Electronic and magnetic origins of unconventional charge density wave in kagome FeGe

Recently, the kagome system of FeGe has emerged as a promising platform for investigating the interplay between magnetism and charge density wave (CDW) phenomena. However, unlike other CDW materials which exhibit soft phonons, the pristine phase of FeGe is found to be dynamically stable, different from the conventional CDW scenario of Fermi surface nesting or van Hove singularity. It is thus critical to understand (i) what are the driving forces of the CDW transition in FeGe? and (ii) how is the magnetism associated with such peculiar CDW transition? Here, supported by density functional theory and tight-binding models, we unravel the triple-well CDW energy landscape of FeGe driven by the downward shift of an entire Ge band, instead of the previously proposed Fe bands. The CDW phase is found to result from a cooperation between the Fermi level pinning phenomenon and the Peierls-like effect, which is distinct from the conventional Peierls effect that drives a double-well transition. Such electronic origin is consistent with experimental observations, using scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES), where the electronic structures near the Fermi level barely change across the CDW transition. Moreover, we demonstrate that the antiferromagnetic coupling, which weakens the Fe-Ge hybridization by exchange splitting and lowers the position of Ge-bands with respect to the Fermi level, plays a more critical role than changes in local moments in driving the CDW transition. Our work thus reveals unconventional CDW mechanism for FeGe and related systems.