Polar charge and orbital order in 2H-TaS2

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.