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  Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

Keune, W., Hong, S., Hu, M. Y., Zhao, J., Toellner, T. S., Alp, E. E., et al. (2018). Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization. Physical Review B, 98(2): 024308. doi:10.1103/PhysRevB.98.024308.

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PhysRevB.98.024308.pdf (Publisher version), 3MB
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PhysRevB.98.024308.pdf
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 Creators:
Keune, W.1, Author
Hong, Sampyo2, 3, Author
Hu, Michael Y.4, Author
Zhao, J.4, Author
Toellner, T. S.4, Author
Alp, E. Ercan4, Author
Sturhahn, W.5, Author
Rahman, T. S.2, Author
Roldan Cuenya, Beatriz2, 6, Author           
Affiliations:
1Department of Physics, University of Duisburg-Essen, 47057 Duisburg, Germany, ou_persistent22              
2Department of Physics, University of Central Florida, Orlando, Florida 32816, USA, ou_persistent22              
3Division of Physical Sciences, Brewton-Parker College, Mount Vernon, Georgia 30445, USA, ou_persistent22              
4Advanced Photon Source, Argonne National Laboratory, Lemont, Illinois 60439, USA, ou_persistent22              
5Seismological Laboratory, California Institute of Technology, Pasadena, California 91125, USA, ou_persistent22              
6Interface Science, Fritz Haber Institute, Max Planck Society, ou_2461712              

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 Abstract: Isotope-selective 57Fe nuclear resonant inelastic x-ray scattering (NRIXS) measurements and atomic-layer resolved density functional theory (DFT) calculations were used to investigate the effect of interfaces on the vibrational (phonon) density of states (VDOS) of (001)-oriented nanoscale Fe/Ag and Fe/Cr multilayers. The multilayers in the experiment contained isotopically enriched 57Fe monolayers as probe layers located either at the Fe/Ag or Fe/Cr interfaces or in the center of the Fe films. This allows probing of the vibrational dynamics of Fe sites either at the buried interfaces or in the center of the Fe films. For Fe/Ag multilayers, distinct differences were observed experimentally between the Fe-partial VDOS at the interface and in the center. At the Fe/Ag interface, the high-energy longitudinal-acoustic (LA) phonon peak of Fe near ∼35 meV is suppressed and slightly shifted to lower energy, and the low-energy part of the VDOS below ∼20 meV is drastically enhanced, as compared to the Fe-specific VDOS in the center Fe layers or in bulk Fe. Similar phenomena are found to a less degree in the Fe/Cr multilayers. The measured Fe-partial VDOS was used to determine the Fe site-selective vibrational thermodynamic properties of the multilayers. Our theoretical findings for the layer-dependent VDOS of the multilayers are in qualitative agreement with the experimental results obtained by NRIXS. For Fe/Ag multilayers, which are characterized by a large atomic mass ratio, the experimental and theoretical results demonstrate phonon confinement in the Fe layers and phonon localization at the Fe/Ag interfaces due to the energy mismatch between Ag and Fe LA phonons. These phenomena are reduced or suppressed in the Fe/Cr multilayers with their about equal atomic masses. Moreover, direction-projected Fe VDOS along the (nearly in-plane) incident x-ray beam was computed in order to address the intrinsic vibrational anisotropy of the Fe/Ag multilayer. We have also performed spin-resolved electronic band structure (DFT) calculations, predicting an enhanced magnetic moment (μFe=2.8μB) of the interfacial Fe atoms and a high electron spin polarization (79%) at the Fermi energy for the Fe/Ag interface, as compared to the case of Fe center layers. This is a result of charge transfer from Fe to Ag at the interface. On the contrary, Cr tends to donate electrons to Fe, thus reducing the interfacial Fe moment (μFe=1.9μB). This implies strong chemical bonding at the Fe/Ag and Fe/Cr interfaces, affecting the interfacial VDOS.

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Language(s): eng - English
 Dates: 2018-06-172018-03-312018-06-222018-07-202018-07
 Publication Status: Issued
 Pages: 16
 Publishing info: -
 Table of Contents: -
 Rev. Type: Peer
 Identifiers: DOI: 10.1103/PhysRevB.98.024308
 Degree: -

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Title: Physical Review B
  Abbreviation : Phys. Rev. B
Source Genre: Journal
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Publ. Info: Woodbury, NY : American Physical Society
Pages: 16 Volume / Issue: 98 (2) Sequence Number: 024308 Start / End Page: - Identifier: ISSN: 1098-0121
CoNE: https://pure.mpg.de/cone/journals/resource/954925225008