Polar charge and orbital order in 2H-TaS2

TL;DR

This paper investigates the charge and orbital order in 2H-TaS2, revealing a polar ordered state driven by displacement waves and phase shifts, distinct from the chiral order observed in similar materials.

Contribution

It demonstrates the emergence of a polar charge and orbital ordered state in 2H-TaS2, contrasting with the chiral order in related compounds, and highlights the role of lattice symmetry.

Findings

A polar charge and orbital order arises in 2H-TaS2.

The transition is driven by interactions between polarized displacement waves.

Bulk experiments are essential to distinguish between chiral and polar phases.

Abstract

It was recently discovered that in spite of the scalar nature of its order parameter, the charge order in 1T-TiSe2 can be chiral. This is made possible by the emergence of orbital order in conjunction with the charge density modulations. Here we show that a closely related charge and orbital ordered state arises in 2H-TaS2. In both materials, the microscopic mechanism driving the transition is based on the interaction between three differently polarized displacement waves. The relative phase shifts between these waves lead both to the formation of orbital order and to the breakdown of inversion symmetry. In contrast to 1T-TiSe2 however, the presence of a mirror plane in the lattice of 2H-TaS2 prevents the distorted structure in this material from being chiral, and a polar charge and orbital ordered state arises instead. It is stressed that bulk experiments are indispensable in differentiating between the chiral and polar phases.