Can Coherent Pulses Reveal the Low Energy Charge Modes in Interacting Electron Systems
Peter Abbamonte
University of Illinois at Urbana-Champaign
The canonical property of an interacting electron system is competition among several ground states, i.e. the close proximity of multiple quantum phase boundaries. The signature of a nearby phase boundary is the presence of low energy (~ 10 meV) electronic modes that exhibit symmetry characteristics of the nearby phase, the best known example being the “stripe” excitations in high temperature superconductors. A complete understanding of an interacting system, in a sense, consists of devising a description of these low energy modes.
Charge excitations are manifested in the dynamic structure factor, S(k,w), which can easily be determined from a theory by evaluation of a charge correlation function. One might think, then, that much could be learned about interacting systems by measuring S(k,w) with meV inelastic x-ray scattering. Unfortunately, however, very few of the electrons in a real material participate in these modes; most are tied up in irrelevant, high energy excitations (i.e. plasmons). Further, current analyzer-based meV instruments provide extremely low detection efficiency. So, sadly, the most important piece of information in the science of correlated electron systems still remains hidden.
In this talk I will describe a possible alternative approach to IXS using time-domain, phase-space mapping of scattered, coherent x-ray pulses. This method makes use of the fact that IXS, in contrast to diffraction processes, is incoherent, i.e. violates Liouville’s theorem, i.e. broadens phase space area, which can be detected with interferometry techniques. The advantage of this method is that the entire scattered pulse can be used simultaneously, so could be much more efficient than frequency-domain IXS. In addition, the effective energy resolution is equal to the inverse of the repetition rate of the source so can be extremely high. I will discuss some possible experimental implementations based on frequency-resolved, optical gating (FROG) methods, and the relative merit of FEL vs. ERL based sources.
