Observing nematic fluctuations in a spin-orbit coupled system using x-ray photon correlation spectroscopy

A seminal coherent x-ray study of nematic fluctuations in a quantum material (QM) characterized by strong spin-orbit couplings is presented. This study was made possible by the brightness of the upgraded Advanced Photon Source (APS). Nematic order is emerging as a fundamental unifying tenet across a broad range of QMs in which electrons acquire a long-range orientational order to break a rotational symmetry as prerequisite for novel charge and/or spin ordered ground states. The main goal of this experiment was to study the dynamics in a one 5d-electron system, BNOO (Ba₂NaOsO₆), associated with nematic fluctuations which manifest as orthorhombic distortions, a typical scenario observed in many QMs. Using x-ray photon correlation spectroscopy, nematic fluctuations were observed in the vicinity of Bragg peaks well above a nematic transition which slowed down as this transition was crossed only to speed up again as BNOO entered a lower-T novel ground state. These results set a benchmark for future studies of mesoscale quantum fluctuations at modern x-ray sources with enhanced coherence.