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Abstract
We developed a multi-line time-domain interferometry (TDI) system using 14.4 keV Mössbauer gamma rays with natural energy widths of 4.66 neV from 57Fe nuclei excited using synchrotron radiation. Electron density fluctuations can be detected at unique lengths ranging from 0.1 nm to a few nm on time scales from several nanoseconds to the sub-microsecond order by quasi-elastic gamma-ray scattering (QGS) experiments using multi-line TDI. In this report, we generalize the established expression for a time spectrum measured using an identical single-line gamma-ray emitter pair to the case of a nonidentical pair of multi-line gamma-ray emitters by considering the finite energy width of the incident synchrotron radiation. The expression obtained illustrates the unique characteristics of multi-line TDI systems, where the finite incident energy width and use of a nonidentical emitter pair produces further information on faster sub-picosecond-scale dynamics in addition to the nanosecond dynamics; this was demonstrated experimentally. A normalized intermediate scattering function was extracted from the spectrum and its relaxation form was determined for a relaxation time of the order of 1 μs, even for relatively large momentum transfer of ~31 nm−1. The multi-line TDI method produces a microscopic relaxation picture more rapidly and accurately than conventional single-line TDI.
Highlights
It is important to understand the microscopic dynamics in condensed matter when examining the physical properties and functionalities of systems
Conventional quasi-elastic gamma-ray scattering (QGS) with 14.4 keV gamma rays from 57Fe nuclei uses a limited energy component of ~neV width in the white spectrum of synchrotron radiation (SR) (~eV) because gamma ray emitters with single-line excitation profiles are used
We show the QGS time spectra that were obtained at 237.5 K at 14 nm−1 and 31 nm−1 in Fig. 3(d,e), respectively
Summary
It is important to understand the microscopic dynamics in condensed matter when examining the physical properties and functionalities of systems. TDI systems were developed using double-line emitters, and the measurement efficiency was improved[10] In both single-line and double-line TDI methods, unfavourable fluctuations in the constant Doppler velocity due to the mechanical motion of the velocity transducer lead to additional large-scale broadening of the energy widths of the probe gamma rays; this causes “pseudo-relaxation” in the resulting S(q, t). This broadening significantly reduces the potential efficiency of TDI, when the sample’s dynamics are relatively slow (Γ < Γ0). Suppression of the speed-up effect is another reason why higher measurement efficiency has been demonstrated by the multi-line method to date when compared with that of the single-line method[11]
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