**Fig. 1. **(a) Schematic view of X-ray scattering. The wavevectors of the incident and scattered X-rays are ${\overrightarrow{k}}_{i}$ and ${\overrightarrow{k}}_{f}$, respectively. In most cases, elastic scattering occurs while keeping the magnitude of the scattering wavevector equal to that of the incidence wave vector, as shown in the inset figure. Momentum transfer of X-rays is defined as $\overrightarrow{Q}$ = ${\overrightarrow{k}}_{f}$ − ${\overrightarrow{k}}_{i}$ , and if two wavevectors exist in the x-z plane (scattering plane), $\overrightarrow{Q}$ = $2{\overrightarrow{k}}_{z}$. Actually, $\overrightarrow{Q}$ is the wavevector transfer since the momentum of photon is defined as $\hslash \overrightarrow{k}$, however we can ignore the constant $\hslash $ and consider the wavevector transfer as the momentum transfer (as a physical quantity). (b) Scattering angle related to correlated lengths of scatterers such as nano-structures and atomic structures.

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