Electronic origin of x-ray absorption peak shifts

TL;DR

This paper uses advanced first-principles simulations to explain how ultrafast laser pulses cause energy shifts in x-ray absorption and magnetic circular dichroism peaks in Ni and Co, revealing electronic origins of these shifts.

Contribution

It demonstrates that peak shifts in transient XAS and MCD are due to laser-induced changes in band occupation, providing a new electronic interpretation for ultrafast demagnetisation phenomena.

Findings

Ni shows energy shift in XAS peaks but not in MCD.

Co exhibits a shift in MCD peaks with minimal XAS change.

The effect depends on pump pulse parameters and band filling differences.

Abstract

Encoded in the transient x-ray absorption (XAS) and magnetic circular (MCD) response functions resides a wealth of information of the microscopic processes of ultrafast demagnetisation. Employing state of the art first principles dynamical simulations we show that the experimentally observed energy shift of the L3 XAS peak in Ni, and the absence of a corresponding shift in the dichroic MCD response, can be explained in terms of laser induced changes in band occupation. Strikingly, we predict that for the same ultrashort pump pulse applied to Co the opposite effect will occur: a substantial shift upwards in energy of the MCD peaks will be accompanied by very small change in the position of XAS peaks, a fact we relate to the reduced $d$-band filling of Co that allows a greater energetic range above the Fermi energy into which charge can be excited. We also carefully elucidate the dependence of this effect on pump pulse parameters. These findings (i) establish a electronic origin for early time peak shifts in transient XAS and MCD spectroscopy and (ii) illustrate the rich information that may be extracted from transient response functions of the underlying dynamical system.