Real Space Structural Correlations in Quantum Materials*

The exotic electrons states found in quantum materials typically involve interactions among orbital, charge, or spin degrees of freedom, all of which can be coupled to structure in important ways. This coupling does not need to change the long-range crystal structure, and short-range structure is sometimes a necessary part of the description of crystalline materials. While the long-range crystal structure produces sharp Bragg peaks from x-ray scattering, local correlations will produce broader diffuse scattering between or underneath the Bragg peaks. Measured from single crystals, this diffuse scattering can be analyzed to understand local order in a range of materials, including ionic conductors and solid solutions, and it can also provide insight into correlated electron systems where short-range structures contribute to interesting electronic properties. I will discuss how recent advances in neutron and x-ray instrumentation have significantly improved the collection of diffuse scattering data, enabling novel modes of analysis such as 3D-ΔPDF [1].

The insulator-metal transition in (V,Mo)O2 provides an example of how short-range structural order can produce a bulk electronic effect [2]. For VO2, a structural phase transition is accompanied by a sharp change in resistivity, but at 19% Mo concentration, the electronic phase transition occurs without a change in long-range crystal structure. X-ray scattering experiments show that there is, however, a change in local structure, clearly indicated by weak but well-defined rods of diffuse scattering. These rods indicate well-defined local correlations in two dimensions that are poorly defined along the third direction. Analysis of the diffuse scattering, including both 3D-ΔPDF methods and simulations, reveals a frustrated short-range structure embedded within the long-range crystal structure that accounts for the bulk insulator-metal transition.

[1] Krogstad et al, Nat. Mat. 19, 63-68 (2020).

[2] Davenport et al, Phys. Rev. Lett. 127, 125501 (2021).