Ultrafast x-rays reveal mesoscopic fluctuations and topological defects in materials driven out of equilibrium

Abstract: Excitation of materials with ultrafast light pulses can induce novel states of matter not accessible in thermodynamic equilibrium. However, general theoretical arguments suggest that fast non-adiabatic changes in materials should induce topological defects leading to non-ergodic and glass-like behavior. Visualizing the dynamics of these transient states at the nanoscale can shed light on their return to equilibrium and metastability of competing orders in systems with intertwined phases and further inspire rational ways to control materials’ functionality.

In this talk I will show two examples where these transient nonequilibrium states are characterized by spatial fluctuations and manifest strong x-ray diffuse scattering. In the first part, I will show results on the ultrafast dynamics of the charge density waves (CDW) of LaTe3 from the SwissFEL facility. A sophisticated analysis of the intensity reveals self-similarity and exponents that evidence photoinduced topological defects, or dislocations of the incommensurate CDW. These defects behave sub-diffusively and are responsible for the slow equilibration and glassy behavior of the CDW. In the second part of the presentation, I will describe recent ultrafast x-ray experiments at the Linac Coherent Light Source (LCLS) facility on the quantum paraelectric phase of SrTiO3 under strong-field THz excitation. We observe strong absorption of THz radiation by transverse optical and acoustic phonons with wavelengths in the 10 nm range, indicating that these modes are strongly polar and modulate the ferroelectric polarization spatially at this lengthscale. We further observe softening in the transverse acoustic branch with cooling, which suggests the proximity to a hidden phase of SrTiO3 where the polarization is spatially-modulated at the nanoscale and is hidden to other kinds of probes without wavevector selectivity. These results highlight the importance of nanoscale fluctuations in materials out of equilibrium and the power of XFELs to probe their properties.

Bio: Mariano Trigo is a Staff Scientist in the Stanford Institute for Materials and Energy Sciences and PULSE Institute at SLAC National Accelerator Laboratory. He received his PhD in Applied Physics from The University of Michigan and joined SLAC in 2010 after a postdoc at Stanford University. He uses x-ray free electron lasers and ultrafast pulses to study quantum materials. At SLAC he has developed new techniques to study the fluctuations and collective modes of materials as well as their interactions using x-ray free electron lasers.