Resonant x-ray fluorescence holography: Three-dimensional atomic imaging in true color

We propose resonant x-ray fluorescence holography as a method for selectively imaging atoms of a specific atomic number as near neighbors around a fluorescing atom of a different atomic number. Theoretical calculations for ${\mathrm{FeNi}}_{3}(001)$ show that the images of Ni atoms neighboring a fluorescing Fe atom can be selectively reconstructed via the differential contrasts from several holograms obtained as the incident x-ray energies are varied across the Ni K absorption edge. Thus, this technique should provide a unique way for determining the chemical order and disorder around a given atom, through resonant selection of its atomic number or ``true color.''