Testing the Ginzburg-Landau approximation for three-flavor crystalline color superconductivity

It is an open challenge to analyze the crystalline color superconducting phases that may arise in cold dense, but not asymptotically dense, three-flavor quark matter. At present the only approximation within which it seems possible to compare the free energies of the myriad possible crystal structures is the Ginzburg-Landau approximation. Here, we test this approximation on a particularly simple "crystal" structure in which there are only two condensates $<us > \sim \Delta \exp(i {\bf q_2}\cdot {\bf r})$ and $<ud > \sim \Delta \exp(i {\bf q_3}\cdot {\bf r})$ whose position-space dependence is that of two plane waves with wave vectors ${\bf q_2}$ and ${\bf q_3}$ at arbitrary angles. For this case, we are able to solve the mean-field gap equation without making a Ginzburg-Landau approximation. We find that the Ginzburg-Landau approximation works in the $\Delta\to 0$ limit as expected, find that it correctly predicts that $\Delta$ decreases with increasing angle between ${\bf q_2}$ and ${\bf q_3}$ meaning that the phase with ${\bf q_2}\parallel {\bf q_3}$ has the lowest free energy, and find that the Ginzburg-Landau approximation is conservative in the sense that it underestimates $\Delta$ at all values of the angle between ${\bf q_2}$ and ${\bf q_3}$.