Rotational symmetry breaking at the incommensurate charge-density-wave transition in Ba(Ni, Co)2(As, P)2: Possible nematic phase induced by charge/orbital fluctuations

Nematic phase transitions in high-temperature superconductors have a strong impact on the electronic properties of these systems. BaFe2As2, with an established nematic transition around 137 K induced by magnetic fluctuations, and BaNi2As2, a nonmagnetic analog of BaFe2As2 with a structural transition in the same temperature range, share a common tetragonal aristotype crystal structure with space-group type I4/mmm. In contrast to BaFe2As2 where collinear stripe magnetic order is found for the orthorhombic low-T phase, a unidirectional charge density wave together with (distorted) zigzag chains are observed for the triclinic low-T phase of BaNi2As2. Here we show that between the high-and low-T phases of Ba(Ni, Co)(2)(As, P)(2) an additional phase with broken fourfold symmetry and d(xz) orbital order exists which is a promising candidate for charge/orbital-fluctuation-induced nematicity. Moreover, our data suggest that the enhanced T-c found for Ba(Ni, Co)(2)(As, P)(2) for higher Co or P substitution levels might result from suppression of the (distorted) zigzag chains by reducing the contribution of the d(xy) orbitals.

Automated summary

Nematic phase transitions in high-temperature superconductors significantly affect their electronic properties. BaFe2As2 and BaNi2As2, sharing a common crystal structure, exhibit different characteristics in the nematic transitions. The presence of an additional phase with broken symmetry and orbital order in Ba(Ni, Co)(2)(As, P)(2) suggests a potential role of charge/orbital fluctuations in inducing nematicity.