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Description
Extensive experimental efforts have been dedicated to understand the low-pressure phase diagram of La$_3$Ni$_2$O$_7$ with the ultimate goal of connecting the low-pressure with the high-$T_c$ superconducting high-pressure phase diagram. The bilayer nickelate in its $C_4$ symmetry-breaking orthorhombic structural phase features a spin-density wave at $T_{SDW}\approx150$ K and another density wave at $T_{DW}\approx130$ K. Using unrestricted Hartree-Fock calculations for the single-$\mathbf{Q}=(\pi,0)$ order in the orthorhombic ambient-pressure structure of La$_3$Ni$_2$O$_7$, we find that a nematic double-stripe spin-density wave that breaks $C_2$ symmetry emerges at $T_{SDW}$, which is followed by a unidirectional charge-density wave at $T_{DW}/T_{SDW}\sim0.9$ originated from the magnetic fluctuations inside the spin-density wave state. The charge-density wave is 2-3 orders of magnitude weaker than the spin-density wave and is, therefore, more easily suppressed by external fields or pressure.