Rotational symmetry breaking at the incommensurate charge-density-wave transition in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Ba</mml:mi><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>Ni</mml:mi><mml:mo>,</mml:mo><mml:mi>Co</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mo>(</mml:mo><mml:mi>As</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">P</mml:mi><mml:mo>)</mml:mo></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>: Possible nematic phase induced by charge/orbital fluctuations
Michael Merz, Liran Wang, Thomas Wolf, Peter Nagel, C. Meingast, S. Schuppler
Abstract
Nematic phase transitions in high-temperature superconductors have a strong impact on the electronic properties of these systems. $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, with an established nematic transition around 137 K induced by magnetic fluctuations, and $\mathrm{Ba}{\mathrm{Ni}}_{2}{\mathrm{As}}_{2}$, a nonmagnetic analog of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ 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 $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ 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 $\mathrm{Ba}{\mathrm{Ni}}_{2}{\mathrm{As}}_{2}$. Here we show that between the high- and low-$T$ phases of $\mathrm{Ba}{(\mathrm{Ni},\mathrm{Co})}_{2}{(\mathrm{As},\mathrm{P})}_{2}$ 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 ${T}_{\mathrm{c}}$ found for $\mathrm{Ba}{(\mathrm{Ni},\mathrm{Co})}_{2}{(\mathrm{As},\mathrm{P})}_{2}$ 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.