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THE PHASE DIAGRAM FOR THE SINE-GORDON MODEL WITH TWO UMKLAPP TERMS

We study the Landau free energy for a uniaxial ordering, taking into account two Umklapp terms of comparable strengths (those of the third and fourth order). Exploring the analogy with the well-known nonintegrable classical mechanical problem of two mixed nonlinear resonances, we complete the previous studies of the corresponding phase portrait by calculating numerically periodic solutions, including those far from the separatrices. It is shown that in the physical range of parameters only periodic configurations are absolutely stable. We determine for the first time the complete thermodynamic phase diagram and show that, in contrast to some earlier claims, the wave number of the ordering does not pass through the devil's staircase, but through a finite number of steps which decreases as the amplitudes of the Umklapp terms increase.