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
Abstract
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 non-magnetic 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) zig-zag 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 dxz 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) zig-zag chains by reducing the contribution of the dxy orbitals.