Core Competency: Condensed Matter Physics
Most condensed matter exhibits complex dynamics. Viscoelastic flow, polymer reptation (creeping), protein folding, crystalline phase transitions and domain switching, and countless other collective processes show both fast and slow responses. These processes are often sensitive functions of temperature, applied fields, and fabrication parameters. These complex dynamics may be crucial for material function, as in synthesized structures such as gels and high-impact polymers and in natural structures such as proteins, whose folding dynamics involves a hierarchy of time scales. Complex dynamics also may be deleterious, as in ferroelectric DRAMS whose slow response components limit their practical switching times. Using time- or energy-resolved light-scattering techniques, we can currently probe the full range of these time scales, from femtoseconds through kiloseconds. Present-day measurements with visible and near-visible wavelengths probe responses on the distance scale of the wavelength or longer, that is, at least 0.1 microns and generally much longer. But in almost all materials, intermolecular interactions are short-range (angstroms) and correlations in quantities such as polymer or ferroelectric alignment, motions of polymer or protein subunits, or liquid state molecular orientational alignment, extend over nanometers, not tenths of microns or longer. How do systems made out of angstrom-size molecules that only interact with neighbors nanometers away coordinate and organize their macroscopic (micron and larger distance scale) responses? In many cases, we don’t know. For example, do the essentially universal polymer responses that we observe on a hierarchy of time scales (assumed to be present in protein folding and other biopolymer behavior as well) reflect the responses on a corresponding hierarchy of length scales — fast motions of small segments, slower motions of larger structural elements, even slower motions of whole polymer chains? If so, why do small-molecule viscoelastic liquids (including honey and molasses) show almost identical dynamical responses, with the same temperature-dependent trends, even though there are only microscopic-sized units? Our measurements reveal macroscopic dynamics, but not the mesoscopic (nanoscale) dynamics that give rise to them. Our theoretical and computational methods are inadequate for description of many-body complex dynamics, including complex molecules and structures and including the slow, as well as fast, time scales.The Linac Coherent Light Source (LCLS) provides a unique opportunity to observe nanoscale dynamics in condensed matter systems over a wide range of time scales using x-ray photon correlation spectroscopy (XPCS) and x-ray transient grating spectroscopy (XTGS). The LCLS is uniquely suited to experiments in the following areas:
The common theme in these examples is the interplay of multiple length and time scales in the dynamics. While the time scales of interest vary greatly (10-13 to 103 s), in all three cases the length scales are in the atomic to nanoscopic range (10-1 to 103 nm). Three types of experimental techniques are envisioned: a) Using the very high time-averaged coherent x-ray flux from the LCLS to carry out XPCS measurements over time scales from 10-3 to 103 seconds; b) Using the extremely high-peak coherent x-ray flux from the LCLS to carry out XPCS measurements using a split-pulse technique over time scales from 10-12 to 10-6 seconds; c) Using the short pulse width from the LCLS to carry out XTGS measurements of stimulated dynamics over time scales from 10-12 to 10-6 seconds.