Successive orthorhombic distortions in Kagome metals

A hallmark of correlated electron systems is an ordering of charge, orbital, and spin degrees of freedom, as well as their strong interactions. Kagome metals serve as a representative example, especially highlighted by the recent discovery of 3Q charge density wave (CDW) and loop-current orders. Since CDWs are highly sensitive to details of the Fermi surface, various magnetic or structural instabilities compete in high-symmetry settings such as in hexagonal Kagome metals.

Here, we present our structural study on Ruthenium-based Kagome metals, including the first successful growth of single crystals. In synchrotron X-ray diffraction, we observe that the parent hexagonal structure transitions into a primitive orthorhombic structure via a critical regime of a short-range ordered state. This behavior is consistent with structural modeling—our optical birefringence measurements further evidence the orthorhombic distortion. As a consequence, this class of Kagome metals exhibits non-collinear and non-coplanar spin textures at low temperatures due to staggered single-ion anisotropy, likely arising from quadrupolar order. Our work highlights versatile structural instabilities in Kagome metals, offering a new platform to explore the intertwined evolution of lattice, orbital, and spin degrees of freedom.