy on the two opposite-spin sublattices, either through structural changes in local crystallographic symmetry or spontaneous emergence of local staggered orbital order from electron correlations in multi-orbital systems. Here, we demonstrate on the two-dimensional square lattice that a $d$-wave AM can emerge spontaneously in the single-orbital extended Hubbard model, without invoking crystallographic anisotropy and multi-orbital physics. We carry out mean-field studies on the concrete single-orbital $t\text{−}U\text{−}V$ model, with $U$ and $V$ the on-site and nearest-neighbor Coulomb interactions, obtaining the mean-field ground states, analyzing their properties, and determining the phase diagram in the $U\text{−}V$ plane. The $d$-wave AM with interesting spin-transport behavior is found to be stabilized in a wide region of the phase diagram when the system is doped away from half filling, actualized by the coexistence of on-site antiferromagnetic order and complex $d$-wave nearest-neighbor spin bond orders. Our findings provide an alternative route to achieve AM and substantially expand the range of candidate AM materials.

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• Magnetic order
• Altermagnets
• Square lattice
• Strongly correlated systems
• Extended Hubbard model
• Mean field theory