Umklapp-driven first-order transition into a charge-density wave phase

Abstract: Using phenomenological Landau free energy, we systematically analyze influence of so-called umklapp contributions on the phase diagram of a solid hosting a commensurate charge-density wave phase. The umklapp terms may change the transition into the ordered state from continuous (second-order) to discontinuous (first-order). Additional elements, such as critical and tricritical points, emerge on the phase diagram under suitable conditions. The proposed mechanism is generalized for the case of nearly-commensurate charge-density wave, as well as spin-density wave orders. Several charge-density-wave-hosting alloys are used as experimentally available examples illustrating the formalism.