= Gauge-coupling sign in Ginzburg-Landau superconductivity
{title2=$D_\eta=\nabla-i\eta qA/\hbar$}
For <energy> with kinetic term $\hbar^2|D_\eta\psi|^2/(2m)$, $\eta=\pm1$, variation gives $-\hbar^2D_\eta^2\psi/(2m)+(\alpha+\beta|\psi|^2)\psi=0$ and current $j=\eta q\hbar\,\operatorname{Im}(\psi^*\nabla\psi)/m-q^2|\psi|^2A/m$. Thus changing the sign of the covariant <derivative> changes the phase-gradient term of the current. The displayed <energy>, field equation and current must use the same convention.
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