Competing itinerant and local spin interactions in kagome metal FeGe

Two-dimensional kagome metals with corner-sharing triangles provide a platform for exploring strong electron correlations and band topology due to their geometrically frustrated structure. In these systems, comparable energy scales among spin, lattice, and electronic dynamics lead to competing quantum phases such as charge density wave (CDW), magnetic order, and superconductivity. Kagome metal FeGe, for instance, shows various phases: A-type antiferromagnetic (AFM) order around 400 K, a CDW phase with AFM moment below 100 K, and a c-axis double cone AFM near 60 K. Using neutron scattering, we detected gapless incommensurate spin excitations related to the double cone AFM at temperatures surpassing the mentioned phase transitions. Commensurate spin waves adhere to the Bose population factor, while incommensurate ones show deviation, peaking at T_Canting, indicative of a second-order magnetic phase transition. Density functional theory calculations suggest the incommensurate structure originates from the nested Fermi surfaces and spin density wave order formation. The temperature variation of these excitations hints at a connection between spin density wave and CDW order, possibly because of flat electronic bands near the Fermi level.