Frequency spectra of quantum beats in nuclear forward scattering: the enhancement of zero-frequency peak through nano-structuring

Frequency spectra of quantum beats (QBFS) in nuclear forward scattering (Nfs) are analyzed and compared to Moessbauer spectra. The most characteristic combinations of magnetic and quadrupole interactions are considered in detail for 57Fe. Pure magnetic Zeeman splitting corresponds to a eight-line spectrum of Qb, six of which show the same energy separation as the six lines in Moessbauer spectra. Two other lines (called 2' and 3') are the lower-energy satellites of the lines 2 and 3. As the quadrupole interaction Eq appears, the satellites remain unsplit in the quantum beat frequency spectra, as well as the first (zero-frequency) and the (largest frequency) lines. Each of the lines 3 and 5 generates the doublet, and the lines 2 and 4 generate the triplets according to the obtained relationship useful to fit fruitfully the Nfs spectra. In the QBFS of thin absorbers of GdFeO3 we demonstrate the enhanced spectral resolution compared to Moessbauer spectra. Small particle size in an antiferromagnet (α-Fe2O3) was found to affect the QBFS via enhancement of intensity around zero-frequencies. It is shown, that the QBFS are normalized to the height of zero-frequency peak, which is enhanced by 66 % in the middle panel. This enhancement corresponds in Moessbauer spectra of thick sample (ζ=33) to the broad U-shaped component between -20 to 20 mm/s. With growing the particle size by sample annealing both the U-component and the QBFS zero-frequency peak enhancement are suppressed