Hard X-rays as pump and probe of atomic motion in oxide glasses

Abstract

Nowadays powerful X-ray sources like synchrotrons and free-electron lasers are considered as ultimate tools for probing microscopic properties in materials. However, the correct interpretation of such experiments requires a good understanding on how the beam affects the properties of the sample, knowledge that is currently lacking for intense X-rays. Here we use X-ray photon correlation spectroscopy to probe static and dynamic properties of oxide and metallic glasses. We find that although the structure does not depend on the flux, strong fluxes do induce a non-trivial microscopic motion in oxide glasses, whereas no such dependence is found for metallic glasses. These results show that high fluxes can alter dynamical properties in hard materials, an effect that needs to be considered in the analysis of X-ray data but which also gives novel possibilities to study materials properties since the beam can not only be used to probe the dynamics but also to pump it.

Figure 1

Relaxation dynamics of the atoms as a function of the incident X-ray beam intensity. (a) Normalized intensity auto-correlation functions measured in vitreous silica at T = 295 K and wave-vector Q_p = 1.5 Å⁻¹ for different intensities of the flux of the X-ray beam. From left to right: F₀ ≈ 1 · 10¹¹ ph/s (red squares), F₁ ≈ 3 · 10¹⁰ ph/s (orange down-triangles), F₂ ≈ 1.2 · 10¹⁰ ph/s (cyan up-triangles) and F₃ ≈ 3.6 · 10⁹ ph/s (blue circles). Lines are fits with a Kohlrausch-Williams-Watts function. The corresponding decay times are reported in Fig. 2d while the shape parameter β is found to be ~1.4 ± 0.1, independent on the incoming flux. (b) Same data rescaled by the incoming flux. (c) Normalized intensity auto-correlation functions measured in Cu₆₅Zr_27.5Al_7.5 metallic glass at T = 413 K and Q_p = 2.5 Å⁻¹ for F₀ ≈ 1 · 10¹¹ ph/s (red squares), and F₁ ≈ 3 · 10¹ ph/s (orange down-triangles).

Figure 2

Tuning of the atomic motion. (a) Normalized intensity auto-correlation functions measured in vitreous silica at T = 295 K and Q_p = 1.5 Å⁻¹ with fixed lagtime per frame, Δt = 6.15 s, but with different sleeping times Δt_s and exposure times Δt_e. From left to right: Δt_e = 5 s and Δt_s = 0 s (red), Δt_e = 2.5 s and Δt_s = 2.5 s (orange), Δt_e = 0.5 s and Δt_s = 4.5 s (purple). The legend illustrates the acquisition mode per frame with full coloured boxes for the exposure times Δt_e (beam on), empty boxes both for the sleeping times Δt_s (beam off) and the constant readout time of the CCD Δt_r (beam off, grey boxes). (b) Same data as in panel (a) but now normalized by the mean flux <F> = F_0·Δt_e/Δt. (c) (g₂(t) − 1)/c measured with Δt_e = 0.5 s and Δt_s = 4.5 s (same purple data as in panel (a) reported as a function of the time t times the mean flux. The data are compared with the g₂(t) measured with the same exposure time Δt_e = 0.5 s and no sleeping time (Δt_s = 0 s, cyan). (d) Decay time for different combinations of the incident flux and the lagtime. The data are shown as a function of the inverse mean flux. Also included is data for a second SiO₂ sample (SiO₂ bis) and for vitreous GeO₂. The legend indicates the flux used for each set of measurements.

Figure 3

Instantaneous, reversible and stationary dynamics. (a) Two-time correlation function measured in vitreous silica at T = 295 K and Q_p = 1.5 Å⁻¹ by varying the intensity of the incoming flux. Left to right in frame number: F₀ ≈ 1 · 10¹¹ ph/s, F₁ ≈ 3 · 10¹⁰ ph/s, F₀ ≈ 1 · 10¹¹ ph/s, F₂ ≈ 1.2 · 10¹⁰ ph/s, and F₀ ≈ 1 · 10¹¹ ph/s. Each frame corresponds to Δt = 6.15 s. (b) Characteristic decay times τ as a function of the flux intensities used in panel (a). (c) Shape parameters β as a function of the flux intensities used in panel (a).

Figure 4

Wave-vector dependence of the X-ray induced dynamics. Wave-vector dependence of the characteristic decay time in vitreous silica measured at T = 295 K and for F₀ ≈ 1 · 10¹¹ ph/s (red circles). The grey triangles are taken with F₁ ≈ 3 · 10¹⁰ ph/s and rescaled by the factor 2.74 corresponding to the X-ray intensity difference between the two measurements.