Ultrafast Pair Distribution Function: Tracking Length Scale Dependent Transient Disorder Through a Phase Transition

Even for seemingly crystalline and well-ordered materials, functional properties can be highly sensitive to local disorder and structural features that are only correlated over nanoscale distances. The pair distribution function (PDF) method, a total scattering technique, has played an integral role in characterizing local structure in equilibrium studies. By Fourier transforming diffraction patterns to real space, subtle changes in diffuse scattering are converted to shifts and/or broadenings of PDF peaks directly corresponding to interatomic distances in the material. The development of X-ray free electron laser (XFEL) facilities provides high brilliance 100 fs X-ray pulses that could be used to apply the PDF method to picosecond structural dynamics. Representing a x10⁹ increase in temporal resolving power over a typical synchrotron experiment, this could be employed to understand the implicated role of disorder in non-equilibrium phase transitions.



In this talk, I will discuss our success at applying this ultra-fast PDF (uf-PDF) method in a feasibility study to track an optically pumped transition in low-temperature dimerized CuIr₂S₄ at the LCLS XFEL. Optical pumping is shown to destroy strong Ir-Ir dimers and drive the material to a new (hidden) transient state not accessible at equilibrium. This process is structurally uncorrelated above length scales of ~ 2 unit cells, with the resulting internal strain driving a recovery of longer-range order and an evolution in the average crystallographic structure over a ~40 ps timescale.