Symmetry breaking and ascending in the magnetic kagome metal FeGe

Abstract

Spontaneous symmetry breaking-the phenomenon where an infinitesimal perturbation can cause the system to break the underlying symmetry-is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. In the paramagnetic state at 460K, we confirm that the crystal structure is indeed hexagonal kagome lattice. On cooling to TN, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of 10^(-4) and the associated splitting of the double degenerate phonon mode of the pristine kagome lattice. Upon further cooling to TCDW, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes more symmetric gradually upon further cooling. Increasing the crystalline symmetry upon cooling is unusual, it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter, hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system and sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.

20 pages with 10 figures, replaced with journal version

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Cited by in corpus (14)

• Intriguing Low-Temperature Phase in the Antiferromagnetic Kagome Metal FeGe
• Spin-charge-lattice coupling across the charge density wave transition in a Kagome lattice antiferromagnet
• FeGe as a building block for the kagome 1:1, 1:6:6, and 1:3:5 families: hidden d-orbital decoupling of flat band sectors, effective models and interaction Hamiltonians
• Kagome metals
• Charge Dynamics of an Unconventional Three-Dimensional Charge Density Wave in Kagome FeGe
• Electronic band structures of topological kagome materials
• Disordered charge density waves in the kagome metal FeGe
• Emergent properties and the multiscale characterization challenge in condensed matter, from crystals to complex materials: a Review
• Lattice dynamics study of electron-correlation-induced charge density wave in antiferromagnetic kagome metal FeGe
• MPd5 kagome superconductors studied by density functional calculations
• Competing Lattice Instability and Magnetism on the Surface of Kagome Metals
• Nonlinear optical response in kagome lattice with inversion symmetry breaking
• Interacting spin and charge density waves in kagome metal FeGe
• Electronic and structural properties of Rh- and Pd-based kagome layered shandites from first principles