| 1. |
- Zeiger, Paul, 1994-, et al.
(författare)
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Control of the phonon band gap with isotopes in hexagonal boron nitride
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The isotopic mass of constituent elements of materials has a well-known effect on the energy of vibrational modes. By means of monochromated scanning transmission electron microscopy we have experimentally studied the phonon bandstructure of hexagonal BN, where a phonon band gap appears between in-plane optical phonon modes and the lower energy part of the phonon spectrum. The size of the phonon band gap can be manipulated by the isotopic mass of the boron. While in 11BN the phonon band gap is about 7 meV wide, in 10BN the gap nearly closes, being an order of magnitude smaller (below 0.5 meV). This opens exciting options for manipulating terahertz wave propagation through isotopically structured devices having otherwise no interfaces between chemically distinct components.
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| 2. |
- Zeiger, Paul M., 1994-, et al.
(författare)
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Lessons from the harmonic oscillator : Reconciliation of the frequency-resolved frozen phonon multislice method with other theoretical approaches
- 2023
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Ingår i: Physical Review B. - : American Physical Society. - 2469-9950 .- 2469-9969. ; 108:9
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Tidskriftsartikel (refereegranskat)abstract
- We compare the frequency-resolved frozen phonon multislice (FRFPMS) method [introduced in P. M. Zeiger and J. Rusz, Phys. Rev. Lett. 124, 025501 (2020)] with other theoretical approaches used to account for the inelastic scattering of high-energy electrons, namely, the first-order Born approximation and the quantum excitation of phonons model. We show that these theories lead to similar expressions for the single inelastically scattered intensity as a function of momentum transfer for an anisotropic quantum harmonic oscillator in a weak phase object approximation of the scattered waves, except for a too small smearing of the scattering potential by the effective Debye-Waller factor (DWF) in the FRFPMS method. We propose that this issue can be fixed by including an explicit DWF smearing into the potential and demonstrate numerically that in any realistic situation, a FRFPMS approach revised in this way correctly accounts for the single inelastically scattered intensity and the correct elastic scattering intensity. Furthermore, our simulations illustrate that the only requirement for such a revised FRFPMS method is the smallness of mean-squared displacements for all atomic species in all frequency bins. The analytical considerations for the FRFPMS method also explain the 1/ω2 scaling of FRFPMS spectra observed by P. M. Zeiger and J. Rusz [Phys. Rev. B 104, 104301 (2021)] by the use of classical statistics in the molecular dynamics simulation. Moreover, we find that the FRFPMS method inherently adds the contributions of phonon loss and gain within each frequency bin. Both of these issues related to the frequency scaling can be fixed by a system-independent postprocessing step.
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