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Anharmonic softening of Raman active phonons in iron-pnictides: Estimating the Fe isotope effect due to anharmonic expansion

Författare och institution:
Mats Granath (Institutionen för fysik (GU)); Johan Bielecki (Institutionen för teknisk fysik, Kondenserade materiens fysik, Chalmers); Joakim Holmlund (Institutionen för teknisk fysik, Kondenserade materiens fysik, Chalmers); Lars Börjesson (Institutionen för teknisk fysik, Kondenserade materiens fysik, Chalmers)
Publicerad i:
Physical Review B, 79 ( 23 ) s. 235103
Artikel, refereegranskad vetenskaplig
Sammanfattning (abstract):
We present Raman measurements on the iron-pnictide superconductors CeFeAsO1-xFx and NdFeAsO1-xFx. Modeling the Fe-As plane in terms of harmonic and a cubic anharmonic Fe-As interactions, we calculate the temperature dependence of the energy and lifetime of the Raman active Fe B-1g mode and fit to the observed energy shift. The shifts and lifetimes are in good agreement with those measured also in other Raman studies which demonstrate that the phonon spectrum, at least at small wave numbers, is well represented by phonon-phonon interactions without any significant electronic contribution. Even at zero temperature there is a non-negligent effect of interactions on the phonon energy, which for the Fe B-1g mode corresponds to 6 cm(-1) or 3% of the total energy of the mode. We also estimate the anharmonic expansion from Fe (56 -> 54) isotope substitution to Delta a approximate to 5.1 x 10(-4) angstrom and Delta d(Fe-As) approximate to 2.5 x 10(-4) angstrom and the shift of harmonic zero-point fluctuations of bond lengths less than or similar to 3 x 10(-5) angstrom(2), giving a total relative average decrease in electronic hopping integrals of vertical bar delta vertical bar/t less than or similar to 2.0 x 10(-4). For a nonphonon-mediated weak-coupling superconductor this gives an isotope exponent alpha similar to 10(-2). The results pose a serious challenge for any theory of superconductivity in the pnictides that does not include electron-phonon interactions to produce a sizable Fe isotope effect.
Ämne (baseras på Högskoleverkets indelning av forskningsämnen):
Fysik ->
Den kondenserade materiens fysik
Materialteknik ->
Annan materialteknik ->
Funktionella material
Annan teknik ->
Övrig annan teknik ->
Materialfysik med ytfysik
Ytterligare information:
Article Number: 235103
Postens nummer:
Ingår i post nr:
Posten skapad:
2010-01-08 15:21
Posten ändrad:
2013-10-29 15:25

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