Ferroaxial and nematic transitions in the charge density wave phase of 1T-TiSe$_2$

TL;DR

This study clarifies the symmetry-breaking nature of the charge density wave in 1T-TiSe2, revealing a ferroaxial order with nematic features, resolving previous conflicting reports about its chiral character.

Contribution

The paper identifies the bulk broken symmetry in 1T-TiSe2 as ferroaxial, using symmetry-resolved elastoresistivity and elastocaloric measurements to distinguish it from chiral CDW states.

Findings

Ferroaxial order breaks vertical mirror symmetries but preserves inversion.

Detection of antisymmetric elastoresistivity coefficients indicating a toroidal moment.

Diverging nematic susceptibility signals rotational symmetry breaking inside the ferroaxial CDW state.

Abstract

Charge density waves (CDWs) with multi-component order parameters can break unexpected symmetries through the interplay of nearly degenerate instabilities. In the widely investigated material 1T-TiSe$_2$, a central question is whether the observed CDW has a chiral character, which would manifest as the spontaneous breaking of mirror and inversion symmetries. Previous experiments have reported conflicting results about the broken symmetries in the CDW phase of 1T-TiSe$_2$. Here, we resolve this controversy by identifying the bulk broken symmetry as ferroaxial, corresponding to the breaking of vertical mirrors while preserving inversion symmetry. Using symmetry-resolved elastoresistivity, we detect the spontaneous emergence of intrinsic off-diagonal elastoresistivity coefficients that satisfy an antisymmetric relation ($m_{xx-yy,xy} \approx -m_{xy,xx-yy}$), providing an unambiguous bulk transport signature of a macroscopic electric toroidal moment. Simultaneous elastocaloric measurements reveal that the onset of ferroaxial order occurs just below the CDW transition. As the temperature is lowered further, a diverging nematic susceptibility signals a distinct rotational symmetry-breaking instability inside the ferroaxial CDW state. Our findings demonstrate that the proposed ``chiral'' CDW in 1T-TiSe$_2$ is actually a centrosymmetric ferroaxial state, reconciling previous surface-sensitive observations with bulk symmetry constraints.