Low temperature phase transitions inside the CDW phase in the kagome metals AV3Sb5 (A = Cs, Rb, K): Significance of mixed-type Fermi surface electron correlations

To understand the multistage phase transitions in V -based kagome metals inside the charge -density -wave (CDW) phase, we focus on the impact of the "mixed -type" Fermi surface because it is intact in the CDW state on the "pure -type" Fermi surface. On the mixed -type Fermi surface, moderate spin correlations develop, and we reveal that uniform (q = 0) bond order is caused by the paramagnon interference mechanism, which is described by the Aslamazov-Larkin vertex correction. A dominant solution is E2g-symmetry nematic order, in which the director can be rotated arbitrarily. In addition, we obtain A1g-symmetry order, which leads to the change in the lattice constants without symmetry breaking. The predicted E2g and A1g channel fluctuations at q = 0 can be observed by the elastoresistance measurements. These results are useful to understand the multistage phase transitions inside the 2 x 2 CDW phase. The present theory has a general significance because mixed -type Fermi surfaces (with multiorbital van Hove singularities) exist in various kagome lattice systems.