 D-wave superconductivity

Pairing with d-wave orbital structure, such as a crystal gap proportional to $k_x^2-k_y^2$. Its sign changes under a quarter-turn rotation, but a compensating overall phase change can leave gauge-invariant observables fourfold symmetric. Gap nodes and phase signs therefore do not by themselves establish nematicity or time-reversal breaking. A phase-sensitive test of d-wave pairing probes relative signs through interference.