By Michael Leitner
Diffusion in solids at reasonable temperatures is a well known phenomenon. even though, direct experimental facts concerning the liable atomic-scale mechanisms has been scarce, as a result of problems in probing the suitable size- and time-scales. the current thesis bargains with the applying of X-ray Photon Correlation Spectroscopy (XPCS) for answering such questions. this is often a longtime approach for the examine of gradual dynamics on length-scales of some nanometres. The scattered depth within the diffuse regime, i.e. resembling atomic distances, is especially low, although, and so it has thus far been thought of most unlikely to exploit XPCS for this problem.
Threefold growth is pronounced during this paintings: It proposes a few structures chosen for top diffuse depth, it optimizes the photon detection and knowledge overview strategies, and it establishes theoretical types for interpretating the consequences. jointly those advances allowed the 1st winning atomic-scale XPCS test, which elucidated the position of most popular configurations for atomic jumps in a copper-gold alloy.
The progress in on hand coherent X-ray depth including next-generation X-ray resources will open up a large box of software for this new method.
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Extra info for Studying Atomic Dynamics with Coherent X-rays
Example text
1. Third, in actual experiments the incoming wave is not an ideal flat, monochromatic wave and the detector has a finite aperture, which can be pictured as if the sample is illuminated by a number of plane waves with no phase relation and therefore with no interference effects on accessible timescales between them. This leads to a partial washing-out of the interference pattern, which is treated in detail in Sect. 2. The net effect is just that the magnitude of the term in the measured auto-correlation function due to interference is diminished: g (2) (q, t) = 1 + β where β < 1 is the so-called coherence factor.
3 is bounded and that the relative contribution of V1234 to the integral in Eq. 3 becomes negligible when the size of the sample gets much bigger than the correlation length ξ. Second, in the above derivation the instantaneous intensity as the squared modulus of the amplitude, which itself is given by the Fourier transformation of the scatterer density, is the quantity of interest. However, in the experiment this instantaneous intensity is detected in quanta, and in X-ray physics (contrary to most optical measurements) this effect is not negligible, see Chap.
In Fig. 13 it can be seen that the particles move on atomic distances with a timescale of about 107 MCsteps, whereas the initial decay (not shown) happens at about 10 MCsteps. The simulations presented here are in this aspect the state of the art (the accessible timescales), see for example Berthier and Kob [1]. The other difference is the spatial scale: While simulations are restricted to a 48 4 Characteristics of Diffusion in Selected Systems few thousand particles, in reality systems are much larger.