Discovery of an Intermediate Nematic State in a Bilayer Kagome Metal ScV6Sn6

TL;DR

This paper reports the discovery of an intermediate nematic phase in the bilayer kagome metal ScV6Sn6, characterized by spontaneous in-plane anisotropy without time-reversal symmetry breaking, revealing complex nematic behavior between charge density wave transitions.

Contribution

It identifies a new nematic phase in ScV6Sn6, expanding understanding of symmetry-breaking phenomena in kagome metals and their interplay with charge density waves.

Findings

Intermediate nematic phase exists between T* and T_CDW.

The phase shows spontaneous in-plane anisotropy without time-reversal symmetry breaking.

Multiple nematic states are observed at different temperature scales.

Abstract

Nematicity, where rotational symmetry of the crystal lattice is spontaneously broken, is a ubiquitous phenomenon in correlated quantum matter, often intertwining with other orders to produce a richer spectrum of phases. Here we report a new phase transition in high-quality ScV6Sn6 bilayer kagome metal at a temperature T^*, occurring seven Kelvins below the charge density wave (CDW) transition at T_CDW, as indicated by thermodynamic, transport, and optical measurements. This emerging intermediate phase does not exhibit spontaneous time-reversal-symmetry breaking, as evidenced by zero-field Sagnac interferometer experiments. However, it displays a strong, spontaneous (strain- and field-free) anisotropy in the kagome plane between T^* and T_CDW, as revealed by transport and optical polarization rotation measurements. Additionally, a pronounced depolarization effect detected by the Sagnac interferometer further confirms its nematic nature. This intermediate nematic phase, alongside the recently discovered intra-unit cell nematic order at much lower temperatures, presents a diverse landscape of nematicities at multiple length and temperature scales, distinguishing it from those observed in kagome metals AV3Sb5. Our findings highlight ScV6Sn6 and the broader RM6X6 intermetallic family as fertile platforms for realizing symmetry-breaking phases driven by a unique interplay of competing CDW instabilities, kagome physics, and Van Hove singularities.