  We study theoretically the superconductor diodes, where the magnitude of the
critical current changes as the direction is reversed, in terms of a
generalized Ginzburg-Landau model with the higher-order terms in the momentum
of the order parameter. This theory is applied to Rashba spin-orbit coupled
systems, where analytical relations between the nonreciprocal critical currents
and the system parameters are achieved. Numerical calculations with mean-field
theory are also obtained to study broader parameter regions. These results
offer a rather general description and design principles of superconductor
diodes.
