Atomic Vibrational Density Of States Of Crystalline Beta-Fesi2 And Amorphous Fesi2 Thin Films

Nuclear resonant inelastic x-ray scattering of 14.4125 keV synchrotron radiation was used to measure directly the partial vibrational density of states (VDOS), g(E), of crystalline beta-(FeSi2)-Fe-57 and amorphous(a-)(FeSi2)-Fe-57 thin films prepared by codeposition in ultrahigh vacuum. The structure of the samples was characterized by x-ray diffraction and Mossbauer spectroscopy. The VDOS of beta-FeSi2 extends up to E(max)similar to65 meV and exhibits a strong peak at Esimilar to36 meV and weaker bands centered at about 25, 43, and 53 meV. These characteristic features coincide with positions of prominent IR and Raman spectral lines reported in the literature. The measured VDOS shows good agremeent with the theoretical VDOS of crystalline beta-FeSi2 computed by using the density functional theory combined with the direct method. Contrary to the crystalline phase, the VDOS of a-FeSi2 shows a broad peak at similar to30 meV with little structure, and a deviation from Debye behavior at small excitation energies (<15 meV). This is revealed as a peak in the reduced VDOS, g(E)/E-2, at E(bp)similar to10 meV, which is interpreted as "boson peak." Above the boson peak (30 meVless than or similar toEless than or similar to60 meV) g(E)/E-2 was observed to be approximately proportional toexp(-E/E-0), with E-0=7.4 meV being close to E-bp.