Unidirectional Diagonal Order and 3D Stacking of Charge Stripes in Orthorhombic Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4

TL;DR

This study reveals how crystal symmetry influences the direction of charge stripes in certain nickelate materials, showing unidirectional stripe order and a specific stacking pattern that melts below the charge ordering temperature.

Contribution

It demonstrates the direct link between orthorhombic crystal symmetry and unidirectional charge stripe order in Pr and Nd nickelates, including the observation of a 3-layer stacking order.

Findings

Charge stripes run along the short a-axis in orthorhombic crystals.

Charge ordering temperature is approximately 240K across different compounds.

A 3-layer stacking order of charge stripes exists and melts about 40K below T_CO.

Abstract

The interplay between crystal symmetry and charge stripe order in Pr1.67Sr0.33NiO4 and Nd1.67Sr0.33NiO4 has been studied by means of single crystal x-ray diffraction. In contrast to tetragonal La1.67Sr0.33NiO4, these crystals are orthorhombic. The corresponding distortion of the NiO2 planes is found to dictate the direction of the charge stripes, similar to the case of diagonal spin stripes in the insulating phase of La2-xSrxCuO4. In particular, diagonal stripes seem to always run along the short a-axis, which is the direction of the octahedral tilt axis. In contrast, no influence of the crystal symmetry on the charge stripe ordering temperature itself was observed, with T_CO 240K for La, Pr, and Nd. The coupling between lattice and stripe degrees of freedom allows one to produce macroscopic samples with unidirectional stripe order. In samples with stoichiometric oxygen content and a hole concentration of exactly 1/3, charge stripes exhibit a staggered stacking order with a period of three NiO2 layers, previously only observed with electron microscopy in domains of mesoscopic dimensions. Remarkably, this stacking order starts to melt about 40K below T_CO. The melting process can be described by mixing the ground state, which has a 3-layer stacking period, with an increasing volume fraction with a 2-layer stacking period.