Novel opportunities for sub-meV inelastic X-ray scattering at high-repetition rate self-seeded X-ray free-electron lasers

TL;DR

This paper proposes enhancements to inelastic X-ray scattering techniques using high-repetition rate free-electron lasers, aiming for sub-meV energy resolution and significantly increased flux for advanced condensed matter studies.

Contribution

It introduces optimized X-ray optics and increased spectral flux strategies to achieve sub-meV resolution in IXS at high-repetition rate XFELs, expanding capabilities for condensed matter research.

Findings

Achieves potential for 0.1 meV energy resolution in IXS.

Proposes a method to increase spectral flux by up to three orders of magnitude.

Demonstrates feasibility of high-flux, ultra-high-resolution IXS at XFEL facilities.

Abstract

Inelastic X-ray scattering (IXS) is an important tool for studies of equilibrium dynamics in condensed matter. A new spectrometer recently proposed for ultra-high-resolution IXS (UHRIX) has achieved 0.6~meV and 0.25~nm$^{-1}$ spectral and momentum transfer resolutions, respectively. However, further improvements down to 0.1~meV and 0.02~nm$^{-1}$ are required to close the gap in energy-momentum space between high and low frequency probes. We show that this goal can be achieved by further optimizing the X-ray optics and by increasing the spectral flux of the incident X-ray pulses. UHRIX performs best at energies from 5 to 10 keV, where a combination of self-seeding and undulator tapering at the SASE-2 beamline of the European XFEL promises up to a hundred-fold increase in average spectral flux compared with nominal SASE pulses at saturation, or three orders of magnitude more than possible with storage-ring based radiation sources. Wave-optics propagation shows that about $7\times 10^{12}$~ph/s in a $90$-$\mu$eV bandwidth can be achieved on the sample. This will provide unique new possibilities for dynamics studies by IXS.