= D-wave bond order
{title2=$\Delta_x=-\Delta_y,\quad\Delta_{\mathbf k}\propto\cos(k_xa)-\cos(k_ya)$}
For nearest-neighbor spin-singlet pairing on a square lattice, opposite signs of the pairing amplitude on the two bond directions give the displayed momentum-dependent <superconducting gap function>. It changes sign under a quarter-turn and has nodes on the zone diagonals. This sign structure is distinct from a nonnegative gap whose magnitude is merely anisotropic. A real pure $d_{x^2-y^2}$ pattern does not by itself require broken time reversal.
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