Femtosecond X-ray induced changes of the electronic and magnetic response of solids from electron redistribution

Abstract

Resonant X-ray absorption, where an X-ray photon excites a core electron into an unoccupied valence state, is an essential process in many standard X-ray spectroscopies. With increasing X-ray intensity, the X-ray absorption strength is expected to become nonlinear. Here, we report the onset of such a nonlinearity in the resonant X-ray absorption of magnetic Co/Pd multilayers near the Co L edge. The nonlinearity is directly observed through the change of the absorption spectrum, which is modified in less than 40 fs within 2 eV of its threshold. This is interpreted as a redistribution of valence electrons near the Fermi level. For our magnetic sample this also involves mixing of majority and minority spins, due to sample demagnetization. Our findings reveal that nonlinear X-ray responses of materials may already occur at relatively low intensities, where the macroscopic sample is not destroyed, providing insight into ultrafast charge and spin dynamics.

Ultrafast Demagnetization Dominates Fluence Dependence of Magnetic Scattering at Co M Edges

“…Likewise, for pulse durations on the order of the demagnetization time, a significant contribution of ultrafast demagnetization to the fluence dependence of scattering should be expected also at the L edges, as shown in Ref. [10].…”

“…In line with our reasoning, this experiment has shown a stronger effect for the shorter pulse lengths, corroborating the dominantly coherent nature of the XMCD-SAS fluence dependence at the Co L 3 edge. Recently, Higley et al have shown that ultrafast demagnetization plays a nonnegligible role at the Co L 3 edge when monitoring x-ray absorption using 40 fs x-ray pulses [10]. This illustrates the competition of different mechanisms in the quenching of XMCD-based signals as a function of fluence.…”

“…X-ray-induced ultrafast demagnetization was found to be of relevance only for pulse durations significantly longer than 100 fs, given moderate fluences of up to 30 mJ=cm 2 [5]. Recently, this mechanism gained renewed attention, even for x-ray pulse durations of only 40 fs, i.e., significantly below the typical demagnetization time constants of around 100-300 fs [10]. In the case of the Co M 3;2 edges, an almost complete loss of resonant scattering signal was reported for a fluence of 5000 mJ=cm 2 in a 100 fs pulse.…”

HIV-1 Entry and Prospects for Protecting against Infection

“…However, for those bnAbs that have been recovered by in vitro cultures, many have been found to be IgG1. A notable exception are antibodies targeting the MPER region, which have been isolated predominantly as IgG3 [ 154 , 155 ]. The flexibility of IgG3′s long hinge region has been suggested to favor access to the MPER epitope in very close proximity to the membrane.…”

Bi-cross validation of spectral clustering hyperparameters

“…X-ray free electron lasers (X-FELs) (Ishikawa et al, 2012) are remarkable instruments capable of producing highly coherent X-ray pulses less than 20 fs in duration. Since their inception, X-FELs have made contributions to a diverse range of disciplines spanning from condensed matter (Higley et al, 2019) and atomic molecular optics (Yang et al, 2018) to structural biology (Nogly et al, 2018) and femtosecond chemistry (Hong et al, 2015). Compared to thirdgeneration light sources, X-FELs require high-throughput data systems (Thayer et al, 2016) for writing to the disk on a per-pulse basis.…”