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

TL;DR

This paper uses Landau theory to analyze how umklapp interactions can induce a first-order transition into charge-density wave phases, revealing complex phase diagram features like tricritical points.

Contribution

It introduces a systematic phenomenological framework for understanding how umklapp effects influence phase transitions in charge-density wave systems, including nearly-commensurate and spin-density wave orders.

Findings

Umklapp terms can turn second-order transitions into first-order.

Phase diagrams may feature critical and tricritical points.

Application to real alloys demonstrates the formalism's relevance.

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.