Studying structural heterogeneities in quantum materials with x-ray diffuse scattering: Stacking faults in RuCl₃, and charge density waves in La_2-xBa_xCuO₄

The properties of quantum materials are well-known to depend on both the idealized unit cell structure and structural distortions and disorder over different length scales. Modern synchrotron x-ray techniques combine high-energy x-rays, advanced area detectors, and high-performance computational pipelines to accurately map diffuse scattering over large regions of reciprocal space. This scattering encodes information about the deviations of the real structure from the idealized crystal lattice with very high precision. In this talk I will outline the application of this high precision x-ray diffuse scattering to two canonical quantum materials.

In the layered Kitaev magnet alpha-RuCl₃, stacking faults give rise to characteristic rods of diffuse scattering perpendicular to the plane of the 2D layers. We present advanced single crystal three dimensional difference pair distribution function (3D-deltaPDF) analysis, allowing us construct a quantitative model of the stacking faults in the material. Including this disorder within realistic theoretical models of RuCl₃ is likely to be key to understanding the observed variation in the electronic properties of different RuCl₃ crystals.

We also present an analysis of the structural response to the famous charge density wave (CDW) phase in La_1.875Ba_0.125CuO₄, based on our data set covering several thousand CDW peak positions. Our model shows that the CDW leads to longitudinal modulation of the atomic positions within the copper planes, as well as inducing a ‘breathing’ type modulation of the surrounding La/Ba layers. Despite the similarity of the data collection in these two projects, the analysis technique and the underlying physics are very different, highlighting the wealth of information that may be found in diffuse x-ray scattering.