Towards novel probes for valence charges via X-ray optical wave mixing

TL;DR

This paper explores X-ray optical wave mixing, combining theory and experiments to probe valence charge dynamics, but finds no evidence of the nonlinear effect previously claimed, highlighting challenges and future potential.

Contribution

It provides the first combined theoretical and experimental analysis of X-ray parametric down-conversion for probing valence charges, challenging prior claims and outlining future research directions.

Findings

No evidence of the nonlinear effect was observed.

Simulated scattering patterns reveal characteristic signatures.

The study challenges previous proof-of-principle claims.

Abstract

We present a combined theoretical and experimental study of X-ray optical wave mixing. This class of nonlinear phenomena combines the strengths of spectroscopic techniques from the optical domain, with the high-resolution capabilities of X-rays. In particular, the spectroscopic sensitivity of these phenomena can be exploited to selectively probe valence dynamics. Specifically, we focus on the effect of X-ray parametric down-conversion. We present a theoretical description of the process, from which we deduce the observable nonlinear response of valence charges. Subsequently, we simulate scattering patterns for realistic conditions and identify characteristic signatures of the nonlinear conversion. For the observation of this signature, we present a dedicated experimental setup and results of a detailed investigation. However, we do not find evidence of the nonlinear effect. This finding stands in strong contradiction to previous claims of proof-of-principle demonstrations. Nevertheless, we are optimistic to employ related X-ray optical wave mixing processes on the basis of the methods presented here for probing valence dynamics in the future.