Emergent Tetragonality in a Fundamentally Orthorhombic Material

TL;DR

This study reveals an unexpected emergence of tetragonal symmetry in an orthorhombic material near a charge density wave bicritical point, driven by electronic effects despite the crystal structure remaining orthorhombic.

Contribution

It demonstrates that electronic properties can exhibit emergent tetragonality near a phase transition without structural symmetry change in an orthorhombic material.

Findings

Emergent tetragonality observed near the bicritical point.

Divergence in in-plane elastoresistivity anisotropy.

Structural symmetry remains orthorhombic under strain.

Abstract

Symmetry plays a key role in determining the physical properties of materials. By Neumann's principle, the properties of a material are invariant under the symmetry operations of the space group to which the material belongs. Continuous phase transitions are associated with a spontaneous reduction in symmetry. (For example, the onset of ferromagnetism spontaneously breaks time reversal symmetry.) Much less common are examples where proximity to a continuous phase transition leads to an increase in symmetry. Here, we find an emergent tetragonal symmetry close to an apparent charge density wave (CDW) bicritical point in a fundamentally orthorhombic material, ErTe$_3$, for which the CDW phase transitions are tuned via anisotropic strain. The underlying structure of the material remains orthorhombic for all applied strains, including at the bicritical point, due to a glide plane symmetry in the crystal structure. Nevertheless, the observation of a divergence in the anisotropy of the in-plane elastoresistivity reveals an emergent electronic tetragonality near the bicritical point.