时间：7月27日（周四） 10:00—11:00

地点：勤园20-518

Two-dimensional kagome metals consisting of corner-sharing triangles offer a unique platform for studying strong electron correlations and band topology due to its geometrically frustrated lattice structure. The similar energy scales between spin, lattice, and electronic degrees of freedom in these systems give rise to competing quantum phases such as charge density wave (CDW), magnetic order, and superconductivity. For example, kagome metal FeGe first exhibits A-type collinear antiferromagnetic (AFM) order at T_N = 400 K, then establishes a CDW phase coupled with AFM ordered moment below T_CDW =100 K, and finally forms a c-axis double cone AFM structure around T_Canting = 60 K. Here we use neutron scattering to demonstrate the presence of gapless incommensurate spin excitations associated with the double cone AFM structure at temperatures well above T_Canting and T_CDW that merge into gapped commensurate spin waves from the A-type AFM order. While commensurate spin waves follow the Bose population factor and can be well described by a local moment Heisenberg Hamiltonian, the incommensurate spin excitations first appear below T_N where AFM order is commensurate, start to deviate from the Bose population factor around T_CDW, and peaks at T_Canting, consistent with a critical scattering of a second order magnetic phase transition, as a function of decreasing temperature. By comparing these results with density functional theory calculations, we conclude that the incommensurate magnetic structure arises from the nested Fermi surfaces of itinerant electrons and the formation of a spin density wave order. The temperature dependence of the incommensurate spin excitations suggest a coupling between spin density wave and CDW order, likely due to flat electronic bands near the Fermi level around T_N and associated electron correlation effects.

Brief BIO of Pengcheng Dai

Pengcheng Dai obtained his B.S. in 1984 from Zhengzhou University and his Ph. D in 1993 from University of Missouri. He did 3 years postdoc at Oak Ridge National Laboratory up graduation, and became a staff member at Oak Ridge National Laboratory from 1996 till 2001. In 2001, he moved to The University of Tennessee as an associated professor, became full professor in 2006. In 2008, he was promoted to Tennessee Advanced Materials Laboratory chair professor. In 2013, he moved from The University of Tennessee to Rice University, and is currently Sam and Helen Worden professor of physics at Rice. His group has focused on using neutron scattering as a probe to study correlated electron materials over the past two decade. He is a APS, AAAS, and NSSA fellow, and won sustained prize of neutron scattering society of America in 2016 and The Heike Kamerlingh Onnes Prize in 2022. He is currently a divisional associate editor of PRL. He published over 300 papers with google citation of > 23600.